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Chevrolet Corvette 1991 - G - TESTS W/CODES 1990-91 ENGINE PERFORMANCE General Motors Self-Diagnostics - ECM
Chevrolet Corvette 1991 - INTRODUCTION
NOTE: Due to the limited production of the ZR-1 Corvette, the testing procedures in this article do not apply to the ZR-1 model.
Most engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the computer system as a possible cause of problems, perform checks and inspections covered in BASIC TESTING article. Failure to do so may result in lost diagnostic time.
If no faults were found while performing BASIC TESTING PROCEDURES, proceed with DIAGNOSTIC PROCEDURE. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to TESTS W/O CODES article in this section for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES article in this section.
Chevrolet Corvette 1991 - SELF-DIAGNOSTIC SYSTEM
All vehicle are equipped with either an Electronic Control Module (ECM) or Powertrain Control Module (PCM). Unless specifically stated, references to ECM also apply to PCM equipped vehicles. Control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate the SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in control module memory. To retrieve stored codes, see RETRIEVING CODES (NON-SCAN) in this article. Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.
Chevrolet Corvette 1991 - HARD FAILURES
Hard failures cause SERVICE ENGINE SOON light to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, control module will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but most likely degraded driveability will be encountered.
Chevrolet Corvette 1991 - INTERMITTENT FAILURES
Intermittent failures cause SERVICE ENGINE SOON light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in control module memory. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from control module memory. Intermittent failures may be caused by sensor, connector or wiring related problems. - INTERMITTENTS in the TESTS W/O CODES article in this section.
Chevrolet Corvette 1991 - DIAGNOSTIC PROCEDURE
Diagnosis of the computerized engine control system should be performed in the following order:
- Make sure all engine systems not related to the computer system are operating properly. DO NOT proceed with testing unless all other problems have been repaired. DIAGNOSTIC CIRCUIT CHECK must be performed prior to utilizing trouble code charts. - the BASIC TESTING article in this section.
- If trouble codes were displayed (other than Code 12), decide whether codes are hard or intermittent trouble codes. Hard codes will cause the SERVICE ENGINE SOON light to illuminate continuously while engine is running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION in this article. For diagnosing hard codes, proceed to appropriate trouble code chart in this article. For diagnosing intermittent codes, see INTERMITTENTS in TESTS W/O CODES article in this section. Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes.
- If no trouble codes were displayed and a driveability problem exists, go to SYMPTOMS in TESTS W/O CODES article in this section. The comments there will send you to the proper system or component to check in SYSTEM/COMPONENT TESTS article in this section.
- After any repairs are made, clear any trouble codes and perform FIELD SERVICE MODE check in BASIC TESTING article in this section.
Chevrolet Corvette 1991 - RETRIEVING CODES (NON-SCAN)
- Turn ignition on. DO NOT start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Data Link (ALDL) connector attached to control module wiring harness. Most ALDL connectors are located under dash on driver's side of vehicle. For location of ALDL, see COMPONENT LOCATIONS illustration in SYSTEM/COMPONENT TESTS article in this section. Turn ignition on with engine not running. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. See Fig. 1.
NOTE: Inserting jumper wire into test and ground terminals of ALDL connector with engine running will cause fuel injected vehicles to enter field service mode. Flashes of the SERVICE ENGINE SOON light will not indicate codes if this is done. See FIELD SERVICE MODE in BASIC TESTING article in this section. 
Fig. 1: Chevrolet Corvette 1991 - Component Locations - ALDL Connector Terminal Identification - SERVICE ENGINE SOON light should begin to flash codes. Each code will be repeated 3 times. If codes are not flashed or SERVICE ENGINE SOON light does not illuminate, perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
Chevrolet Corvette 1991 - READING TROUBLE CODES
The control module stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Trouble codes may be read by counting flashes of the SERVICE ENGINE SOON light, or by reading the output of a diagnostic "Scan" tester connected to the ALDL connector. The tester is faster to use, more accurate, and capable of reading information which otherwise would necessitate testing individual control module and sensor/solenoid connector terminals with a digital voltmeter. See SCAN TESTER USAGE and tables under SCAN DATA in this article.
NOTE: When using a "Scan" tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.
If "Scan" tester is not available, it is possible to read flashes of the SERVICE ENGINE SOON light by grounding the diagnostic test terminal "B" of the ALDL with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, identifies Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest numbered code. Each code is displayed 3 times. Codes will continue to repeat as long as ALDL test terminal is grounded.
NOTE: Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others may require operation for 5 minutes or longer at normal operating temperature, road speed and load. Therefore, some codes may not set in a service bay operational mode and may require road testing vehicle in order to duplicate condition under which code will set.
Chevrolet Corvette 1991 - TROUBLE CODE DEFINITION
Chevrolet Corvette 1991 ECM/PCM TROUBLE CODE DEFINITION
Code No. Circuit Affected 12 (1) No RPM Reference Pulse 13 Open Oxygen Sensor Circuit 14 CTS Signal Voltage Low 15 CTS Signal Voltage High 21 TPS Signal Voltage High 22 TPS Signal Voltage Low 23 MAT Sensor Signal Voltage High 24 Vehicle Speed Sensor Circuit 25 (IAT) MAT Sensor Signal Voltage Low 33 MAP Sensor Signal Voltage High 34 MAP Sensor Signal Voltage Low 41 Cylinder select Error 42 EST Circuit Open Or Grounded 43 ESC Error 44 Lean Exhaust Indication 45 Rich Exhaust Indication 46 Anti-Theft System Fault 51 Faulty PROM, MEM-CAL or ECM/PCM 52 Low Engine Oil Temperature 53 System Overvoltage 54 Fuel Pump Voltage Low 62 High Engine Oil Temperature
(1) Display of a Code 12 is normal when no reference pulses are received by control module (engine not running).
NOTE: Trouble code charts should only be used if SERVICE ENGINE SOON light is illuminated (indicating a current problem exists). Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime control module-related Codes 51, 52 or 55 are displayed with another code, start with 50-series code first, then proceed to low profile numbered codes.
Chevrolet Corvette 1991 - HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
During any diagnostic procedure, it must be determined if codes are hard failure codes or intermittent failure codes. Diagnostic charts will not usually help analyze intermittent codes. To determine hard codes and intermittent codes, proceed as follows:
- MANUALLY enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See CLEARING TROUBLE CODES.
- Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. SERVICE ENGINE SOON light should go out. Run warm engine at specified curb idle for 2 minutes and note SERVICE ENGINE SOON light.
- If SERVICE ENGINE SOON light comes on, MANUALLY enter diagnostic mode. Read and record trouble codes. This will reveal hard failure codes. Codes 13, 15, 24, 44, 45 and 55 may require a road test to reset hard failure after trouble codes were cleared.
- If SERVICE ENGINE SOON light does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under DIAGNOSTIC PROCEDURE.
Chevrolet Corvette 1991 - CLEARING TROUBLE CODES
Turn ignition switch to ON position and ground diagnostic test terminal "B" at ALDL connector. Turn ignition switch to OFF position and remove control module fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. If fuse cannot be located, pigtail at battery can be disconnected. When power to ECM is removed, degraded driveability may be exhibited until control module "relearns" optimum operational parameters.
Chevrolet Corvette 1991 - ECM/PCM LOCATION
Control module is located in the left rear corner of the engine compartment, next to the battery. For illustration of engine control module locations, see COMPONENT LOCATIONS in the appropriate SYSTEM/COMPONENT TESTS article in this section.
Chevrolet Corvette 1991 - DIAGNOSTIC MATERIALS Diagnostic Aids
Diagnostic aids (located in many trouble code charts) are additional tips used to help diagnose trouble codes when inspected circuit checks out okay. Diagnostic aids may help lead to a definitive solution to that trouble code problem.
Chevrolet Corvette 1991 - FIELD SERVICE MODE CHECK
If ALDL test terminal "B" is grounded with engine running, SERVICE ENGINE SOON light will indicate operational mode of engine. This test confirms proper operation of fuel system and verifies closed loop operation. Clear codes and perform this test after any repair is completed. Field service mode check can be found by proceeding to FIELD SERVICE MODE CHECK in BASIC TESTING article in this section.
Chevrolet Corvette 1991 - SPECIAL TOOLS (DIAGNOSTIC)
NOTE: Special "Scan" testers plugged into the ALDL may be used to read trouble codes and check voltages in the system on the serial data line (terminal "E", or terminal "M" on P-4 systems). These testers can save a great deal of time. For additional information, see SCAN TESTER USAGE and tables under SCAN DATA in this article.
The computerized engine control system is most easily diagnosed using a "Scan" tester; however, other tools may aid in diagnosing problems if a "Scan" tester is unavailable. These tools are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights (for both TBI and PFI) and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.
Chevrolet Corvette 1991 - SCAN TESTER USAGE
NOTE: Prior to connection of "Scan" tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received by "Scan" tester will be accurate. This is done by performing DIAGNOSTIC CIRCUIT CHECK located in BASIC TESTING article in this section. If vehicle does not pass diagnostic circuit check, information received by "Scan" tester may be invalid.
The "Scan" tester is a specialized tester which, when plugged into ALDL, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to backprobe sensors and connectors. "Scan" testers cut down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See tables under SCAN DATA in this article. They may also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication that output signals have been sent to devices by the control module. It does not indicate if devices have responded properly to that signal. This will need to be verified at output device using a voltmeter or test light.
NOTE: Code 12 should always exist when ALDL is grounded with key on and engine not running, but may not be indicated by all makes of "Scan" testers.
If trouble codes are not present, this is not an indication that there is not a problem. Driveability related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Sensors that are out of specification WILL NOT set a trouble code but WILL cause driveability problems. Using a "Scan" tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See tables under SCAN DATA in this article.
NOTE: Information obtained by "Scan" tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, it will be necessary to verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off, remove tester, turn ignition on and ground ALDL test terminal "B". If same codes are not flashed by SERVICE ENGINE SOON light that were indicated by "Scan" tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.
Chevrolet Corvette 1991 - SCAN DATA
NOTE: Information contained in the following tables is typical of readings taken on vehicle with engine idling, upper radiator hose hot, closed throttle, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Data parameters are updated every 1 1/4 seconds. On systems using P-4 computers, parameter updates are more often. Not all devices and systems are used on all models and the following lists represent only the most commonly used parameters. For additional information, refer to owners manual furnished with tester.
Chevrolet Corvette 1991 PORT FUEL INJECTION
Tester Position Units Measured Nominal Value A/C Clutch On/Off Off (On With A/C) A/C Request Yes/No No/Yes (With Request) AIR Divert Sol. On/Off On (Air To Switching Sol.); Off (Air To Atmosphere) AIR Switching Sol. On/Off On (To Exhaust Manifold); Off (To Catalytic Converter) BARO Volts 3-4.5 Battery Voltage Volts 13.5-14.5 Block Learn Counts 118-138 (128 Normal) Canister Purge Sol. On/Off On/Engine Cold (Idle Some) Clear Flood On/Off See Tester Manual Coolant Fan On/Off Off below 216?F (102?C) Coolant Temp. ?C 85-105? (Norm. Temperature) Crank RPM RPM 100-900 Cross Counts Counts 0-255EGR Solenoid On/Off On When Energized EGR Duty Cycle 0-100% 0/Closed-100/fully Open Fan Relay On/Off On When Energized Fan Request On/Off On With Request Fuel Back-Up Yes/No Yes When Engaged IAC Counts 0-50 Ignition/Crank On/Off On With Ignition/Crank Injector Pulse Width Mil./Sec .8-3.0 INT (Integrator) Counts 110-145 (128 Normal) Knock Retard (ESC) Counts 0-255 Knock Signal Yes/No Yes When Knock Exists MAT ?C 10-90? MAP Volts 1 (Idle) to 4.5 (WOT) Open/Closed Loop Status Ol/Cl Closed/Open During Extended Idle O2 Sensor Millivolts 100 (Lean) To 999 (Rich) P/N Switch P/N/RDL Park/Neutral P/S Switch Norm/Hi Normal PROM I.D. PROM # Original Factory Number RPM RPM Spec. +/-25 RPM Drive (A/T); Spec. +/-50 RPM Neut. (M/T) Spark Advance No. Deg. Varies TCC On/Off Off (On With Command) TPS Volts 1.25 (Idle) to 5.0 (WOT) Throttle Angle 0-100% 0 (Idle) To 100 (WOT) Trouble Codes Code # No Codes Upshift Light (Man. Trans.) On/Off Off VSS or MPH MPH 0-Actual Water Injection On/Off On When Injecting 1st Gear Switch On/Off On/1st Gear Only 3rd Gear Switch On/Off On/3rd & 4th Gear 4th Gear Switch On/Off On/4th Gear
Chevrolet Corvette 1991 - TEST PROCEDURE SUMMARY
If no hard fault codes are present, driveability symptoms exist or intermittent codes exist, see TESTS W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures.
Chevrolet Corvette 1991 - DIAGNOSTIC FLOW CHARTS FOR CODES: 13 TO 62
NOTE: Mini-schematics are supplied courtesy of General Motors Corp.
Chevrolet Corvette 1991 - CODE 13, OPEN OXYGEN SENSOR CIRCUIT
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This tests if problem still exists. Vehicle cannot enter closed loop mode if oxygen sensor circuit is open. Code 13 indicates an open in the O2 sensor circuit. Code will set if:
- Engine is at normal operating temperature.
- Neither Code 21 nor Code 22 are stored.
- O2 sensor voltage is constant within a specified range (.34-.55 volt).
- Throttle angle is greater than idle.
- A precalibrated amount of time has elapsed since start-up.
- All conditions have existed for a precalibrated amount of time.
- Determines if O2 sensor, wiring or control module is at fault. If wiring is good, grounding oxygen sensor wire will cause .45 volt reference supplied by the control module to pull low.
- This tests O2 sensor circuit wiring. Use only a high impedance (10-megohm minimum) digital voltmeter.
Chevrolet Corvette 1991 - Diagnostic Aids
Control module will not go into closed loop if Code 13 is set. Code 13 may set if vehicle runs out of fuel or stalls while vehicle is in motion. If oxygen sensor ground becomes loose, a false oxygen sensor reading will occur. This can result in a Code 13 being set.
Chevrolet Corvette 1991 CODE 13 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color O2 Signal A16 Purple O2 Ground A22 Tan
Chevrolet Corvette 1991 - CODE 14, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE LOW
NOTE: This chart assumes engine cooling system is functioning properly (not overheating). Test numbers refer to test numbers on diagnostic chart.
- Code 14 indicates the control module has seen low coolant sensor voltage signal (high temperature) at control module terminal for a precalibrated period of time. This checks if conditions for Code 14 still exist.
- This tests for grounded sensor signal line between control module and coolant sensor.
Chevrolet Corvette 1991 - Diagnostic Aids
After the engine is started, temperature should rise steadily to about 190?F (88?C), then stabilize when thermostat opens. At normal operating temperature, signal voltage at control module terminal should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. When Code 14 is set, control module will turn on electric cooling fan(s), if equipped. Paste-up, place TEMPERATURE-TO-RESISTANCE VALUES table after flow chart. See CODE 14 EXAMPLE
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Chevrolet Corvette 1991 CODE 14 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color CTS Signal C16 Yellow CTS Ground C10 Black
Chevrolet Corvette 1991 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature ?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Corvette 1991 - CODE 15, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 15 indicates control module has seen high resistance in coolant sensor circuit. This could be due to high resistance (cold temperature) or high voltage at coolant sensor terminal at control module for a precalibrated period of time. This checks if conditions for Code 15 still exist.
- This test simulates conditions for a Code 14. If control module recognizes the low voltage signal, "Scan" tester will display greater than 130?C. This indicates the control module and wiring are not at fault.
- This test determines if coolant sensor ground or signal circuit is open.
Chevrolet Corvette 1991 - Diagnostic Aids
After the engine is started, temperature should rise steadily to about 190?F (88?C), then stabilize when thermostat opens. At normal operating temperature, voltage at control module sensor signal line should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor temperature-to-resistance table. When Code 14 is set, control module will turn on electric cooling fan(s), if equipped. Paste-up, place TEMPERATURE-TO-RESISTANCE VALUES table after flow chart. See CODE 14 EXAMPLE
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Chevrolet Corvette 1991 CODE 15 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color CTS Signal C16 Yellow CTS Ground C10 Black
Chevrolet Corvette 1991 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature ?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Corvette 1991 - CODE 21, THROTTLE POSITION SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test checks if code is the result of a hard failure or an intermittent condition.
- This test simulates conditions for a Code 22. If control module recognizes the change of state, the control module and wiring are okay.
- This step isolates a faulty sensor, control module or open sensor ground circuit. If sensor ground is shared by another sensor, there may be an accompanying code related to that sensor.
Chevrolet Corvette 1991 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate, as throttle angle is increased. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Chevrolet Corvette 1991 CODE 21 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color TPS Signal C15 Dark Blue TPS Ground C10 Black TPS Reference C7 Gray
Chevrolet Corvette 1991 - CODE 22, THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test checks if code is the result of a hard failure or an intermittent condition.
- This test simulates conditions for a Code 21. If control module recognizes the change of state, the control module and wiring are okay.
- This simulates a high signal voltage to check for an open in the TPS signal line to control module. "Scan" tester should recognize this signal and display high TPS voltage.
Chevrolet Corvette 1991 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate, as throttle angle is increased. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article.
Fig. 10: Chevrolet Corvette 1991 - Component Locations - Code 22, Schematic, TPS Signal Voltage Low
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 11: Chevrolet Corvette 1991 - Component Locations - Code 22, Flow Chart, TPS Signal Voltage Low
Chevrolet Corvette 1991 CODE 22 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color TPS Signal C15 Dark Blue TPS Ground C10 Black TPS Reference C7 Gray
Chevrolet Corvette 1991 - CODE 23, MAT SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if code is the result of a hard failure or an intermittent condition. Code 23 will set if engine has been running for a precalibrated period of time, has reached operating temperature and signal voltage indicates a MAT temperature less than -22?F (-30?C).
- This simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the control module and wiring are not at fault.
- This checks for continuity of sensor signal and ground circuits. If ground circuit is shared by other sensors and ground circuit is open, accompanying codes related to those sensors may be present.
Chevrolet Corvette 1991 - Diagnostic Aids
If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a "Scan" tester. A Code 23 will result if signal and ground circuits become open. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Chevrolet Corvette 1991 CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color MAT Signal C4 Tan MAT Ground C5 Black/Pink
Chevrolet Corvette 1991 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature ?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Corvette 1991 - CODE 24, VEHICLE SPEED SENSOR (VSS)
The speed sensor, which is a Permanent Magnet (PM) generator, provides the control module with vehicle speed information. The PM generator, mounted in the transmission, produces a pulsing voltage signal whenever the vehicle speed is more than 3 MPH. The voltage level and pulses increase with vehicle speed. The control module converts the pulsing voltage to MPH, which is used by the control module in calculations to determine vehicle adjustments.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- A Code 24 will set when MPH reads zero, transmission is not in Park or Neutral, engine speed indicates vehicle is in a cruise mode (1200-4400) RPM, TPS indicates closed throttle and high manifold vacuum is sensed by the MAP sensor. All of these conditions must be met for 2-5 seconds. The PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
- Before replacing the control module, PROM/MEM-CAL should be checked for correct application.
Chevrolet Corvette 1991 - Diagnostic Aids
A faulty or misadjusted park/neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter. Code 24 may set if vehicle is power braked (brakes applied and throttle depressed) for more than 10 seconds.
NOTE: Disregard Code 24 that sets while wheel are not turning.
Chevrolet Corvette 1991 - CODE 25, (IAT) MAT SENSOR SIGNAL VOLTAGE LOW
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if the code is a hard failure or an intermittent condition. Code 25 will set if a MAT temperature greater than 266?F (130?C) is sensed for more than a precalibrated period.
- This simulates condition for Code 23. if control module recognizes the open circuit and "Scan" tester displays a temperature of less than -30? C, control module and wiring are okay.
Chevrolet Corvette 1991 - Diagnostic Aids
If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor should read close to each other, when measured with a "Scan" tester. A Code 25 will result if sensor signal circuit is shorted to ground. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Chevrolet Corvette 1991 CODE 25 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color (IAT) MAT Signal C4 Tan (IAT) MAT Ground C5 Black/Pink
Chevrolet Corvette 1991 TEMPERATURE-TO-RESISTANCE VALUES (1)
Temperature ?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
Chevrolet Corvette 1991 - CODE 32, EGR SYSTEM ERROR
EGR vacuum is regulated by an ECM-controlled solenoid. The ECM will turn the EGR on and off (duty cycle) by grounding and removing ground to circuit No. 435. The duty cycle is calculated by the ECM based on coolant temperature, airflow and engine RPM. There should be no EGR under the following conditions:
- Vehicle is in Park or Neutral
- Throttle position sensor input is less than a specified value or TPS is indicating Wide Open Throttle (WOT).
With the ignition on and engine stopped, the EGR solenoid is de-energized. By grounding the ALDL test terminal, the solenoid should energize.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Grounding the ALDL test terminal should cause the EGR solenoid to close, allowing vacuum to be applied. Vacuum should hold.
- Determines if the electrical control part of the system is at fault or if the connector or solenoid are at fault.
- By plugging the EGR valve side and removing ground from the ALDL test terminal, solenoid valve should open and allow vacuum to bleed off through vent.
- With the engine not running and vacuum applied to valve, valve should move to the fully open position.
- System uses negative backpressure to open EGR valve. Valve should close when the engine is cranked.
Fig. 18: Chevrolet Corvette 1991 - Component Locations - Code 32, Schematic, EGR System Error
Fig. 19: Chevrolet Corvette 1991 - Component Locations - Code 32, Flow Chart, EGR System Error
Chevrolet Corvette 1991 - CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 33 and determines if it is the result of a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high for greater than a precalibrated period of time, TPS voltage indicates throttle is closed and neither Code 21 or 22 is present.
- This step simulates conditions for a Code 34. If the control module recognizes and sets Code 34, low MAP signal, the control module and 5-volt reference and MAP signal circuits are not at fault. If ground circuit is shared with other sensors and ground circuit becomes open, additional codes related to these sensors may be set.
Chevrolet Corvette 1991 - Diagnostic Aids
With the ignition switch in the ON position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the BARO readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. Code 33 will result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Chevrolet Corvette 1991 - CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This confirms Code 34 and determines if code was caused by a hard failure or an intermittent fault. Code 34 will set when ignition is on and MAP signal voltage is low. On some systems, engine must be running to set code.
- Jumpering MAP signal to 5-volt reference at MAP harness connector will determine if sensor is at fault or if there is a problem with the control module or wiring.
- "Scan" tester may not display 12 volts. The important thing is the control module recognizes the voltage as greater than 4 volts (high MAP voltage signal), indicating the control module and MAP signal circuit are not at fault.
Chevrolet Corvette 1991 - Diagnostic Aids
With the ignition switch in the ON position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Compare BARO readings with a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Chevrolet Corvette 1991 - CODE 41, CYLINDER SELECT ERROR
The ECM used for this engine can also be used for other engines. The difference is in the MEM-CAL. If a Code 41 sets, the incorrect MEM-CAL has been installed or MEM-CAL is faulty and must be replaced.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Turn ignition off and clear codes. Start engine and run for one minute. If code resets, check for faulty connection due to MEM-CAL not locked in place or incorrectly installed. If code does not recur, code is intermittent. See DIAGNOSTIC AIDS.
Chevrolet Corvette 1991 - Diagnostic Aids
Check MEM-CAL to be sure locking tabs are secure. Also check the pins on both the MEM-CAL and ECM to verify proper contact. Check the MEM-CAL part number for proper application. If the correct MEM-CAL is installed and is defective, it is possible the ECM will also need replacing.
Fig. 24: Chevrolet Corvette 1991 - Component Locations - Code 41, Flow Chart, Cylinder Select Error
Chevrolet Corvette 1991 - CODE 42, EST CIRCUIT OPEN OR GROUNDED
Code 42 indicates the ECM has seen an open or short to ground in HEI EST or by-pass circuit.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
- This test checks for a normal EST ground path through the ignition module. If EST circuit is shorted to ground, reading will be less than 500 ohms.
- As test light voltage touches by-pass circuit, the module should switch. This will cause the ohmmeter to switch from hundreds of ohms to thousands of ohms. This test assures that the module "switched".
- If module did not switch, this step will test for a short in circuit No. 423, an open in circuit No. 424, and a faulty ignition module connection or module.
- This step confirms that Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.
Chevrolet Corvette 1991 - Diagnostic Aids
The "Scan" tester does not have the ability to help diagnose a Code 42 problem. See INTERMITTENTS in the TESTS W/O CODES article in this section.
Chevrolet Corvette 1991 - CODE 43, ESC ERROR WITHOUT ESC MODULE
- If tapping on engine does not produce a knock signal on "Scan" tester, try tapping the engine closer to the sensor before proceeding.
- The ECM supplies a 5-volt reference signal on the knock sensor line. With knock sensor connected this signal is pulled low to about 2.5 volts. Knock signal, which is an AC signal, rides on this 2.5-volt DC signal.
- This test determines if the knock sensor is faulty or if the ESC portion of MEM-CAL is faulty.
Chevrolet Corvette 1991 - Diagnostic Aids
Check knock sensor signal circuit for a potential open or short to ground. An open knock signal circuit would cause the 5-volt reference to remain high at ECM knock signal terminal. A short to ground in the knock signal circuit would cause the 5-volt reference signal to pull low to near zero volts. Resistance to ground through the knock sensor should be 3300-4500 ohms. Also check for proper installation of MEM-CAL.
Chevrolet Corvette 1991 - CODE 44, LEAN EXHAUST INDICATION
Oxygen sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Checks to see if O2 sensor is registering a lean condition. Code 44 is set when O2 sensor voltage signal at control module is low (less than .3 volt) for a precalibrated period and system is operating in "closed loop".
Chevrolet Corvette 1991 - Diagnostic Aids
Using the "Scan" tester, observe the Block Learn Memory (BLM) value at different RPMs. If conditions for a Code 44 exist, the block learn value will be around 150.
Chevrolet Corvette 1991 - Oxygen Sensor (O2) Wire
O2 sensor wire may be mispositioned and laying against the exhaust manifold. Check for ground between sensor and wire connector.
Chevrolet Corvette 1991 - Fuel Contamination
Water, even small amounts, near the in-tank fuel pump inlet can be delivered to the injector. The water may cause a lean exhaust and set Code 44.
Chevrolet Corvette 1991 - Fuel Pressure
System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see the appropriate BASIC TESTING article in this section.
Chevrolet Corvette 1991 - Exhaust Leaks
If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.
Chevrolet Corvette 1991 - Misfire Or Stall
If engine misfires or stalls while vehicle is moving, including running out of fuel, a Code 44 may set. If Code 44 is intermittent, see INTERMITTENTS in the TESTS W/O CODES article in this section.
Fig. 29: Chevrolet Corvette 1991 - Component Locations - Code 44, Schematic, Lean Exhaust Indication
Chevrolet Corvette 1991 CODE 44 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color O2 Signal A16 Purple O2 Ground A22 Tan
Chevrolet Corvette 1991 - CODE 45, RICH EXHAUST INDICATION
Oxygen sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
Code 45 indicates a rich exhaust and diagnosis should begin with these items: fuel pressure, leaking injector, HEI shielding (ground), vapor canister fuel saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Test checks to see if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than idle, O2 sensor signal at control module is greater than .7 volt for a precalibrated period and time since engine start is one minute or more.
Chevrolet Corvette 1991 - Diagnostic Aids
If other codes of lower number are set with Code 45, use those charts first. Malfunction in the MAP or TPS sensor circuits can cause a Code 45 to set. If other codes are not set, Code 45, rich exhaust, is most likely caused by one of the following:
Chevrolet Corvette 1991 - Fuel Pressure High
If fuel pressure is too high, air/fuel ratios will be rich. For fuel pressure checking procedure, see BASIC TESTING article in this section. The control module can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will set.
Chevrolet Corvette 1991 - Ignition Ground
If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses to be picked up by control module on reference line of EST harness. Additional pulses result in a higher than actual engine speed signal. The control module will increase injector pulse width ("on" time) to match the increased RPM signal. "Scan" tester will show higher than actual RPM, which can help in diagnosing this problem.
Chevrolet Corvette 1991 - Evaporative Fuel Canister
Fuel saturation of the charcoal canister will cause a rich air/fuel ratio. If full of fuel, check canister control valves and hoses.
Chevrolet Corvette 1991 - Map Sensor
An output that causes the control module to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the control module to substitute a fixed value for the MAP sensor. If the condition disappears, substitute a different MAP sensor and continue testing.
Chevrolet Corvette 1991 - TPS
An intermittent TPS output will cause the system to operate rich due to a false indication of engine acceleration.
Chevrolet Corvette 1991 - O2 Sensor Contamination
O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White powdery coating to cover the exterior of the O2 sensor. The false high signal voltage (low oxygen content sensed) produced is interpreted by the control module as a rich mixture, causing the control module to set Code 45.
Chevrolet Corvette 1991 - EGR Problem
EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. Also check for shorted or leaking injector, or fuel contaminated oil. If Code 45 is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
Fig. 31: Chevrolet Corvette 1991 - Component Locations - Code 45, Schematic, Rich Exhaust Indication
Chevrolet Corvette 1991 CODE 45 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color O2 Signal A16 Purple O2 Ground A22 Tan
Chevrolet Corvette 1991 - CODE 46, PASS-KEY SYSTEM
The Personal Automotive Security System (PASS-KEY) is designed to disable vehicle operation if an incorrect ignition key or starting procedure is used. The ECM applies and monitors a 5-volt signal to the Central Control Module (CCM). When CCM determines proper key and starting technique is being used, it will complete the PASS-KEY circuit, modifying the ECM-monitored voltage signal and enabling the starter relay. ECM will then allow enablement of the fuel injector circuits and permit vehicle to start and run. If monitored circuit No. 229 becomes open or shorted to ground, Code 46 will set and vehicle will not start.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- If the engine cranks and a Code 46 is stored, it indicates the CCM, which modifies the ECM-monitored signal, is not operating or circuit No. 229 is open or shorted to ground. If CCM is okay and circuit No. 229 is not open or shorted to ground, the ECM may be at fault, but this is not likely.
- If Code 46 is stored and the engine will not start, it indicates an anti-theft system problem, an incorrect ignition key or starting procedure is used.
NOTE: If condition is intermittent, see INTERMITTENTS in the TESTS W/O CODES article in this section.
Fig. 33: Chevrolet Corvette 1991 - Component Locations - Code 46, Schematic, Pass-Key System
Fig. 34: Chevrolet Corvette 1991 - Component Locations - Code 46, Flow Chart, Pass-Key System
Chevrolet Corvette 1991 - PASS-KEY Diagnosis
PASS-KEY is a sophisticated system which interfaces the CCM, ECM and starter with a power source, starter enable relay, ignition switch and instrument cluster. Before replacing the CCM, check the system for the following common problems.
- Check ignition key pellet sensing contacts in the ignition lock cylinder. Look into cylinder lock. If contacts are damaged, replace cylinder lock.
- Check ECM and GAGES fuses
- Check security indicator bulb in the instrument panel.
- A defective resistor pellet within the ignition key or incorrect resistance value of key (15 different assigned values) will cause vehicle not to start. Key must be correct electrically and mechanically. If incorrect key is used to try to start vehicle, decoder will not allow vehicle to start for 2-4 minutes, even if correct key is inserted.
NOTE: Testing and servicing CCM requires special test equipment and documentation.
Chevrolet Corvette 1991 - CODE 51, FAULTY PROM/MEM-CAL
Check that all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace control module.
Chevrolet Corvette 1991 - CODE 52, LOW ENGINE OIL TEMPERATURE
The engine oil temperature sensor is a thermistor, similar to a coolant or air temperature sensor. When engine oil temperature is cold, sensor resistance is high, therefore, ECM will see a high monitored voltage signal on circuit No. 901. As temperature increases, resistance decreases and the ECM sees a lower monitored voltage on the circuit.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 52 will set if engine oil temperature sensor indicates less than -31?F (-35?C) and engine has been running for more than 30 minutes.
- Code 52 will set when open occurs in sensor, wire or connection. This determines if the ECM and wiring are okay.
- Determines if the engine oil temperature signal or sensor ground circuit is open.
Chevrolet Corvette 1991 - Diagnostic Aids
If Code 52 was set, the engine oil life monitor system has been calculating the engine oil life with false information. The oil life monitor must be reset and engine oil and filter changed. A "Scan" tester will read engine oil temperature comparable to MAT or CTS when the engine is cold. Temperature reading on tester should rise as engine oil temperature increases. If Code 52 is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
To reset oil life monitor, turn ignition on. Depress ENG MET button on trip monitor and release. Within 5 seconds, depress and release ENG MET button again. Within 5 seconds, depress and hold the RANGE button on trip monitor. The CHANGE OIL light should flash.
Hold the RANGE button until the CHANGE OIL light stops flashing. When the light goes out, the engine oil life monitor is reset. This should take about 10 seconds. If the light does not reset, turn the ignition off and repeat procedure.
Chevrolet Corvette 1991 - CODE 53, SYSTEM OVERVOLTAGE
This code indicates a basic charging system problem. Code 53 will set when voltage at control module terminal is greater or less than specification for a precalibrated time. If voltage at ECM battery voltage terminal is not within specification, check and repair charging system.
Chevrolet Corvette 1991 CHARGING SYSTEM SPECIFICATIONS
Application Minimum Charge Maximum Charge 5.7L Y Body 8.0 17.1
Fig. 37: Chevrolet Corvette 1991 - Component Locations - Code 53, Flow Chart, System Overvoltage
Chevrolet Corvette 1991 - CODE 54, FUEL PUMP VOLTAGE LOW
The ECM monitors fuel pump pressure switch circuit voltage for fuel system adjustments. This signal is also used to store a trouble code if the fuel pump relay is defective or fuel pump voltage is lost while the engine is running. There should be about 12 volts on the ECM fuel pump voltage monitor circuit for 2 seconds after the ignition is turned on, or any time reference pulses are being received by the ECM.
Code 54 will set if the voltage on the voltage monitor circuit is low for 2-4 seconds since the last reference pulse is cycled off. However, if the voltage detected is low with the engine running, the light will only remain on while the condition exists. If no fault is found, see INTERMITTENTS in TESTS W/O CODES article in this section.
Fig. 38: Chevrolet Corvette 1991 - Component Locations - Code 54, Schematic, Fuel Pump Voltage Low
Chevrolet Corvette 1991 - CODE 61, SECONDARY PORT THROTTLE VALVE SYSTEM
The secondary port throttle valves are vacuum operated, and controlled by an ECM actuated solenoid. Vacuum to operate the valve actuators is supplied by the engine and a self-starting electric vacuum pump.
When the driver has the engine power switch in the "FULL" engine power position and pushes down on the accelerator pedal far enough, the ECM turns "ON" the secondary port throttle valve solenoid allowing vacuum to open the valves.
If vacuum in the vacuum reservoir drops below 41 kPa of pressure (12" Hg of vacuum), while the secondary port throttle valves are commanded open, a vacuum switch within the electric vacuum pump will turn the pump "ON" to supplement engine vacuum.
The ECM monitors vacuum to the valve actuators through a vacuum sensor in line with the actuators. During "FULL" engine power operation the ECM will set a Code 61 if vacuum to the actuators drops below 21 kPa. During "NORMAL" engine power operation (secondary operation disabled) a Code 61 will set if any vacuum is present at the valve actuators. When Code 61 is set, engine speed is limited to 3000 rpm.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 61 will set when:
- Secondary port throttle valves are commanded closed and vacuum sensor signal between .74 volt and 4.89 volts.
- Secondary port throttle valves are commanded open and vacuum sensor signal voltage is less than 1.70 volts.
- If the vacuum pump runs constantly with the ignition "ON" and engine "OFF" or if it does not operate at all, this problem must be corrected before using this chart.
- The output test will not function if either the vehicle is moving or the engine speed is above 2500 rpm. These constraints are imposed by the Tech 1, not the ECM. The output test will be enabled for only 5 seconds.
- If GAUGES fuse is OK, carefully inspect all circuitry for obvious problems prior to removing plenum.
Fig. 39: Chevrolet Corvette 1991 - Component Locations - Code 61 Schematic
Fig. 40: Chevrolet Corvette 1991 - Component Locations - Code 61 Flow Chart
Chevrolet Corvette 1991 - CODE 62, HIGH ENGINE OIL TEMPERATURE
The engine oil temperature sensor is a thermistor, similar to a coolant or air temperature sensor. When engine oil temperature is cold, sensor resistance is high and the ECM will see a high monitored voltage signal on engine oil temperature sensor circuit. As temperature increases, resistance decreases and the ECM sees a lower monitored voltage engine oil temperature sensor circuit.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 62 will set if engine oil temperature sensor indicates a temperature greater than 291?F (144?C) and engine has been running for more than 30 minutes.
Chevrolet Corvette 1991 - Diagnostic Aids
If Code 62 was set, the engine oil life monitor system has been calculating the engine oil life with false information. The oil life monitor must be reset and engine oil and filter changed. A "Scan" tester will read engine oil temperature comparable to MAT or CTS when the engine is cold. Temperature reading on tester should rise as engine oil temperature increases. If Code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
To reset oil life monitor, turn ignition on. Depress ENG MET button on trip monitor and release. Within 5 seconds, depress and release ENG MET button again. Within 5 seconds, depress and hold the RANGE button on trip monitor. The CHANGE OIL light should flash.
Hold the RANGE button depressed until the CHANGE OIL light stops flashing and goes out. When the light goes out, the engine oil life monitor is reset. This should take about 10 seconds. If the light does not reset, turn the ignition off and repeat the procedure.
