HARD FAILURES
Hard failures cause CHECK ENGINE light to glow and remain on until problem 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 unit will use a substitute value in its calculations to continue engine operation. In this condition, commonly known as limp-in mode, vehicle runs but driveability will not be optimum.
INTERMITTENT FAILURES
Intermittent failures may cause CHECK ENGINE light to flicker or glow and go out after intermittent fault goes away. However, corresponding trouble code will be stored in ECU memory. Not all trouble codes will illuminate CHECK ENGINE light. If related fault does not reoccur within a certain time frame, related trouble code will be erased from ECU memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TESTS W/O CODES article in the ENGINE PERFORMANCE Section.
Note. Follow proper diagnostic routine when diagnosing system. See DIAGNOSTIC ROUTINE table.
| Procedure | Order |
|---|---|
| Basic Diagnostic Procedures | 1st |
| Entering Diagnostics | 2nd |
| Retrieving Trouble Codes | 3rd |
| Symptoms (1) | 4th |
| Intermittents (1) | 5th |
| (1) Symptoms and intermittents are covered in TESTS W/O CODES article in the ENGINE PERFORMANCE Section. | |
| (1) | Symptoms and intermittents are covered in TESTS W/O CODES article in the ENGINE PERFORMANCE Section. |
DIAGNOSTIC ROUTINE
CHECK ENGINE LIGHT
Note. Not all trouble codes activate CHECK ENGINE LIGHT.
All models are equipped with a CHECK ENGINE light. As a bulb check, light glows when ignition is turned on and engine is not running.
On California models, CHECK ENGINE light glows when a fault is detected with engine running. A corresponding trouble code will set in ECU memory. CHECK ENGINE light also glows if ECU or crank angle sensor malfunctions.
On Federal models, CHECK ENGINE light glows only when ECU or crank angle sensor malfunctions with engine running.
Mode I
This is normal operating mode. In this mode, CHECK ENGINE light and Red Light Emitting Diode (LED) inspection light in ECU stay on when ignition is turned on, engine off (bulb check). If engine is started, CHECK ENGINE light and Red LED in ECU should turn off, unless a fault or trouble code is present (California) or if ECU or crank angle sensor is malfunctioning (California and Federal).
Mode II
This is self-diagnostic mode for retrieving fault or trouble codes. See RETRIEVING CODES. With engine running, Mode II can monitor air/fuel mixture feedback control. CHECK ENGINE light and red LED in ECU will flash on (lean) and off (rich) more than 5 times every 10 seconds with engine speed at 2000 RPM when system is in closed loop. CHECK ENGINE light and red LED do not flash (remain on or off) if system is in open loop.
RETRIEVING CODES
Use Mode II to retrieve trouble codes from self-diagnostic system. Read codes using either CHECK ENGINE light or Red LED inspection light on ECU. Trouble codes will flash after selecting Mode II.
Trouble codes are indicated by number of flashes from CHECK ENGINE light or Red LED inspection light. For example, 3 long (.6 second) flashes followed by 2 short (.3 second) flashes of CHECK ENGINE light or Red LED indicate a Code 32.
CLEARING CODES
Note. Ensure all diagnostic codes are accessed from ECU memory before disconnecting battery or switching from Mode II to Mode I.
Memory Erase
Stored memory can be erased by disconnecting battery or selecting Mode I after Mode II has been accessed.
SUMMARY
If no hard fault codes (or only pass codes) are present, driveability symptoms exist or intermittent codes exist, proceed to TESTS W/O CODES article in the ENGINE PERFORMANCE Section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.