DIAGNOSTIC TROUBLE CODES (DTCs)
The Engine Control Module (ECM) monitors several different engine control circuits. If a malfunction occurs, ECM will record a Diagnostic Trouble Code (DTC). The next time engine is started and the same malfunction is detected ECM will turn on MIL signaling a DTC is in ECM memory.
After 3 consecutive drive cycles without a malfunction, ECM will turn off MIL, however DTC will be stored in ECM memory. If ECM does not detect a that fault for 40 drive cycles, DTC will be purged from ECM memory. ECM will purge a DTC for a misfire or fuel trim malfunction when fault is not detected for 80 drive cycles of engine with engine speed, coolant temperature, etc. similar to when malfunction first occurred.
Hard faults cause MIL 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. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but driveability will be affected.
VISUAL INSPECTION
Before proceeding with self-diagnostics, visually check the following and prepare vehicle as follows
- Perform all necessary safety precautions to prevent personal injury or vehicle damage.
- Block drive wheels. Set parking brake. Place shift lever in Park(A/T) or Neutral (M/T). DO NOT move shift lever during test unless specifically directed to do so.
- Check condition of air cleaner and air ducting.
- Check all vacuum hoses for leaks, restrictions and proper routing.
- Check EEC system wiring harness connections for corrosion, damaged pins, loose wires and improper routing.
- Check ECM, sensors and actuators for physical damage.
- Check engine coolant and oil level.
- Check battery condition.
- Check exhaust system for leaks.
- Turn off all lights and accessories. Ensure vehicle doors are closed when measuring voltage.
CLEARING CODES WITHOUT SCAN TOOL
Note. Clearing code procedure will clear the following: Diagnostic trouble codes. 1st trip diagnostic trouble codes. Freeze frame data. 1st trip freeze frame data. System readiness codes. Test values. Ensure necessary freeze frame data and diagnostic trouble codes are recorded prior to clearing diagnostic information. Turning ignition off when in Mode 2 will not clear codes. Clear code procedure must be used. All diagnostic information will be erased if battery is disconnected for more than 24 hours.
Enter diagnostic Mode 2 as described in RETRIEVING CODES . After retrieving codes in Mode II, turn test mode selector fully clockwise. Wait at least 2 seconds. Turn test mode selector fully counterclockwise. This will be erase DTCs from memory by switching PCM mode selector from diagnostic Mode 2, back to diagnostic Mode 1.
SELF-DIAGNOSTICS USING SCAN TOOL
Note. Procedures given are generic and may vary depending on equipment used. See scan tool manufacturers instructions for specifics.
Using Scan Tool
- Ensure scan tool is properly connected. Select vehicle and engine selection menu (optional). Select year, engine, model and any additional information requested by scan tool.
- Turn ignition on. Select DIAGNOSTIC TROUBLE CODES. Select ERASE. All codes should now be cleared from ECM memory. If problem has not been corrected or fault is still present, hard code will immediately be reset in ECM memory.