Description: Clearing the Check Engine Light (CEL) or Malfunction Indicator Light (MIL) on vehicles.
1. General Procedure:
- Using scan tools, technicians send requests to vehicles' ECUs (Electronic Control Units) to clear the CEL/MIL.
- If unsuccessful, a common solution is to recommend a driving cycle, allowing the light to extinguish naturally over time.
2. Potential Issues:
- An unsuccessful SID $04 request might indicate a severe vehicle problem, or the ECU could reply with a 7F22 code, preventing the MIL from turning off.
- Certain vehicle systems like ABS and SRS might prevent MIL clearance if Active DTCs are present.
3. OBD2 Regulations:
- Requires three consecutive successful trips to turn the MIL off, while the DTC remains stored.
- If the fault isn't repeated for 40 (or 80) trips, the code is deleted.
4. Clearing Procedure Requirements:
- All tools need the ignition key to be in the ON position to clear the CEL/MIL.
- This procedure holds for all vehicles, preventing code clearance with the engine running.
- Recent changes allow the request to be sent to the ECU if the vehicle isn't moving. However, some manufacturers might send a negative response, necessitating engine shutdown.
5. SAE J1979 Regulation:
- Some ECUs might not respond during running conditions. However, certain automakers recommend sending erase commands with the Key On, Engine Running (KOER).
- ECUs are mandated to respond when the ignition is ON and the engine isn't running, following the SAE J1979 standard.
- ECUs unable to perform operations under certain conditions, like with the engine running, will either ignore the request (with SAE J1850 and ISO 9141-2 interfaces) or send a negative response (with ISO 14230-4 interfaces).
