Symptoms of P0666
- Check-engine light, sometimes with several unrelated codes at once.
- Warning lights, a no-start, or intermittent electrical faults.
- Communication or module-reset problems on a scan tool.
Common causes of P0666
- Low battery voltage, poor grounds, corroded connectors, or blown fuses.
- Wiring or network (CAN bus) faults between modules.
- A module software/programming issue, or rarely a failed controller.
Useful checks before replacing parts
- Compare internal module temperature data with ambient conditions, engine compartment heat, battery voltage, freeze-frame data, and communication or processor codes.
- Inspect module mounting, heat shields, connector water intrusion, power and ground stability, harness shorts, and nearby exhaust or cooling issues.
- Verify whether temperature data is plausible cold and hot before deciding the module has an internal sensor fault.
Related OBD-II codes
These nearby computer entries can help compare wording and pin down the fault area.