P0668Powertrain
Powertrain · SAE
ECU Internal Temperature Sensor — Low Voltage
Meaning
The ECU has detected that the signal voltage of the internal temperature sensor within the control module is below the minimum acceptable threshold. A low voltage corresponds to an unrealistically high temperature reading or indicates an internal short to ground in the sensor circuit.
Common causes
- Internal failure of the temperature sensor integrated within the ECU
- Problem with the internal reference voltage inside the ECU
- Actual ECU overheating due to incorrect mounting position or insufficient ventilation
- Internal short circuit in the ECU sensor circuit
- Unstable or out-of-specification ECU supply voltage
Symptoms
- Check-engine light (MIL) on
- Possible ECU protective mode activation
- Anomalous engine behavior not explained by other codes
- Spontaneous ECU reset in rare cases
- Sometimes no noticeable driving symptoms
Severity
Moderate to high: a faulty internal ECU temperature sensor can compromise self-diagnosis and thermal management of the module; verify installation conditions first.
What to do
- Check ECU supply voltage (typically 12–14 V with engine running)
- Verify ECU mounting position and security: must not be exposed to excessive heat sources
- Check ECU connectors for integrity and absence of moisture
- Verify the ECU ground connection
- If all external parameters are correct, the ECU requires replacement or specialist repair