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P codepowertrainMedium severity

P0666PCM/ECM/TCM Internal Temperature Sensor Circuit

The temperature sensor built inside the engine or transmission control module is sending an implausible or faulted signal.

Definition

The PCM/ECM/TCM contains an internal temperature sensor used to protect the module's driver circuits, compensate sensor readings, and in some applications substitute for intake or transmission temperature values. P0666 sets when the internal sensor's signal is erratic, open, or otherwise outside its valid operating window during the module's self-test. Because the sensor is inside the module, the fault usually indicates an internal module problem rather than external wiring, though poor module power or ground connections can also corrupt the reading. Symptoms range from a MIL with no driveability complaint to derated operation or cooling-fan strategy changes, and the vehicle will fail an OBD emissions inspection with the code current.

Common causes

  • Internal failure of the control module's temperature sensing circuit
  • Poor module power or ground connections skewing the sensor's reference
  • Module overheating from a failed mounting location, blocked airflow, or missing heat shielding
  • Water intrusion or corrosion inside the module connector or case
  • Voltage spikes from jump-starting, welding on the vehicle, or a failing charging system

Common misdiagnoses

  • Replacing the engine coolant temperature (ECT) sensor, which is a separate external sensor
  • Replacing the intake air temperature (IAT) sensor
  • Replacing the transmission fluid temperature sensor
  • Chasing engine cooling system problems (thermostat, fans) when the fault is inside the module

Troubleshooting steps

  1. 1.Verify battery and charging system health

    Confirm a resting voltage near 12.6 V and charging voltage of roughly 13.5-15 V, since low or noisy system voltage can trigger internal module faults.

  2. 2.Load-test module power and ground circuits

    Perform voltage-drop tests on each PCM power feed and ground (target under 0.1 V drop under load) and repair any corroded terminals or ground studs.

  3. 3.Compare the internal temperature PID to reality

    On a scan tool, the module internal temperature should track ambient after a cold soak and rise gradually with runtime; a pegged, frozen, or wildly jumping value confirms an internal fault.

  4. 4.Inspect the module and connectors for damage

    Look for water staining, green corrosion in the connector cavities, and check that the module's mounting location isn't exposed to exhaust heat or a missing shield.

  5. 5.Check for software updates

    Search the manufacturer's TSB and calibration database, as some applications have reflashes that address false internal-sensor codes.

  6. 6.Replace or repair the control module if the fault persists

    With clean power and grounds confirmed, a recurring P0666 requires module replacement (with programming/immobilizer relearn) since the sensor is not serviceable separately.

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