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P codepowertrainMedium severity

P0668PCM/ECM/TCM Internal Temperature Sensor Circuit Low

The control module's internal temperature sensor signal is stuck at the low end of its range, reading far colder than plausible.

Definition

The PCM/ECM/TCM monitors its built-in temperature sensor to protect output drivers and compensate internal circuits, and it flags a fault when the signal drops below the minimum valid threshold — equivalent to an impossibly cold reading. P0668 typically indicates an internal open or short in the sensing circuit, since the sensor and its wiring live entirely inside the module; degraded module power or ground supplies are the only meaningful external contributors. The MIL will be on, the module substitutes a default temperature value, and driveability is usually unaffected, though a current code fails an OBD emissions inspection. A persistent fault requires module replacement after supplies are confirmed good.

Common causes

  • Internal failure (open or short) of the module's temperature sensing circuit
  • High resistance or corrosion in the module's power or ground circuits
  • Circuit-board damage from moisture intrusion into the module
  • Electrical transients from jump-starts, welding, or charging-system faults
  • Extreme cold soak beyond design limits (rare, usually sets and clears)

Common misdiagnoses

  • Replacing the ECT sensor for a code that has nothing to do with coolant temperature
  • Replacing the IAT sensor or its wiring
  • Repairing external temperature-sensor harnesses that are not part of this circuit
  • Replacing the thermostat

Troubleshooting steps

  1. 1.Read the internal temperature PID

    A value pegged at the scale minimum (often around -40 degrees) with the engine warm confirms the signal is railed low rather than genuinely cold.

  2. 2.Test module power and ground circuits

    Voltage-drop each feed and ground under load; more than 0.1 V of drop can skew internal analog references and must be repaired first.

  3. 3.Inspect the module connector and case

    Look for water staining, corrosion, or physical damage that could have compromised the board.

  4. 4.Clear the code and monitor

    If the code returns immediately or the PID stays railed low with good supplies, the fault is internal to the module.

  5. 5.Replace and program the control module

    The internal sensor is not serviceable, so a confirmed hard fault requires a replacement module with the correct calibration and security relearn.

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