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P0666 — Control Module Internal Temperature Sensor Circuit

Generic OBD-II · all makesPowertrain (engine & transmission)

P0666 indicates the control module (PCM/ECM/TCM) has detected a fault in its own internal temperature sensor circuit. The sensor monitors the module's internal temperature to prevent heat-related damage, and the code sets when the signal is out of range, shorted, or open. It does not indicate an engine coolant or ambient air temperature problem.

Symptoms

What causes it — most likely first

1. Internal temperature sensor circuit fault

The temperature sensor and its associated circuitry are inside the control module. An open, short, or out-of-range reading within this internal circuit is the most direct cause of P0666.

2. Control module connector or wiring issue

Corroded, loose, bent, or damaged pins and wiring at the module harness can affect the internal sensor circuit's signal integrity, even though the sensor itself is internal.

3. Excessive heat or poor thermal management

If the module is mounted in a high-heat location, lacks proper heat sinking, or has blocked cooling airflow, internal temperatures can exceed expected ranges and trigger the code.

4. Software or calibration issue

Outdated or corrupted control module software may misinterpret the internal temperature sensor signal or set a false DTC; manufacturers may address this with a reprogramming procedure.

5. Control module failure

Complete internal failure of the PCM/ECM is possible but uncommon for this code. It should only be considered after wiring, connections, heat management, and software issues have been ruled out.

How it's diagnosed

  1. Confirm the code with a scan tool, record freeze frame data, clear the DTC, and operate the vehicle to verify it returns.
  2. Inspect the control module connectors and wiring harness for corrosion, contamination, bent pins, or damage; clean or repair as needed.
  3. Verify the module is properly mounted, check for adequate ventilation or heat sinking, and inspect for nearby heat sources or aftermarket components causing excessive heat.
  4. Check for related DTCs and manufacturer technical service bulletins for software updates or reprogramming instructions.
  5. If no external fault is found and the code persists, evaluate the control module itself as a last-resort cause.

Code facts & related codes

SystemPowertrain (engine & transmission)
StandardizationSAE generic — identical on every make
Related codesP0660 — Intake Manifold Tuning Valve Control CP0661 — Intake Manifold Tuning Valve Control CP0662 — Intake Manifold Tuning Valve Control CP0663 — Intake Manifold Tuning Valve Control CP0664 — Intake Manifold Tuning Valve Control CP0665 — Intake Manifold Tuning Valve Control C

Frequently asked

Do I need a new PCM for P0666?

Usually not. PCM replacement is commonly recommended for P0666, but the fault is more often caused by wiring, connector, heat management, or software issues. Replace the module only after external causes and software updates have been ruled out.

Can I drive with P0666 set?

You often can if there are no drivability symptoms, but the vehicle may enter limp mode if the module believes it is overheating. Diagnosis should be performed promptly to avoid potential performance restrictions.

Is the internal temperature sensor replaceable separately?

No. The internal temperature sensor is part of the control module assembly and is not serviced as a separate component. Repairs typically address wiring, software, thermal management, or module replacement.

What does P0666 mean exactly?

It means the control module has detected an electrical fault in the circuit that monitors its own internal temperature, such as a short, open, or out-of-range signal.