P0666 — PCM/ECM/TCM Internal Temperature Sensor « A » Circuit

Overview of the code

The diagnostic trouble code P0666 indicates an issue with the **internal temperature sensor** within the vehicle’s **Powertrain Control Module (PCM)**, **Engine Control Module (ECM)**, or **Transmission Control Module (TCM)**. Specifically, it refers to problems in the circuit related to sensor « A », which is usually the primary internal temperature sensor utilized by the vehicle’s engine management system.

Typically, this sensor monitors the internal temperature of the ECU or TCM to ensure operational safety and efficiency. When the PCM detects irregularities in this circuit, it triggers the P0666 fault code. It’s important to note that **labelling of this code may vary** slightly between manufacturers, but the general meaning remains consistent.

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**Fast Facts**

  • System affected: Engine Management / Transmission Control
  • Severity: Moderate — may impact vehicle operation or shift behavior
  • Main symptoms: Check engine / transmission warning lights, possible rough idle or shifting issues
  • Driveability: Limited — vehicle can often still be driven but with caution

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Manufacturer variations for trouble code P0666

Different vehicle manufacturers may have their unique labels or diagnostic procedure references for this fault. The basic code P0666 generally remains consistent, but some brands assign specific descriptions or use different numberings for similar issues.

Common variations include:

  • BMW: 38752 — « Internal Temperature Sensor Circuit » (sometimes specific to ECU diagnostics)
  • Ford: P0666 — « Internal Temperature Sensor Circuit » (abbreviation varies)
  • Hyundai / Kia: Check references under their generic « Sensor Circuit » fault codes, sometimes noting « ECU internal sensor circuit »
  • Volkswagen / Audi / Seat / Skoda: Similar labels referring to « Internal ECU Temperature Sensor Circuit »

It is essential always to refer to the specific vehicle’s repair manual for accurate label definitions and troubleshooting procedures.

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What does trouble code P0666 mean?

In simple terms, P0666 means that the vehicle’s control module has detected a problem with its **internal temperature sensor** circuit. This sensor is crucial because it provides the ECU/ECM/TCM with vital thermal information, influencing how the engine or transmission operate.

The control module continuously checks the **voltage signals** and **resistance values** from this internal sensor. When it detects **abnormal readings**—such as a circuit section shorted, open, or experiencing inconsistent temperatures—it logs the P0666 fault.

This code typically appears in scenarios where there is a **fault in the sensor circuit wiring**, a **defective internal sensor**, or **issues with the control module itself**. It’s often found during routine diagnostics or when the vehicle exhibits specific symptoms, such as erratic shifting or engine performance issues.

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Severity and risks of trouble code P0666

The P0666 code is generally considered **moderate** in terms of severity. Because this sensor’s readings impact the control units’ decision-making, a malfunction can lead to **suboptimal operational parameters**.

Potential risks include:

– **Incorrect transmission shifting**, possibly affecting driveability and safety
– **Engine management issues** that could result in poor fuel economy or increased emissions
– Potential **damage to other engine components** if overheating or thermal thresholds are not correctly monitored
– In some cases, the vehicle might enter **limp mode** to prevent severe damage, which limits power and speed

While many vehicles can still be driven with this fault, the **risk of further damage or unexpected behavior** warrants prompt diagnosis and repair.

**Can the vehicle be driven?**
Limited — it is generally safe to operate temporarily but should be addressed promptly to prevent further issues

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Symptoms of trouble code P0666

Drivers or technicians may notice various signs when this fault occurs. Some symptoms are subtle, while others are more apparent.

Common symptoms include:

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  • Illumination of the Check Engine or Transmission warning lights
  • Erratic or harsh shifting, especially if related to transmission control
  • Poor engine idling or unusual fluctuations in idle speed
  • Reduced fuel efficiency
  • Engine warning lights or limp mode activation in severe cases
  • Potential transmission slipping or delayed responses
  • Possible diagnostic trouble code retrieval with P0666

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    Most likely causes of trouble code P0666

    This code is generally linked to issues in the **internal temperature sensor circuit** within the control module. Several causes may contribute to this fault, ranked from most to least common:

    1. Faulty Internal Temperature Sensor — the sensor itself can fail due to age, heat, or internal damage, resulting in inconsistent readings.
    2. Damaged or Corroded Wiring or Connectors — corrosion, wear, or broken wiring in the sensor circuit can interrupt signals, confusing the control module.
    3. Internal Control Module Issue — rare but possible, a malfunction or defect within the ECU/ECM/TCM may cause false readings or communication errors.
    4. Malfunctioning Ground or Power Supply — insufficient power or grounding issues can affect the reliability of sensor signals, prompting the fault.
    5. Software or Calibration Error — in some cases, a software glitch or outdated firmware may play a role, requiring updates or reprogramming.

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    How to diagnose trouble code P0666

    Diagnosis of P0666 should be methodical and cautious, using proper testing equipment and referencing manufacturer-specific repair procedures when available.

    Typical steps include:

    1. Visual Inspection: Check the wiring harness and connectors related to the control module and internal sensor for damage, corrosion, or disconnection.
    2. Retrieve Live Data: Use an OBD2 scan tool to monitor the sensor readings and circuit voltages in real time. Look for abnormal or fluctuating signals.
    3. Check Circuit Continuity: Perform a continuity test on the sensor circuit wiring to verify that no open circuits or shorts exist.
    4. Inspect Grounds and Power Supplies: Ensure that the control module’s grounding points are secure and that power supplies are within specifications.
    5. Test the Internal Sensor: If accessible, test the sensor directly (via resistance or voltage checks as specified by the manufacturer).
    6. Assess Control Module Functionality: If wiring and sensors are OK, consider testing or replacing the ECU/ECM/TCM as a last resort.

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    Possible repairs for trouble code P0666

    Addressing P0666 involves rectifying the identified cause and restoring normal sensor operation. Common repairs include:

    • Replacing the Internal Temperature Sensor: This is often a straightforward repair but may involve removing parts of the control module, usually performed by a professional.
    • Repairing or Replacing Damaged Wiring or Connectors: Cleaning, repairing, or replacing worn or corroded wiring harnesses and connectors to ensure good electrical contact.
    • Addressing Ground or Power Issues: Securing or repairing grounding points and verifying battery and alternator health.
    • Updating or Reflashing the Control Module Software: When software glitches are suspected, a firmware update from the OEM may be necessary.
    • Replacing the Control Module: If proven to be faulty, the ECU/ECM/TCM might need replacement, usually performed by a professional.

    **Note:** Always follow manufacturer repair procedures and use OEM parts when possible to ensure proper function and warranty compliance.

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    Vehicles commonly associated with trouble code P0666 in Europe

    While this fault can occur in various vehicles, certain brands and engine families are more frequently reported in diagnostic data.

    Vehicles often affected include:

    In Europe, the code tends to appear mainly in constraints related to vehicles equipped with electronic control modules highly integrated with their internal temperature sensors. Examples include:

    • Volkswagen Group (VW, Audi, Skoda, Seat) — especially models with engine management systems that include internal temperature sensors in their control units.
    • BMW — certain models with integrated control modules designed to monitor internal temperatures.
    • Ford — especially in models where the sensor circuit is embedded within the ECU.
    • Hyundai / Kia — where internal sensors are monitored by the ECU for thermal management purposes.

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    Frequent mistakes with trouble code P0666

    Diagnosing and repairing P0666 can sometimes be hindered by common errors. Awareness of these pitfalls helps ensure a successful outcome.

    Typical mistakes include:

    • Misdiagnosing the circuit or wiring problem as a control module fault: Always verify wiring and sensor integrity before replacing the ECU.
    • Neglecting to perform a thorough visual inspection: Overlooking obvious physical damage to wiring or connectors can lead to unnecessary component replacements.
    • Overlooking software-related issues: Firmware or calibration problems may be misinterpreted as hardware faults.
    • Assuming the sensor is always the problem without performing live data analysis: Relying solely on DTCs without verifying actual sensor signals can lead to incorrect repairs.
    • Using non-OEM parts for repairs: This can cause compatibility and reliability issues.

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    FAQ — trouble code P0666

    **Can this code disappear on its own?**
    No, unless the underlying fault resolves itself, which is rare. Usually, the code persists until the cause is repaired or the ECU is reset and the fault reappears.

    **Can I keep driving?**
    While the vehicle might still operate normally in the short term, driving with this fault is not recommended. It can lead to further issues, especially with transmission performance or engine management. Seek professional diagnosis soon.

    **Why does the code return after clearing it?**
    If the underlying cause remains unresolved—such as a faulty sensor, damaged wiring, or control module defect—the code will inevitably reappear after clearing it. Proper diagnosis and repair are essential.

    **Is this a common problem?**
    It can happen in vehicles with sensitive internal temperature circuits within their control modules, especially as the vehicles age or if exposed to harsh conditions.

    **Will replacing the control module fix the problem?**
    Only if the fault lies within the ECU/ECM/TCM itself. Often, repairing wiring or replacing the internal sensor suffices. Consult a qualified technician for accurate diagnosis.

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