P0668 — PCM/ECM/TCM Internal Temperature Sensor « A » Circuit Low

The diagnostic trouble code P0668 indicates a problem within the vehicle’s Powertrain Control Module (PCM), Engine Control Module (ECM), or Transmission Control Module (TCM), specifically related to the internal temperature sensor labeled as « A. » In simple terms, this code means that the control unit has detected an unusually low signal from its internal temperature sensor, suggesting a potential fault in the sensor circuit or related components.

Note that labels and terminology may vary between different vehicle manufacturers, but the core concept remains the same: a sensor reading is outside normal parameters, prompting the code to set.

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Manufacturer variations for trouble code P0668

Since different vehicle manufacturers may assign different labels or codes to similar issues, it is important to recognize variants of the P0668 trouble code. Typically, automakers do not change the numerical code, but they may refer to it differently in their diagnostic literature or on scan tools.

• Ford: P0668 — PCM Internal Temperature Sensor Circuit Low
• Honda: P0668 — Internal Temperature Sensor Circuit Low (may be referenced in service info)
• GM (General Motors): Similar code, but possibly labelled as « Engine Coolant Temperature Sensor Circuit Low »
• Volkswagen/Audi/Seat/Skoda: Variants with similar diagnostic logic, but specific labels vary by system

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What does trouble code P0668 mean?

The P0668 code indicates that the vehicle’s onboard computer (such as the PCM, ECM, or TCM) has detected an abnormally low voltage or signal from its **internal temperature sensor**. This sensor is responsible for monitoring the temperature of the control module itself, which is critical for proper operation and to prevent overheating or incorrect data processing.

During normal operation, the sensor provides the control module with a voltage signal corresponding to its measured internal temperature. When this signal falls outside the expected low range — often occurring due to a circuit fault, faulty sensor, or wiring issue — the ECU triggers the P0668 code.

Typically, this code appears in situations where the sensor’s circuit encounters a malfunction, such as poor wiring, damaged sensor, or a fault within the control module itself. It’s often seen when the control unit detects readings that are below the sensor’s minimum voltage threshold, indicating a « low » circuit condition.

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Severity and risks of trouble code P0668

The detection of P0668 is not usually an immediate emergency, but it is important to address promptly. The main risk associated with this code is that the vehicle’s control units may not be able to accurately monitor internal temperatures, which could lead to improper engine management or overheating unnoticed by the driver.

Potential consequences include increased engine wear, inefficiencies, or future component failure if the fault remains uncorrected. In some cases, this may also cause the vehicle to enter a limp mode to protect the engine or transmission from damage.

**Can the vehicle be driven?** — Yes, but with caution. It is advisable not to drive long distances or excessively hard until the issue is diagnosed and repaired, to prevent further damage or system faults.

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Symptoms of trouble code P0668

The presence of P0668 might not always produce obvious symptoms, especially if the internal temperature sensor has a fault that does not immediately impact vehicle operation. However, common signs include:

• Illuminated check engine light
• Erratic or reduced engine performance — particularly during cold starts or in extreme temperatures
• Harsh or uneven idling
• Possible transmission issues if the TCM relies on internal temperature data
• Warning messages or indications on the dashboard related to engine temperature

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Most likely causes of trouble code P0668

Understanding the possible causes helps streamline diagnostics. The causes are generally grouped from most to least common:

1. Open or short circuit in the internal temperature sensor circuit — often due to wiring damage or poor connections.
2. Faulty internal temperature sensor — the sensor component itself may have failed due to age or manufacturing defect.
3. Damage to the control module (PCM, ECM, or TCM) — internal faults within the control unit can trigger false low readings.
4. Corrosion or contamination in the sensor wiring or connector
5. Software glitch or calibration issue in the ECU — although rare, a malfunctioning software update or calibration can cause abnormal readings.

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How to diagnose trouble code P0668

This section outlines a general process to identify the root causality of P0668, ensuring safety and diagnostic efficiency.

1. Perform a visual inspection: Check sensor wiring, connectors for corrosion, damage, or loose connections.
2. Use a scan tool to access live data: Observe the internal temperature sensor voltage reading. It should be within normal range; a reading at or near zero volts indicates a problem.
3. Check the wiring harness continuity: Use a multimeter to test for open or short circuits in the sensor wiring.
4. Inspect the sensor itself: If accessible, test the sensor for proper operation or replace if suspected faulty.
5. Validate control module operation: If wiring and sensor are good, consider testing or reprogramming the ECU if software issues are suspected.
6. Scan for related codes: Look for other codes that may point to wiring or component issues that could be connected to P0668.

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Possible repairs for trouble code P0668

Based on diagnosis, repairs may involve addressing wiring issues, replacing components, or reprogramming the control module. It’s advisable to undertake these repairs systematically, starting with the simplest checks.

• Repair or replace damaged wiring and connectors — A basic repair that most technicians or experienced DIYers can perform.
• Replace the internal temperature sensor — Usually a straightforward task but may require partial disassembly of engine covers or ECU housing in some models; a professional repair recommended if inaccessible.
• Reset or reprogram the control module — Done via software updates or reflash, typically performed by a professional with OEM or compatible diagnostic tools.
• Replace the control module — Only if confirmed faulty through testing; requires professional handling and coding.

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Vehicles commonly associated with trouble code P0668 in Europe

This code is found in various vehicle brands, especially those with sensitive internal temperature monitoring systems. While exact models may vary, some common groups include:

Ford: Focus, Fiesta, Mondeo with common ECUs

Honda: Civic, Accord, CR-V models

GM Vehicles: Astra, Corsa, Insignia

Volkswagen Group: Golf, Polo, Audi A3, Skoda Octavia

While this is not an exhaustive list, it highlights some of the models where this fault may be encountered in European markets.

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Frequent mistakes with trouble code P0668

Diagnosing a fault like P0668 can be prone to errors if not approached carefully. Common mistakes include:

• Ignoring wiring and connector issues — jumping straight to sensor or control replacement without proper inspection.
• Misinterpreting live data readings — assuming a voltage reading is normal without understanding the expected ranges for that specific system.
• Replacing the control module unnecessarily — often a costly step that can be avoided with thorough diagnostics.
• Overlooking related causes — such as power supply problems or software issues that might mimic sensor faults.

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FAQ — trouble code P0668

Can this code disappear on its own? — No, typically this fault persists until the underlying issue is repaired. Intermittent wiring issues may temporarily clear but will likely reappear.

Can I keep driving? — It is possible in the short term, but not recommended. The vehicle may operate with limited driveability, and continued driving could lead to further damage.

Why does the code return after clearing? — Because the root cause, such as a wiring fault or defective sensor, has not been fixed. The control module detects the same low voltage condition and resets the code.

Is this a serious problem? — It depends on symptoms and driving conditions, but prompt diagnosis and repair help avert potential long-term damage to engine or transmission components.