PCM/ECM/TCM Internal Temperature Sensor Circuit Malfunction
The P0666 code indicates a fault in the internal temperature sensor circuit of the Powertrain Control Module (PCM), Engine Control Module (ECM), or Transmission Control Module (TCM). This sensor monitors the module's internal temperature to prevent overheating. A malfunction can cause the module to miscalculate temperature, leading to performance issues or potential damage.
1. Most Common Causes
- Internal failure of the control module (PCM/ECM/TCM) due to damage to the integrated temperature sensor circuit
- Actual module overheating caused by poor heat dissipation or exposure to external heat sources
- Internal short circuit or open circuit within the control module
- Voltage reference or ground anomalies inside the module that interfere with sensor readings
- Environmental or operating conditions exceeding the component's designed thermal limits
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine performance issues (hesitation, stalling)
- Transmission shifting problems
- Reduced fuel economy
3. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm P0666 and check for other codes.
Step 2: Inspect wiring and connectors for damage, corrosion, or loose connections.
Step 3: Test the internal temperature sensor resistance and compare to specifications.
Step 4: If sensor and wiring are okay, consider reprogramming or replacing the PCM/ECM/TCM.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended as the module may overheat or cause drivability issues. Have it diagnosed promptly.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. Repair costs vary: sensor replacement $100-$300, module replacement $500-$1500.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.