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

What P0666 means

P0666 is the standardized powertrain DTC for “PCM/ECM/TCM Internal Temperature Sensor A Circuit.” The code identifies the monitored condition; it does not by itself prove which part failed.

Possible symptoms

• Transmission warning or Check Engine Light
• Harsh/delayed shifts
• Limp mode
• Possible no-shift or engagement issue

Causes to test

These are diagnostic possibilities, not a statistical parts-replacement ranking. Vehicle-specific service information takes priority.

• Electrical circuit or sensor fault
• Fluid level/condition problem
• Solenoid/valve-body or hydraulic fault
• Mechanical transmission/clutch issue
• TCM power, communication, calibration, or adaptation issue

Step-by-step diagnostic workflow

1. Scan the TCM directly; do not rely only on the ECM code list.
2. Check fluid using the transmission-specific procedure and fluid specification.
3. Review input/output speeds, commanded gear, temperature, and companion codes.
4. Inspect external wiring/connectors before internal repair decisions.
5. Follow transmission-specific service information and relearn/adaptation steps after repair.
6. Clear codes only after repair, then complete an appropriate drive cycle and verify the code and symptoms do not return.

Scan data worth checking

• TCM codes
• Input/turbine and output speed
• Commanded gear
• Transmission fluid temperature
• Solenoid/pressure commands where supported

Severity and drivability

Moderate to high depending on drivability. Limp mode, slipping, overheating, or loss of a gear warrants prompt diagnosis.

Stop driving and investigate immediately if there is a flashing MIL, oil-pressure warning, overheating, fuel leakage, severe misfire, major loss of power, abnormal mechanical noise, or a transmission that is slipping badly.

Common diagnostic mistakes

• Treating a generic transmission code as a complete diagnosis
• Using the wrong fluid
• Skipping TCM-specific codes and data

Related generic codes