What Does P0666 Mean?
The PCM/ECM/TCM internal temperature sensor circuit has a general malfunction. This sensor monitors the operating temperature inside the control module itself to protect against overheating and to compensate for temperature-related changes in circuit operation. When this code sets, the module cannot accurately determine its own internal temperature. The driver may notice erratic behavior from the engine or transmission management system, or may not notice any drivability symptoms at all initially.
Common Causes
40%
Faulty PCM/ECM/TCM with degraded internal temperature sensor
25%
Poor power or ground connections to the control module causing signal interference
15%
Control module overheating due to location or cooling fan failure
15%
Water intrusion into control module housing
5%
Software/calibration issue requiring reflash
Diagnostic Steps
1
Monitor the PCM internal temperature PID on the scan tool — if it reads an unrealistic value (e.g., -40°F or 300°F), the internal sensor is clearly faulty.
2
Inspect the PCM housing for signs of water intrusion, corrosion, or physical damage. Check all seals and connector weatherpacks for integrity.
3
Measure voltage and ground at the PCM main power and ground pins — compare to specifications. Voltage should be within 0.1V of battery voltage, and grounds should have less than 0.1V drop.
4
Check the PCM mounting area for excessive heat — the module should not be located near exhaust components or in a poorly ventilated area. Verify any cooling ducting or heat shields are in place.
5
Check for any TSBs or software updates for the PCM. Some manufacturers have released recalibration updates to correct internal sensor reading algorithms.
Estimated Repair Cost
$150 - $1,500
Parts + labor, varies by vehicle and location
The PCM/ECM/TCM internal temperature sensor circuit has a general malfunction. This sensor monitors the operating temperature inside the control module itself to protect against overheating and to compensate for temperature-related changes in circuit operation. When this code sets, the module cannot...
The most common cause of P0666 (PCM / ECM / TCM Internal Temperature Sensor Circuit) is: Faulty PCM/ECM/TCM with degraded internal temperature sensor
Typical repair costs for P0666 range from $150 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Drive cautiously and monitor for erratic engine or transmission behavior. If the PCM is overheating, it could enter a protective shutdown mode or cause intermittent loss of engine/transmission control. Have this diagnosed promptly — if the module overheats and fails while driving, it could stall the engine.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0666 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Powertrain
System
Engine Control Module
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
