PCM/ECM/TCM Internal Temperature Sensor Circuit Low
The P0668 code indicates that the powertrain control module (PCM), engine control module (ECM), or transmission control module (TCM) has detected a low voltage signal from its internal temperature sensor circuit. This sensor monitors the module's internal temperature to prevent overheating. A low voltage reading suggests a short to ground, faulty sensor, or internal module failure, which can lead to incorrect temperature readings and potential module damage.
1. Most Common Causes
- Internal failure of the temperature sensor integrated in the PCM/ECM/TCM
- Short to ground in the internal temperature sensor signal circuit (if externally accessible)
- Internal hardware failure of the control module (processor or ADC)
- Software/calibration corruption of the module
- Physical damage to the module connector or pins (corrosion, bent pins)
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Possible engine performance issues (rough idle, stalling)
- Transmission shifting problems (if TCM involved)
- Module may enter limp mode or fail to operate
3. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm code P0668 and check for other related codes.
Step 2: Visually inspect the PCM/ECM/TCM connectors for corrosion, damage, or loose pins.
Step 3: Test the internal temperature sensor circuit using a multimeter to check for shorts or opens.
Step 4: If circuit tests normal, suspect internal module failure; consider reprogramming or replacement.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive with this code as it can lead to module overheating and failure, potentially causing stalling or transmission issues. Have it diagnosed promptly.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. If the module needs replacement, costs can range from $200 to $1,000+ depending on the vehicle and module.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.