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.
01. Most Common Causes
- Short to ground in the internal temperature sensor circuit
- Faulty internal temperature sensor within the PCM/ECM/TCM
- Corroded or damaged wiring/connectors to the module
- Internal failure of the PCM/ECM/TCM
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine performance issues (rough idle, stalling, poor acceleration)
- Transmission shifting problems (if TCM involved)
- Possible limp mode activation
03. Step-by-Step Diagnosis
Step 1: Use an OBD2 scanner to confirm the code and check for additional related codes.
Step 2: Visually inspect the PCM/ECM/TCM connectors and wiring for damage, corrosion, or loose connections.
Step 3: Test the internal temperature sensor circuit using a multimeter to check for shorts or opens.
Step 4: If wiring and connectors are okay, suspect a faulty module; perform a manufacturer-specific diagnostic test.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as the module may overheat or malfunction, potentially causing engine or transmission damage. Seek repair promptly.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. If the module needs replacement, costs can range from $500-$1500 including labor and programming.