P0668
UniversalThe Powertrain Control Module (PCM), Engine Control Module (ECM), or Transmission Control Module (TCM) has detected a low voltage reading from its internal temperature sensor circuit.
Medium SeverityThis code indicates that the Powertrain Control Module (PCM), Engine Control Module (ECM), or Transmission Control Module (TCM) has detected an issue with its internal temperature sensor, specifically a low voltage signal. This can lead to the module not accurately monitoring its own operating temperature, potentially affecting engine or transmission performance and reliability.
⚠️ Before You Begin (Safety First)
Always ensure your vehicle is turned off and the battery is disconnected before attempting any electrical diagnostic or repair work. Control modules can be sensitive to static electricity and improper handling.
Common Symptoms
- Check Engine Light illumination
- Engine performance issues (e.g., misfires, rough idle, stalling)
- Transmission shifting problems (e.g., harsh shifts, no shifting)
- Reduced fuel economy
- Vehicle entering 'limp mode' to prevent damage
Common Causes
- Internal fault within the PCM/ECM/TCM's temperature sensor
- Short circuit to ground in the sensor circuit wiring
- Faulty PCM/ECM/TCM itself (internal component failure)
- Corrosion or loose connections in the module's wiring harness
- Extreme external heat affecting the module (less common as a direct cause, but can exacerbate issues)
Frequently reported on: All Makes
Diagnostic Steps
- Scan for additional diagnostic trouble codes to help pinpoint the issue.
- Visually inspect the wiring harness leading to the PCM/ECM/TCM for any signs of damage, fraying, or areas where it might be chafing against other components.
- Check the electrical connectors at the module for looseness, corrosion, or bent pins. Ensure they are securely seated.
- If accessible, test the continuity of the sensor circuit wiring from the module connector to other relevant points (requires a wiring diagram).
- Monitor the internal temperature sensor reading with an advanced scan tool if available, though an 'internal' sensor might not always be directly readable as a separate PID.
👨🔧 When to Call a Professional
If basic wiring and connection checks do not resolve the issue, and especially if a faulty PCM, ECM, or TCM is suspected, it is highly recommended to consult a professional technician. Replacing and programming control modules often requires specialized diagnostic tools and expertise.
Commonly Asked Questions
What does P0668 mean?
P0668 indicates that the main computer in your car (PCM, ECM, or TCM) has detected a low voltage signal from its internal temperature sensor circuit, meaning it can't properly monitor its own operating temperature.
Is it safe to drive with P0668?
Driving with P0668 is not recommended. The control module might not be operating correctly, which could lead to unpredictable engine or transmission behavior, potential damage to components, or unsafe driving conditions.
Can I fix P0668 myself?
While basic checks like inspecting wiring and connections can be done by a DIYer, diagnosing and repairing internal module faults or replacing the module often requires specialized tools and expertise, making it an advanced repair.
What are the common repairs for P0668?
Common repairs include fixing a short circuit in the wiring, cleaning corroded connectors, or replacing the affected control module (PCM, ECM, or TCM) if it has an internal fault.
Why does the control module need an internal temperature sensor?
The internal temperature sensor helps the control module monitor its own operating temperature to prevent overheating. If the module gets too hot, it can malfunction, affecting vehicle performance and potentially leading to permanent damage to its internal components.