What it means
Same as P0666 with the signal voltage below the plausible range. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
How it works
The PCM, ECM, or TCM monitors an internal temperature sensor circuit to track the module's operating temperature. P0668 sets when the sensor A circuit signal is below the expected electrical range, which can result from a short to ground or a fault inside the module.
Common causes
Most likely first.
- Sensor internal short
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check engine light on
- Possible limp mode or no-start
- Multiple follow-up faults across modules
- Reset/relearn may be needed after repair
How to diagnose it
- Save freeze-frame data and scan all modules for related supply or communication codes before clearing the fault.
- Check the scan-tool module-temperature reading and note whether it is implausibly low, fixed, or absent.
- Inspect the module connector and accessible harness for chafing, moisture, corrosion, pushed-back pins, or a wire touching ground.
- With the circuit isolated as directed by vehicle service information, check for a short to ground and inspect connector contact condition.
- Verify module power and ground against the manufacturer's specification, then consider an internal sensor or module fault if wiring tests pass.
Common fixes
- Repair chafed wiring that is shorting the sensor circuit to ground.
- Clean, tighten, or repair corroded or displaced connector terminals.
- Restore faulty module power or ground connections.
- Repair or replace the control module if the internal sensor circuit remains low with sound external connections.
- Perform any required module reset or relearn after the repair.
Can I keep driving?
If the vehicle behaves normally, limit driving to necessary short trips and avoid heavy loads because the module may not be able to monitor its temperature reliably. Stop if it enters limp mode, stalls, or develops starting problems.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.