What it means
A general circuit fault on the ECU's internal temperature sensor A - the sensor that monitors module operating temperature for thermal protection.
At a glance
- Usually lights the check-engine light
How it works
The PCM, ECM, or TCM uses an internal temperature sensor to track its own operating temperature and help protect its electronics. P0666 sets when the module detects a fault in the sensor A circuit, rather than a specifically low or high signal.
Common causes
Most likely first.
- Internal temperature sensor or circuit fault
- Module power or ground problem
- Damaged internal connection
- Control module fault
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
- Record freeze-frame data and scan all modules for related power, ground, or communication faults before clearing codes.
- Compare the reported module temperature with the ambient temperature after the vehicle has been sitting, and note whether the reading is missing or implausible.
- Inspect the named control module's connectors, pins, and nearby wiring for looseness, corrosion, moisture, or damage.
- Check the module's power and ground connections under load against the manufacturer's specification, since poor supply can disrupt the sensor circuit.
- If external connections and supplies are sound, follow the vehicle's wiring information to test the internal temperature sensor circuit and confirm whether the module requires repair or replacement.
Common fixes
- Repair a loose, corroded, or damaged module connector.
- Repair wiring or restore a poor module power or ground connection.
- Repair or replace the affected control module if its internal sensor circuit is confirmed faulty.
- Perform any required module configuration, reset, or relearn after repair.
Can I keep driving?
If the vehicle starts and runs normally, a short, gentle trip may be possible, but avoid strenuous driving because the module may not be able to monitor its temperature correctly. Stop if it enters limp mode, stalls, or will not restart.
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.