P0666 code meaning
Control Module Internal Temperature Sensor A Circuit
Powertrain system Generic code P06xx codes Check engine light
What P0666 means
Control modules measure their own circuit board temperature with an internal sensor to protect output stages (overtemperature is P0634) and, in GM transmission modules, as a substitute for fluid temperature. In GM's 6T70/6T75 and related automatics the sensor sits in the transmission module inside the valve body, with no external wiring – so the repair is practically always the module or valve body assembly. P0666 is the general circuit fault of this sensor A. No manufacturer table available to us uses P0666 itself – GM splits the circuit faults into low (P0668) and high (P0669), plausibility is P0667. Expect a warning lamp and, in a transmission module, changed shift quality because a substitute temperature is used.
P0666 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of P0666
- Warning or check engine light
- Changed shift quality (substitute value)
- Possibly transmission fluid temperature codes
Common causes of P0666
- Most likelyModule faulty internally (temperature sensor or measuring circuit)
- PossibleMoisture or corrosion in the module or valve body
- Less likelySupply or ground problems of the module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Transmission control moduleControl module internal temperature sensor
What each part of P0666 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 6 | Area: computer (engine control module) and output circuits. |
| 66 | The exact fault in that area. |
What to do next
- Note P0666 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0666
What does P0666 mean?
P0666 means "Control Module Internal Temperature Sensor A Circuit". Control modules measure their own circuit board temperature with an internal sensor to protect output stages (overtemperature is P0634) and, in GM transmission modules, as a substitute for fluid temperature. It is a generic code, so it means the same on every make.
Is it safe to drive with P0666?
Drive gently and only to a workshop. With P0666 the car can go into limp mode (reduced power) to protect itself.
Will P0666 fail an emissions test?
Yes, it can. P0666 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0666 to fix?
Difficulty: hard. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will P0666 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.