P0666: 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. 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.

Symptoms

• Warning or check engine light
• Changed shift quality (substitute value)
• Possibly transmission fluid temperature codes

Most likely causes

• Module faulty internally (temperature sensor or measuring circuit)
• Moisture or corrosion in the module or valve body
• Supply or ground problems of the module

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the battery and grounds
2. Clear the code and see if it returns
3. If it is a reference voltage code, disconnect sensors one at a time and see if the voltage recovers
4. Check whether a software update is available
5. Do not replace the control module without a full diagnosis

Good to know

• Repair difficulty: hard
• It can trigger limp mode.
• It affects emissions.

What each character of P0666 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 6: Control modules and outputs
• 66: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0666?

It depends on the symptoms. If the car is running normally you can go to a repair shop; if it goes into limp mode, drive as little as possible.

What causes code P0666?

The most common causes are: module faulty internally (temperature sensor or measuring circuit); moisture or corrosion in the module or valve body; supply or ground problems of the module.

Related codes

• P0667Control Module Internal Temperature Sensor A Range/Performance
• P0668Control Module Internal Temperature Sensor A Circuit Low
• P0669Control Module Internal Temperature Sensor A Circuit High
• P0634Control Module Internal Temperature A Too High
• P0711Transmission Fluid Temperature Sensor A Circuit Range/Performance

Sources

• law.cornell.edu: 13 CCR 1968.2
• saabwisonline.com: dtc p0667 p0668 or p0669

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.