P0666 OBD-II Trouble Code: PCM/ECM/TCM Internal Temperature Sensor Circuit

Check Engine Light

P0666 code definition

Either the powertrain control module (PCM), engine control module (ECM) or transmission control module (TCM) has detected an internal temperature malfunction in a related sensor. On many vehicles, the three modules are integrated into one unit, called the PCM.

What the P0666 code means

A circuit failure has caused a potentially catastrophic overheating condition in a sensor.

What causes the P0666 code?

Defective control module or shorted wiring to a module.

What are the symptoms of the P0666 code?

  • Engine stalling
  • No-start condition
  • Stored code
  • Illuminated malfunction indicator lamp (MIL)

How does a mechanic diagnose the P0703 code?

Individual control modules are designed with temperature sensors, that help with self-diagnosis and preservation in case of a malfunction. In some applications, the module that’s in failure mode may be easy to track down.

Begin diagnosis by testing the power and ground circuits for the module in question. Compare your findings with factory specs and make any repairs needed.

Clear any codes and retest the system. If all power and ground circuits are within spec, suspect a failed control module. Remember that replacing a control module will also require you to reprogram it for the vehicle.

Common mistakes when diagnosing the P0666 code

Often, multiple control modules can fail along with the shorted or defective ground wiring to the first module in question.

How serious is the P0666 code?

In most cases, a P0666 code can be enough to prevent a vehicle from running at all.

What repairs can fix the P0703 code?

Replacement of the failed module, or multiple failed modules.

Additional comments for consideration regarding the P0666 code

Newer vehicles are equipped with Controller Area Network (CAN) bus systems. These discrete networks of micro-controllers govern everything such as:

  • Stability control
  • Traction control
  • Antilock brakes
  • Radio
  • Power windows
  • Cruise control

These sophisticated networks can include as many as 20 different processors and micro-controllers, and can be very difficult to service by anyone other than a qualified tech with a specialized scanner such as an Autohex.

A misplaced probe from a digital volt/ohmmeter can be enough to wipe out a controller or even a whole line of controllers, requiring the entire vehicle to be reprogrammed. Should you elect to take this job on yourself, it’s important to have a memory saving device of some sort for backup in case of a system meltdown.

Need help with a P0666 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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