Error Code P0666 is defined as PCM/ECM/TCM Internal Temperature Sensor Circuit. This code is a generic trouble code, which means it applies to all vehicles equipped with OBD-II system, especially those made since 1996 up to present. This includes vehicles from, but not limited to, Audi, Chevrolet, Ford, GMC, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

Internal Temperature sensor/circuit incorporates the PCM (powertrain control module) ECM (engine control module), and the TCM (transmission control module). The job of The purpose of the PCM/ECM/TCM internal temperature sensor is to monitor the temperature of different sensors housed within the control modules. Malfunctions are identified through the process of the control module self-test failures. For some vehicles, the three modules are joined in one consolidated unit, normally called the PCM.

When the PCM, ECM, or TCM detects improper signals in the internal temperature sensor circuit, Error Code P0666 will be stored, and Check Engine light will be activated.

Other codes associated with the Internal Temperature Sensor/Circuit of different modules include:

  • Error Code P0667
  • Error Code P0668
  • Error Code P0669

Common Symptoms

Aside from activating the Check Engine light, this code may also cause the activation of the Transmission Warning light. Other common symptoms include:

  • No start condition
  • Engine stalling
  • Improper shifting

Possible Causes

  • Faulty control module
  • Defective or loose control module ground strap
  • Short in the wire, corroded or damaged connector
  • Defective PCM, ECM, or TCM

How to Check

The first thing that must be done to diagnose this code properly is to check the TSB (technical service bulleting) for the specific vehicle year, model, and power plant. This usually points to the right direction of diagnosis.

Next is to check all the associated wirings and connectors; search for signs of damage such as burnt wires, burnt spots, and bare wires caused by rubbing or scarping. Ground straps and ground wires must also be checked.

Then, check the connectors for security, damaged pins, and corrosion. This process also includes checking the PCM, ECM, and TCM, based on the specific vehicle and control module configuration. The specific tech data for the automobile will assist you with the component location, and control module configuration.

Advanced Steps

Advanced steps for diagnosis vary from one vehicle to another and may require specific advanced equipment and tools, such as a digital multi-meter, and specific technical references for the vehicle. Voltage requirements also vary based on the specific year and model of the vehicle.

Voltage Checks

Battery voltage should be around 12V, and the starter should have battery voltage with the ignition switch in the start position. The presence of voltage with the starter not engaging is a sign of a defective starter or starter solenoid. The lack of voltage means the ignition switch is faulty, or there’s a problem in the wiring.

If this process determines the absence of a power source or ground, then perform a continuity test to check the integrity of the wiring, ignition switch, and other components. Continuity test must be performed with the power removed from the circuit while reading for wiring and connections must be at 0 ohms of resistance, unless otherwise specified in the technical data of the vehicle. Resistance or no continuity means there’s a faulty wiring problem (open or short) and must be repaired or replaced.

How to Fix

Depending on the diagnosis, common repairs for this code include:

  • Repair or replacement of damaged, corroded, or shorted fuses, wires, and connectors
  • Repair or replacement of control modules power and ground circuits
  • Cleaning corrosion from connectors
  • Repairing or replacing faulty ground straps
  • Flashing or replacing the PCM, ECM or TCM
  • Replacement of control module for a new one

This code’s severity varies from the specific nature of the problem, an activated Check Engine light, or Transmission Warning Lamp on the vehicle, to start to no start condition.

When this code appears though, it usually means a serious condition, especially when it means the control module has suffered a fatal problem or a fault in the programming.

Misdiagnosis may result in the replacement of ECM, PCM, or TCM in error when the cause of the problem is simply loose connections. Also, some vehicles have two or more control modules. Thus, make sure you replace the right module.