P0666 Fault Code

PCM / ECM / TCM Internal Temperature Sensor Circuit

• What does that mean?
• Symptoms
• Causes
• Solutions

The P0666 fault code indicates a problem with the PCM (Powertrain Control Module), ECM (Engine Control Module), or TCM (Transmission Control Module) internal temperature sensor circuit. This sensor plays a crucial role in monitoring the temperature within the modules to ensure optimal performance and prevent overheating. When this code is triggered, it often signifies an issue with the temperature sensor itself, the wiring, or the connectors engaging the sensor.

Monitoring the internal temperature is vital because if the modules overheat, it can lead to significant performance issues or even complete failure of the systems involved. It is essential for the vehicle's overall functionality especially in complex modern engines that rely heavily on electronic control systems.

What are the symptoms of the P0666 code?

Vehicles experiencing a P0666 fault code may exhibit the following symptoms:

• Check Engine Light (CEL) illuminated on the dashboard.
• Possible limp mode where vehicle performance is reduced to prevent damage.
• Erratic shifting in automatic transmissions.
• Engine stalling or misfiring due to incorrect temperature readings.
• In some cases, there may be no noticeable symptoms aside from the fault code itself.

What causes the P0666 code?

Some common causes for the P0666 fault code include:

• A malfunctioning internal temperature sensor within the PCM, ECM, or TCM.
• Issues with the wiring harness that connects to the sensor.
• Corroded or damaged connectors that may disrupt the circuit.
• Software glitches in the control module that require reprogramming or updates.
• Overheating caused by improper engine performance or high ambient temperatures.

How to fix P0666?

To resolve a P0666 fault code, the following steps may be taken:

• Perform a thorough diagnostic to verify the code and rule out any related codes.
• Inspect the wiring harness and connectors for signs of damage, corrosion, or loose connections.
• Test the internal temperature sensor for proper operation and replace it if found faulty.
• Check for software updates or reprogramming needs for the PCM, ECM, or TCM.
• Clear the code and conduct a road test to ensure the issue is resolved and the code does not reappear.