P0666Generic code

P0666 Code: PCM/ECM/TCM Internal Temperature Sensor Circuit

The P0666 code means there is a fault in the sensor that measures the internal temperature of the computer. The sensor is built into the circuit board, so diagnosis comes down to ruling out power and ground problems and checking the software.

Severity
Low
Can I keep driving?
Yes, no immediate risk
Estimated cost
$0 to $1,500
Difficulty
Hard
System
Computer and Outputs
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0666 code means

Many computers have a temperature sensor on their circuit board to protect themselves from overheating and to correct some measurements. Because the sensor is inside, it cannot be replaced separately. When its circuit looks wrong to the computer, it stores the P0666 code.

Before you condemn the computer, always check its power supply and grounds. Low voltage, a loose battery terminal or a corroded ground can cause internal errors that disappear as soon as the connection is fixed. It is also worth looking for manufacturer software updates, because many of these codes are cleared by reflashing the computer without replacing it.

If the computer sits in a very hot area (next to the manifold, or with blocked ventilation), check how it is mounted too. Heat shields that went missing after other repairs can cause these codes.

These codes almost never justify replacing the computer on their own. Check power and grounds and look for updates first. If the computer is bolted to the engine or near the exhaust, make sure it still has its heat shield and its original bracket.

P0666 code symptoms

  • Check engine light on.
  • Usually no other symptoms.

Likely causes of the P0666 code

  1. Computer power supply or ground. The most frequent cause of apparent internal errors.
  2. Software. An update may be available.
  3. Real temperature too high because the heat protection is missing.
  4. Internal failure. Consider this only after power, grounds and software have been ruled out.

How to diagnose the P0666 code step by step

  1. Check power and grounds.
  2. Look for software updates.
  3. Check the mounting and the heat protection.

How to fix the P0666 code

  • Correct the power supply and ground problems.
  • Update the software.
  • Replace and program the computer if the code persists.

Technical reference data

  • The sensor is built into the computer circuit board.
  • Reference values: check the manufacturer data for your vehicle.

Brand-specific notes

This code is common on computers mounted in the engine bay, where heat is a factor.

P0666 on Ford

Frequent on Ford computers mounted in the engine bay.

P0666 on GM

Frequent on GM computers mounted in the engine bay.

Frequently asked questions about P0666

Can I drive with a P0666 code?
Yes, normally. There are usually no other symptoms, but have it checked so a real overheating problem does not go unnoticed.
Can the sensor be replaced?
No. It is inside the computer, on the circuit board, so it cannot be changed separately.
Does the computer have to be replaced?
Only if everything else fails. Check power, grounds, software and the heat shield first.
How much does it cost to fix P0666?
Between $0 and $1,500. Fixing a ground or updating software costs little, and replacing and programming the computer is at the top of the range.
What does the P0666 code mean?
It is a generic OBD2 code that means the circuit of the internal temperature sensor in a control module is not reading correctly.
What is the most common cause of P0666?
A power supply or ground problem at the computer. A P0666 code can also come from software or from a missing heat shield.

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Questions about the P0666 code

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