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P0666 – PCM / ECM / TCM Internal Temperature Sensor Circuit (OBD2)

P0666 means "PCM / ECM / TCM Internal Temperature Sensor Circuit". The control unit has detected a fault in the electrical circuit of the engine control unit: the signal or control current is missing or outside the expected window. The code does not say whether the value is too high or too low, so the wiring and the component itself both have to be checked.

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P0666 quick summary

Fault codeP0666
MeaningPCM / ECM / TCM Internal Temperature Sensor Circuit
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityHigh
Can I drive?Depends on symptoms. If the engine runs normally, drive carefully to a workshop; if it stalls, misfires or goes into limp mode, have it recovered.
Repair cost€0 – €1,500

What internal control unit faults mean

These codes mean the engine (or transmission) control unit has detected a fault in its own memory, processor, internal temperature or internal communication.

Control units constantly check themselves: memory checksums, RAM and ROM tests, processor monitoring and internal temperature. If a self-test fails, they store codes such as P0601–P0606, P0613, P0618, P0619 or P0634. Codes P0666–P0669 cover the internal temperature sensor.

An internal fault code does not always mean the control unit is broken. Low or unstable supply voltage, poor grounds, a failed software update or water damage can all cause self-test failures. Some codes can be cleared by reprogramming the latest software.

If the fault is real, the control unit must be repaired or replaced and then programmed and matched to the vehicle (immobilizer, VIN, variant coding).

On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 17050 (P0666). Both numbers mean the same fault.

Symptoms of P0666

  • Check Engine light on
  • Engine stalls or will not start
  • Limp mode or erratic running
  • Communication problems with the scan tool
  • Other modules report lost communication

Most common causes of P0666 (most likely first)

Mechanic's Pro Tip

Before condemning a control unit, check the grounds. A corroded engine ground strap is one of the most common reasons for 'internal' control unit faults – and a fraction of the cost.

  1. 1.Damaged wire or loose, corroded connector at the engine control unit

    Most likely

    Green or white corrosion, pushed-back pins and chafed insulation near hot or moving parts are the most common findings.

  2. 2.Faulty engine control unit

    An internal break or short in the component itself.

  3. 3.Blown fuse or missing supply voltage

    If several codes appeared at the same time, check the shared fuse, relay or ground point first.

  4. 4.Low or unstable supply voltage or poor ground

  5. 5.Water ingress or corrosion in the control unit

  6. 6.Internal hardware failure

  7. 7.Overheating of the control unit

Step-by-step diagnostic guide

  1. 1

    Check battery, supply and grounds

    Measure battery voltage and the supply and ground pins at the control unit under load.

    Multimeter: Voltage drop on the control unit ground: below about 0.1 V.

  2. 2

    Check the circuit with a multimeter

    With the ignition off, unplug the engine control unit and the control unit connector (or back-probe), then check each wire between them for continuity and for shorts to ground and to battery voltage. Compare the component's resistance or supply voltage with the value in the repair manual.

    Multimeter: Wire continuity: below about 1 Ω. Wire to ground with both connectors unplugged: no continuity. Supply/reference with ignition on: usually 5 V for sensors, battery voltage for actuators.

  3. 3

    Read codes and clear

    Read all codes, note them and clear. See whether the internal fault returns immediately or only occasionally.

  4. 4

    Check for software updates

    Check whether a software update or technical service bulletin exists for the code.

  5. 5

    Inspect for water damage

    Check the control unit housing and connectors for water or corrosion.

  6. 6

    Repair or replace and program

    If the fault persists, have the unit repaired or replace it and program/match it to the vehicle.

Frequently asked questions

Can I keep driving with P0666?

Depends on symptoms. If the engine runs normally, drive carefully to a workshop; if it stalls, misfires or goes into limp mode, have it recovered.

How much does it cost to fix P0666?

Typical repair costs are 0 € – 1500 €, depending on the cause and the vehicle. The most common fixes are: damaged wire or loose, corroded connector at the engine control unit; faulty engine control unit; blown fuse or missing supply voltage. Always confirm the cause with the tests above before buying parts.

Does an internal control unit code mean the ECU is dead?

Not always. Supply, ground and software problems cause many of these codes. Check them first.

Can a control unit be repaired?

Often yes. Specialists repair many common control unit faults, which is usually cheaper than a new unit with programming.

What is the difference between P0666 and similar codes?

P0666 belongs to a group of codes for the same component that differ in the type or location of the fault: P0618 – Alternative Fuel Control Module KAM Error; P0619 – Alternative Fuel Control Module RAM/ROM Error; P0634 – PCM / ECM / TCM Internal Temperature Too High; P0667 – PCM / ECM / TCM Internal Temperature Sensor Range/Performance; P0668 – PCM / ECM / TCM Internal Temperature Sensor Circuit Low; P0669 – PCM / ECM / TCM Internal Temperature Sensor Circuit High. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.