Fault code P0666 is called “PCM/ECM/TCM Internal Temperature Sensor “A” Circuit Malfunction” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.
Technical description and explained code P0666
If your OBD-II equipped vehicle has stored code P0666. This means that the transmission/engine/transmission control module (PCM/ECM/TCM) has detected a fault in the internal sensor “A”. Which is related to the internal temperature sensor. The problem with this error is that a fault has been detected in the internal temperature sensor in the module.
Most PCM power circuits are relatively simple and straightforward. When the ignition is turned to the “ON” position, power is applied to the relay. This causes the solenoid to close two points of contact, one of which is permanently connected to the battery power source. When this occurs, the circuit closes and power from the battery is applied to the PCM. Additionally passing through the circuit with a fuse or fusible link.
Relays usually have a five pin design. The primary input pin is supplied with DC battery voltage. The ground output, grounded to the engine or chassis ground.
The secondary input terminal is supplied with battery voltage through a fused circuit when the ignition switch is in the “ON” position. The fourth terminal is the output for the PCM. The fifth, is the signal wire for the Controller Area Network (CAN).
The electronic control systems in vehicles equipped with OBD-II are controlled by a network of computers called control modules. This involves constant communication between the various modules via the Controller Area Network (CAN).
The purpose of the internal PCM / ECM / TCM temperature sensor is to monitor the temperature of the various sensors housed in the control modules. Faults are detected by the control module’s self-test failures. On some vehicles, the three modules are combined into one combined unit, usually called a PCM.
When the PCM, ECM, or TCM detects an internal temperature that is out of specification, code P0666 will be set. The engine warning light or transmission warning light will also illuminate.
Symptoms of vehicle malfunctions
The main signal that an error P0666 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.
It can also be warning signs such as:
- The “Check engine” control lamp on the control panel will light up (the code will be stored as a fault).
- Other related fault codes may also be present.
- Transmission emergency mode to prevent further damage.
- There may be problems starting the engine.
- There may be a drop in engine power.
- In most cases the engine will not start at all.
The diagnosis code P0666 varies from medium to very serious, depending on the specific nature of the problem. The malfunction light may just be illuminated, or the vehicle may not start at all.
Factors that can cause this error code
The error code P0666 can mean that one or more of the following problems have occurred:
- Defective power relay of PCM, ECM or TCM.
- Open or short circuit in CAN circuit.
- Damaged, corroded wiring or connectors.
- Low system voltage.
- Faulty ignition switch.
- Faulty PCM or other control module, also possible programming error.
How to fix or reset OBD-2 code P0666
Some suggested steps for troubleshooting and fix the error code P0666:
- Inspect the wiring for damage, abrasions, breaks, and shorts. If wiring is damaged, repair or replace it.
- Remove all diagnostic trouble codes from the PCM and perform a test drive to see if diagnostic trouble code P0666 returns.
- If the code returns, check the ECM/PCM/TCM relay to make sure it is working properly.
- Check the wiring diagram of the relay and fuse system.
- Clear the trouble codes again and perform a test drive.
- Insert the key into the ignition switch and turn it to the on position. If the Check Engine light does not come on, but the code in memory comes back on, the relay may be faulty.
- If the relay is faulty, it may be necessary to replace and reprogram the PCM.
Diagnose and repair of problems
To exclude all variants of error P0666, you should visually inspect the wiring, check the voltage on the battery. Also check the battery terminals and cables for loose connections.
Make sure the system voltage is as specified in the manual. The charging system is fully functional and the battery is in good condition, begin the diagnostic procedure.
Locate the starting point of the ECM/PCM/TCM supply circuit. This is usually the ignition switch, so use a digital multimeter to check the voltage at this wire or terminal. Compare the reading with the value given in the manual, and repair if necessary if an abnormality is found.
Most, but not all, control modules require a reference voltage of about 9 volts. Voltage requirements depend on the year of manufacture and model of the vehicle.
If the code persists, but the power supply to and from the ignition switch is within the specified range. Note and test the power supply to the fuse box and then to the PCM connector. If abnormalities are found, make the necessary repairs. To ensure that all electrical parameters are in accordance with the manufacturer’s specifications.
The steps described above will help to eliminate error P0666, but in some cases, when the cause is not obvious, intermittent faults may be present. Also, if the PCM is already damaged you may need to have it repaired by a specialized repair shop.
On which vehicles does this problem occur most frequently
Fault code P0666 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:
- BMW
- Dodge
- Ford
- Honda
- Land Rover (Discovery)
- Mazda (6)
- Skoda
- Volkswagen
Fault code P0666 can sometimes be found with other errors. The most common are the following: P0634, P0667, P0668, P0669.
