Fault code P0646 – A/C clutch relay control circuit low

Fault code P0646 is called “A/C Clutch Relay Control Circuit Low” 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 P0646

If your OBD-II equipped vehicle has stored code P0646. This means that the transmission control module (PCM) has detected a low signal level. Which refers to the control circuit of the air conditioner clutch relay (A/C).

Fault code P0646 – A/C clutch relay control circuit low

The air conditioner clutch relay control circuit is designed to control the operation of the air conditioner. The operation is done by supplying power to the air conditioner clutch, after which the air conditioner compressor is turned on.

Depending on the specific vehicle, this circuit can be very complex and include several control modules.

When the transmission control module (PCM) or one of the other supporting control modules detects incorrect signals. Which go through the electric circuit of the air conditioner clutch relay control, code P0646 is set.

Depending on the vehicle and the specific fault, this code may be detected up to eight times before the check engine light comes on. Code P0646 is then set by the PCM. This means that the control module detects a low signal level in the air conditioner relay control circuit.

Symptoms of vehicle malfunctions

The main signal that an error P0646 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check engine” control lamp on the control panel will light up (the code will be stored in the memory as a malfunction).
  2. When this error occurs, the vehicle’s air conditioner may not work properly or not effectively.
  3. The air conditioner may not work at all.

The degree of seriousness of the code P0646 may vary from insignificant, just burning engine control light while the air conditioner is working properly. But the air conditioner may also malfunction or not work at all.

Factors that can cause this error code

The error code P0646 can mean that one or more of the following problems have occurred:

  • Faulty air conditioner clutch or clutch relay.
  • Broken control module ground plate.
  • Open or short circuit in CAN circuit.
  • Blown fuse or jumper.
  • Damaged, corroded wiring or connectors.
  • Low pressure in air conditioning system due to lack of refrigerant.
  • Faulty PCM or other control module, also possible programming error.

How to fix or reset OBD-2 code P0646

Some suggested steps for troubleshooting and fix the error code P0646:

  1. Inspect the wiring for damage, abrasions, breaks, and shorts. If wiring is damaged, repair or replace it.
  2. Remove all diagnostic trouble codes from the PCM, and test drive to see if diagnostic trouble code P0646 returns.
  3. If the problem persists, check the CAN bus pins as well as the related electrical components with a special diagnostic scanner.
  4. Diagnose and, if necessary, replace the air conditioner compressor clutch or clutch relay.
  5. The air conditioner compressor should also be diagnosed.
  6. If everything is OK, the PCM may need to be replaced and reprogrammed.

Diagnose and repair of problems

To exclude all variants of error P0646, you should visually inspect the wiring, check the voltage on the battery. Also check the battery terminals and cables for loose connections.

Make sure that the system voltage corresponds to the value specified in the manual. The charging system is fully operational, and the battery is in good condition.

Checking voltage

The process of checking voltage, no power source or grounding, may require a circuit continuity check. This is done to check the integrity of the wiring, control modules, and other components.

All control module ground cables may need to be included in this process. A continuity check should always be performed with the power off. Normal readings for wiring and connections should be 0 ohms of resistance unless otherwise specified in the specifications.

Resistance or no continuity indicates faulty wiring that is open or shorted and requires repair or replacement. The most common and common error in diagnosing code P0646 is to misdiagnose symptoms and codes.

On which vehicles does this problem occur most frequently

Fault code P0646 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Cadillac
  • Chery (Amulet)
  • Chevrolet (Cruze, Spark)
  • Chrysler
  • Dodge (RAM)
  • Ford (Fiesta, Focus, Fusion)
  • Hyundai
  • Jeep
  • Kia
  • Mazda (3)
  • Opel
  • GAZelle (Next)
  • LADA (Kalina)
  • UAZ (Patriot)
  • VAZ (2114)

Fault code P0646 can sometimes be found with other errors. The most common are the following: P0645, P0647, P06A0, P06A1, P06A2.


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