P0646 OBD-II Trouble Code: A/C Clutch Relay Control Circuit Low

Check Engine Light

What the P0646 code means

The PCM or another supporting control module has registered a fault in the A/C clutch relay control circuit. Other modules can include:

  • Alternative fuel control module
  • Body control module
  • Antilock brake control module
  • Instrument panel control module
  • Fuel injection control module
  • Turbo control module
  • Anti-theft module
  • Cruise control module
  • Traction control module
  • Proximity alert module
  • Climate control module

What causes a P0646 code?

In most cases, the code is due to a defective A/C compressor control relay, or an A/C compressor driver in the PCM, since most A/C relay control circuits are integral with the PCM. Other causes can include corroded, open or shorted wiring in the CAN Bus harness, loose module control straps, or broken ground wires.

What are the symptoms of a P0646 code?

Symptoms may include:

  • Stored trouble code
  • Flashing or illuminated A/C malfunction lamp
  • Illuminated malfunction indicator lamp (MIL). If the code is stored but the MIL is not illuminated, the code may show up as “pending”.

How does a mechanic diagnose a P0646 code?

Newer vehicles are equipped with a CAN bus, an abbreviation for Controller Area Network. CAN is a communications bus that allows multiple processors to communicate with each other without a host computer. Designed for automotive use, the CAN bus network is a complex arrangement of wiring harnesses and connectors that connects two or more automotive control modules, sharing information to control practically every electrical function in the vehicle and networked back into the PCM.

These modules receive input data from various sensors, then pass along output signals to other control modules and system components. Individual module failures are rare, but possible. Follow manufacturer’s recommendations for testing and diagnosing individual modules, and replace as required. Replacement of almost any control modules will require reprogramming.

The technician will need an OBD-II code reader/scanner and a digital volt/ohmmeter for a successful diagnosis. A specialized scanner such as an Autohex or Tech II may also be needed.

  • Begin by visually inspecting all wiring and connectors. Repair/replace any damaged, disconnected, shorted or corroded wiring, connectors or components if needed. Retest the system to see if repairs were successful.

  • If wiring, connectors and components (including fuses) are in good working order, connect the scanner to the diagnostic port. Record all stored codes, and freeze frame data. Clear codes and test drive the vehicle to see if the code returns.

  • If the code doesn’t immediately return, there could be an intermittent condition. Intermittent conditions are tough to diagnose and may need to be allowed to return and worsen in order to get an accurate diagnosis.

  • Some applications involve up to 18 separate control modules. Should you choose to diagnose and repair, begin with a careful visual inspection of system circuitry, connectors and fuses.

  • Look for any engine or transmission ground straps, cables or wires that were left loose after repairs were performed.

  • Obtain a CAN bus system wiring diagram and/or pin out value chart and use the digital volt/ohmmeter to check continuity between individual connectors. Compare your findings with manufacturer’s specs and repair any open/shorted circuits as required.

Common mistakes while diagnosing P0646 code

Symptoms and stored codes are often misinterpreted and misdiagnosed, and repairs don’t address the actual problem.

How serious is a P0646 code?

A P0646 code can lead to A/C or climate control that won’t work properly.

What repairs can fix a P0577 code?

Addition comments for consideration for P0646 code

Unlike many other trouble codes, a code like this should perhaps be left to a professional, due to the sophistication of the circuitry involved. Hazards involved include:

  • Wiping out the memory of the PCM and CAN bus controllers, requiring reprogramming is necessary. A memory saving device must be used as a backup.

  • A single misplaced probe can destroy a string of control modules, requiring reprogramming

  • Disconnecting all control modules and testing continuity could take 40 hours of labor or more

A specialized CAN scanner can help narrow down the problem, but again, should be used by a trained professional.

Need help with a P0646 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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