Skip the auto shop - Our mechanics make house calls

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
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

More related articles

How Long Does a Fuel Filler Neck Last?
Getting gas from the pump to your gas tank is the only way to keep a car going. The fuel filler neck is a metal tube that is welded to the...
How to Find the Keyless Code on a Ford Explorer or Mercury Mountaineer
Many Ford Explorers and Mercury Mountaineers were manufactured with an option known as the Ford keyless entry keypad. Some models refer to it as SecuriCode as well. It...
Veteran and Military Driver Laws and Benefits in Idaho
The state of Idaho offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch in the...


Related questions

Q: Car won't accelerate past 3000rpm

This issue could be caused by a variety of things. I would suggest by checking to be sure that the idle air control module is working properly. Also, check to be sure that the throttle body is delivering...

Q: Help in pinpointing recent car issues/ performance.

The Mass Airflow Sensor will cause general misfire codes. As far as cylinder #4 is concerned, you should still give it a quick inspection. The Mass Air Flow sensor is used to calculate a few different equations for the computer....

Q: Check Engine Light came on gave me a O2 sensor code. A week later the car broke down and won't start. It now has catalytic converter code.

It is helpful if you include the codes numbers. There are numerous O2 sensor and catalytic converter codes. There are two O2 sensors on your car. One is monitors the fuel ratio and the PCM (powertrain control module) uses it...