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

Our certified mechanics come to you · Get a fair and transparent estimate upfront

Red-stars EXCELLENT RATING ON

Cost of diagnosing the P0646 code

P0646 is a diagnostic trouble code (DTC) for "A/C Clutch Relay Control Circuit Low". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

Cars Estimate Credit towards follow-up repair Earliest Availability
Ford$20.00
Toyota$20.00
BMW$20.00
SCHEDULE P0646 DIAGNOSTIC Get a fair and transparent estimate upfront

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.

P0646
trouble codes
Check Engine Light

No more waiting rooms! Our mechanics will come to you to diagnose and fix the P0646 code.

SCHEDULE P0646 DIAGNOSTIC
Get a $20 credit for the follow-up repair

Recent Check Engine Light is on Inspection reviews

Excellent Rating

(6,144)

Rating Summary
5,685
221
51
35
152
5,685
221
51
35
152

Chris

11 years of experience
13 reviews
Chris
11 years of experience
Ford F-150 - Check Engine Light is on - North Salt Lake, Utah
We were having problems with our check engine light, and Chris was exceptionally thorough until he found the problem and could help resolve the situation. We appreciate having someone who cares enough to get the job done right, not just done. We will definitely use Chris' services in the future. Thanks Chris.

Leng

21 years of experience
2 reviews
Leng
21 years of experience
Mazda MPV - Check Engine Light is on - Seattle, Washington
Leng arrived on time and explained the issue that my van is having. I will make a follow up appointment for the fix. Friendly, easy to talk to, and professional - thank you very much!

Okela

13 years of experience
11 reviews
Okela
13 years of experience
Land Rover LR4 - Check Engine Light is on - Brooklyn, New York
Okela was incredible! He is super thorough and took time to walk us through the issues with our LR4. Couldn’t recommend more! Thank you Okela!!

Robert

37 years of experience
188 reviews
Robert
37 years of experience
Land Rover LR2 - Check Engine Light is on - Atlanta, Georgia
Robert arrived on time. Did a diagnostic check quickly. He was friendly, and gave great service. He knows how to work on a Land Rover.


More related articles

P0592 OBD-II Trouble Code: Cruise Control Multi-Function Input "B" Circuit Low
P0592 means that the PCM has detected a fault in the cruise control input circuit, often due to a failed cruise control switch.
P2441 OBD-II Trouble Code: Secondary Air Injection System Switching Valve Stuck Closed Bank 1
P2441 P2441 code definition Secondary Air Injection System Switching Valve Stuck Closed Bank 1 What the P2441 code means P2441 is an OBD-II code for the engine control module (ECM) detecting no exhaust pulse while monitoring the exhaust pressure sensor...
P0877 OBD-II Trouble Code: Transmission Fluid Pressure Sensor/Switch “D” Circuit Low
P0877 means that the transmission fluid pressure that is above or below factory spec due to low fluid level, overheating engine, or faulty sensors.

Related questions

Mitsubishi Triton 4D56 - Engine Light and no start

Hey there. This code suggests that the intake throttle valve is not functioning correctly. When you say you give it gas and it starts, does this mean you are opening the throttle valve and putting gas into the engine manually?...

I have a 2002 Toyota Corolla and I'm having hesitation issues coming from a stop. I have tried everything to try and fix it. I hav

Claimed by Kevin Gainer

Engine light on

Check engine lights are just an indication to the operator that there has been a failure in the system and the computer will store a failure code for the failure. The problem you are having may be a misfire in...

How can we help?

Our service team is available 7 days a week, Monday - Friday from 6 AM to 5 PM PST, Saturday - Sunday 7 AM - 4 PM PST.

1 (855) 347-2779 · hi@yourmechanic.com