AutoCodes – GMC – P0646 GMC

P0646 GMC Code - Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage

Quick Answer

When the Air Conditioning (A/C) switch is pressed, the Heating, Ventilation, and Air Conditioning (HVAC) control module sends an A/C request message to the Engine Control Module (ECM) via serial data. Common causes include Faulty Air Conditioning (A/C) Compressor Clutch Relay, Air Conditioning Compressor Clutch Relay harness is open or shorted, Air Conditioning Compressor Clutch Relay circuit poor electrical connection. Severity is Low - monitor the vehicle and repair when practical. The vehicle can usually be driven if no severe symptoms are present, but symptoms should be checked before extended driving.

What it means: P0646 GMC indicates Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage.
Common causes: Common causes include Faulty Air Conditioning (A/C) Compressor Clutch Relay, Air Conditioning Compressor Clutch Relay harness is open or shorted, Air Conditioning Compressor Clutch Relay circuit poor electrical connection.
Severity: Low - monitor the vehicle and repair when practical.
Can I drive?: The vehicle can usually be driven if no severe symptoms are present, but symptoms should be checked before extended driving.

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Code Popularity 524

How popular is this code, among other codes? Low

Repair Importance Level

Low (Monitor) (1 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Jul 17, 2022

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P0646 GMC Description

When the Air Conditioning (A/C) switch is pressed, the Heating, Ventilation, and Air Conditioning (HVAC) control module sends an A/C request message to the Engine Control Module (ECM) via serial data. The ECM then grounds the A/C compressor clutch relay coil control circuit, closing the relay contacts. The closed relay contacts provide voltage to the compressor clutch causing it to engage. The Diagnostic Trouble Code (DTC) will be set when the ECM detects a short to ground or an open at the A/C compressor clutch relay control circuit.

OBDII Code P0646 GMC - Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage - AutoCodes.com — P0646 GMC Code - Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P0646 GMC?

Faulty Air Conditioning (A/C) Compressor Clutch Relay
Air Conditioning Compressor Clutch Relay harness is open or shorted
Air Conditioning Compressor Clutch Relay circuit poor electrical connection
Faulty Air Conditioning Compressor Clutch
Faulty Engine Control Module (ECM)

How to Fix the DTC P0646 GMC?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0646 GMC code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P0646 GMC code?

Need more information on how to fix the P0646 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P0646 GMC code diagnosed by a professional: Find a repair shop in your area

Information for Specific GMC Models

P0646 2014 GMC Sierra — Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage
P0646 2015 GMC Sierra — Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage
P0646 2016 GMC Sierra — Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage
P0646 2017 GMC Sierra — Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage
P0646 2018 GMC Sierra — Air Conditioning Compressor Clutch Relay Control Circuit Low Voltage

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0646 GMC Code

What does the OBDII code P0646 mean on a GMC vehicle?

The code P0646 indicates a low voltage condition in the air conditioning compressor clutch relay control circuit, which can affect the operation of the A/C system.

What are the common symptoms of P0646 in a GMC?

Symptoms include the A/C system not working, reduced cooling performance, and possibly an illuminated check engine light.

What causes the P0646 code to appear on a GMC?

Potential causes include a faulty A/C compressor clutch relay, damaged wiring or connectors, a failing compressor clutch, or issues with the powertrain control module (PCM).

Can I still drive my GMC with a P0646 code?

Yes, you can drive the vehicle, but the air conditioning system may not function properly, which could be uncomfortable in hot weather.

How do I diagnose the P0646 code on my GMC?

Start by visually inspecting the relay, wiring, and connectors for damage or corrosion. Test the relay and wiring using a multimeter to check for voltage issues.

How is the P0646 code typically fixed on a GMC?

Common fixes include replacing the A/C compressor clutch relay, repairing damaged wiring, cleaning corroded connections, or replacing the compressor clutch if it's defective.

Could a faulty PCM cause the P0646 code in a GMC?

While rare, a malfunctioning PCM could lead to this code if it fails to properly control the A/C compressor clutch relay circuit.

How much does it cost to repair the P0646 code on a GMC?

Repair costs vary, but replacing the relay or fixing wiring issues may cost $50-$200, while replacing a compressor clutch could range from $500-$1000.

Can the P0646 code cause other vehicle problems?

The code primarily impacts the A/C system, but prolonged issues could strain the compressor or other related components.

Do I need special tools to fix the P0646 code on my GMC?

Basic tools like a multimeter and a socket set are usually sufficient for diagnosing and repairing this code, though advanced tools may be needed for PCM-related issues.