Error Code P0646 is defined as A/C Clutch Relay Control Circuit Low, meaning the PCM has determined a fault in the A/C clutch relay control circuit, like caused by defective control relay or driver, or faulty wiring.

This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system. This includes vehicle models from Cadillac, Chevrolet, Chrysler, Dodge, Ford, GM, Ram, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

The job of the A/C (air-conditioning) clutch relay control circuit is to control the air conditioning’s operation. This is made possible by providing power to the A/C to engage the A/C compressor. Based on the specific vehicle, this circuit can be complex and include several control modules.

The PCM identified this code by one of the supporting modules, namely:

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

Error Code P0646 is associated with the A/C (air-conditioning) clutch relay control circuit. When the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) or one of the vehicle’s supporting control modules sees an abnormal signal within the A/C clutch relay control electrical circuit, this code will be set. Based on the vehicle and specific malfunction, this code may be set as many as eight times before the Check Engine light is activated.

Specifically, this code is set when the PCM detects a PCM sees a low voltage situation in the A/C conditioning relay control circuit.

Common Symptoms

This code refers to a fault in the A/C system, meaning A/C operation is affected. Common symptoms for this code are:

  • Malfunctioning or non-operational A/C
  • Flashing of A/C malfunction light

Possible Causes

Defective A/C compressor driver or A/C compressor control relay are the two most common causes for this code. Other possible causes include:

  • Broken ground wire
  • Corroded, shorted, or open wires in the CAN bus harness
  • Loose control module ground strap
  • Defective CAN bus

How to Check

The first step for diagnosis of this code is to refer to the TSB (technical service bulletin) for the specific vehicle, year, model, and power plant. In many cases, manufacturers provide information for easier diagnosis and repairs of common problems.

Next is to inspect the associated wirings; look for signs of damage and defects, such as bare wires, burnt spots, scraping, rubbing, etc.

Then, check the connectors if they’re connected properly. Look for signs of corrosion and damaged pins. All wirings connectors and connections to components, including the control module and ground straps must be checked. This circuit can be complex and includes multiple relays, fuse-able links, and fuses in some cases.

Advanced Step

Advanced equipment like a digital multimeter and specific technical references may be used for specific vehicles. Voltage requirements usually vary from the vehicle’s specific year and model.

Voltage Checks

Voltage check process is troubleshooting guidelines specific and must be done in proper sequence. Most options and equipment depends on the number of control modules involved in the troubleshooting process. Thus, it’s important to check with the reference list.

If the process shows the absence of a power source or ground, the wiring’s integrity may be checked, including control modules and other components. All other control module ground straps must be included in this process. Continuity test must be always performed with the power removed from the circuit, and the normal readings for wiring and connections must be 0 ohms of resistance unless otherwise technical data specifies. Resistance or no continuity means faulty wiring, which can be open or short. They must be repaired or replaced.

How to Fix

Depending on the diagnosis, common repairs for this code include:

  • Replacement of the A/C clutch relay
  • Repair or replacement of faulty wiring
  • Repair or replacement of ground straps
  • Replacement of control module
  • Replacement of the blown fuse or fuse-able link (if applicable)
  • Evacuate and recharging of the A/C system
  • Cleaning connectors off of corrosion
  • Replacement of the A/C clutch
  • Replacement of the A/C compressor
  • Replacement or re-flashing of PCM

In many cases, mechanics will simply replace the entire wiring harness rather than attempt to repair different components. This is because a full diagnosis can be extremely time-consuming, and full replacement is usually the cost-effective choice.

The most common mistake in addressing this code is replacing the A/C components when the fault is simply in the wiring, ground straps, or other components.