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The A/C compressor clutch is responsible for pressurizing your car’s A/C system. It is connected to a relay that lets the engine control module (ECM) or powertrain control module (PCM) activate or deactivate the A/C. Once the compressor clutch is engaged, the A/C system can run and deliver cool air throughout the cabin.

If the PCM cannot control the A/C compressor clutch relay, its diagnostics will log a P0645 code.

What Does the P0645 Code Mean?

checking car airconditioning system
The Diagnostic Trouble Code (DTC) P0645 stands for “A/C Clutch Relay Control Circuit.”

The Diagnostic Trouble Code (DTC) P0645 indicates “A/C Clutch Relay Control Circuit.”

The PCM controls the A/C compressor clutch relay through the control circuit driver. This solid-state device provides the ground for the relay control circuit. The driver lets the module turn the clutch off or on, delivering power to the clutch and engaging the compressor. Depending on the vehicle, the A/C compressor clutch relay control circuit can have multiple control modules.

The control circuit driver has a fault line that is under constant observation by the PCM. When the controller turns the clutch on, the control circuit’s voltage will be around zero volts. It should match the positive voltage from the battery that powers the A/C compressor clutch.

AC Compressor Clutch Isolated
The A/C compressor clutch is responsible for pressurizing your car’s A/C system.

The PCM and its supporting controllers regularly check the voltage signal in the A/C compressor clutch relay control circuit. If one of the control modules doesn’t get the right response from the relay, it will set the generic code P0645.

Note: The definition of code P0645 may differ according to the vehicle manufacturer. Check the appropriate repair manual or repair database for the exact code definition.

What are the Common Causes of the P0645 Code?

What are the Common Symptoms of the P0645 Code?

How to Diagnose the P0645 Code

P0645 is a generic diagnostic trouble code (DTC) and applies to many OBD-II vehicles built from 1996 onward. While it’s a generic code, the steps you need to take for diagnosis and repair may vary between vehicles.

Analyzing and repairing a faulty A/C compressor clutch relay control circuit can prove complicated. If you’re not confident about conducting diagnostic tests on your vehicle, it’s a good idea to bring it to a professional mechanic. Otherwise, you may go ahead and diagnose the issue yourself. To help you get started, here are some videos that may give you an idea of what the process might involve:

How to Fix the P0645 Code

Attempting to fix a P0645 code may become a frustrating task if you don’t have the right tools and know-how to test the A/C compressor. In most cases, it’s best to leave the job to professionals.
For DIYers with advanced technical knowledge and hands-on experience who prefer to test and replace their vehicle’s A/C compressor on their own, it will be helpful to rely on the appropriate repair manual or refer to an online repair database.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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