P0645 OBD-II Trouble Code: A/C Clutch Relay Control Circuit

P0645 code definition

If your PCM has stored a P0645 code, it means that a problem has been identified in the A/C clutch relay control circuit.

What the P0645 code means

This code can be identified by the PCM, or by one of the following supporting modules:

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

If there is a fault in the A/C clutch relay control circuit, the system will record a trouble code, and your Check Engine Light will come on. This may not happen right away – with some vehicles, it can take as many as eight failures before the Check Engine Light is activated.

What causes the P0645 code?

Usually, the cause is a defective A/C compressor driver or A/C compressor control relay. Other causes could be:

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

What are the symptoms of the P0645 code?

You may not notice any symptoms other than an illuminated Check Engine Light. You could also see an illuminated or flashing A/C Malfunction light. If the Check Engine Light has not come on, the system may have stored the code as pending. If you are driving in hot weather, though, you may find that your air conditioning works poorly, if at all.

How does a mechanic diagnose the P0645 code?

The mechanic will use a code reader and ohmmeter in order to diagnose the code. Then, he or she will:

• Visually examine all the connectors and wiring, and replace any that are shorted out, open, disconnected or otherwise damaged
• Re-test the system to determine if the repairs have been effective
• Clear out the codes and see if they return when the vehicle is operating
• Test drive the vehicle
• Possibly use a specialized scanner like an Autohex, because of the complexity of the wiring
• Use a CAN scanner to identify pin values and determine how the control module is operating
• Check for corroded or loose connectors that could be causing trouble codes to be stored
• Compare findings with the manufacturer’s values

Frequently, a mechanic will simply replace an entire wiring harness rather than attempt to repair various components. This is because the diagnostics can be so complicated and time-consuming. A full replacement is often more cost-effective for the customer.

Common mistakes when diagnosing the P0645 code

The most common mistake when diagnosing the P0645 code is to misdiagnose the symptoms and codes. This is because of the complexity involved. Often, the wrong components are repaired or replaced.

How serious is the P0645 code?

That really depends on how much you value your air conditioning. Some drivers never use it, being content to just drive around with their windows down. For others, air is an absolute must. Suffice it to say, though, that this is not a problem that is ever going to endanger you or your passengers, so you can take your time having it dealt with, if you choose to deal with it at all.

What repairs can fix the P0645 code?

Corroded or otherwise damaged wires can be replaced, or if the source of the problem is not easily identifiable, the entire wiring harness can be replaced. Sometimes this is the more affordable option.

Need help with a P0645 code?

Having to do without air conditioning in your car is inconvenient. If you prefer to drive in comfort, and you’re A/C isn’t working properly, contact us at YourMechanic. You can reach us at 1-800-701-6230, or request an appointment using our online form. We will send a qualified mobile mechanic to the location of your choosing to fix you’re A/C. We’ll also be happy to provide you with a free quote.