Fix P0645: Mechanic’s solutions that WORKED

The OBD2 code P0645 signifies an issue within the air conditioning (A/C) clutch relay circuit, indicating that the circuit is open. This code is critical because it affects the vehicle’s ability to regulate cabin temperature, leading to discomfort, especially in extreme weather conditions. An open circuit means there’s a break in the electrical path that prevents power from reaching the A/C compressor’s clutch, stopping it from engaging and the A/C from cooling the cabin.

P0645 quick overview

Meaning	P0645: A/C Clutch Relay Circuit Open
Is it serious?	Yes, especially in terms of comfort. While it may not affect the engine’s immediate performance, it can lead to an uncomfortable cabin environment.
Possible causes	– Faulty A/C compressor clutch – Open or short in the A/C clutch relay circuit – Bad A/C clutch relay – Issues with the Powertrain Control Module (PCM)
How to diagnose?	Inspect the A/C compressor clutch and connector. Test the A/C clutch relay and circuit for continuity. Check the PCM and related wiring for issues. Verify operation of the A/C system through diagnostic scanner. Review the A/C system pressure to ensure it’s correctly charged.

How to fix P0645 – learn from mechanics

I looked into P0645 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chrysler

Vehicle Information:
2011 Chrysler 200 Touring 2.4L

Issue + Repair History:

• Brought in for A/C service, system had no refrigerant.
• Leaking from condenser, high-pressure sensor, accumulator, and A/C compressor.
• Replaced all 4 components, A/C came on and blew cold initially.
• Three days later, A/C started blowing hot again, compressor not turning on.
• Detected OBD2 codes P0645 (A/C circuit open) in ECM, TCM, and HVAC module.
• No voltage to compressor, good signals from high-pressure switch.
• Replaced A/C compressor under warranty and still experienced P0645 code.
• Installed new PCM from dealership and had it reprogrammed, P0645 resolved, but no power to A/C compressor.

Other OBD2 Codes + Test Results:

• OBD2 Codes: P0645

Mechanic Suggestions:

• Double-check connector at compressor for proper fit.
• Verify A/C request, ambient sensors, and clutch engagement with scanner.
• Ensure battery was disconnected during PCM replacement.
• Check if PCM is requesting compressor activation and if TIPM is turning it on.
• Check PID values, particularly evaporator temperature sensor reading.

Final Fix:

• Replaced A/C compressor again and A/C connector, A/C blowing cold. A/C is fixed.

Case #2: Jeep

Vehicle Information:
2002 Jeep Grand Cherokee Limited 4.0L

Issue + Repair History:

• Experienced OBD2 code P0645.
• Able to operate clutch using Snap-on scanner.
• Losing signal from ECM to trigger relay to power clutch.
• Alldata does not show code for 2002, using 2001 flow charts.
• Clutch not engaging due to bad evaporator sensor, not sending signal to PCM.

Other OBD2 Codes + Test Results:

• OBD2 Code: P0645

Mechanic Suggestions:

• Check if the A/C pressure transducer is reading the correct pressure.
• Ensure the A/C system is properly charged.
• If Alldata does not provide information, call them for assistance.

Final Fix:

• Replaced the evaporator sensor, clutch now engaging and A/C working.

Case #3:

Vehicle Information:
2008 Ford F-150 XLT 5.4L

Issue + Repair History:

• No compressor clutch operation, OBD2 code P0645.
• Pin 18 out of PCM not grounding A/C clutch relay.
• Manually grounding wire activates A/C.
• Unplugging A/C pressure transducer with truck running shows reference voltage on pin 29 and 10, ground on pin 58.
• Discovered misfire code which disabled A/C, fixed by replacing bad coil, cleared all codes, A/C working.

Other OBD2 Codes + Test Results:

• OBD2 Code: P0645
• A/C pressure: Good
• A/C relay: Good
• Clutch cycle switch: Bypassed

Mechanic Suggestions:

• Check for any other codes in PCM, as Fords often disable A/C with misfire codes stored.
• Disconnect the TPS to check for shorted or stuck sensors causing the problem.
• Verify high/low pressure switches and related wiring.
• Commonly encountered shorted sensors include DPFE, which may affect engine performance.
• Check the pressure the PCM is seeing, as the sensor may be faulty.

Final Fix:

• Replaced faulty coil causing misfire, cleared all codes, A/C resumed working.

Diagnostic Step	Brief Explanation
Inspect the A/C compressor clutch	Check for physical damage or electrical issues.
Test the A/C clutch relay and circuit	Ensure there is continuity in the circuit and the relay works.
Check the PCM and related wiring	Look for any signs of damage or issues that could affect the circuit.
Verify operation of the A/C system	Use a diagnostic scanner to check for A/C request and clutch engagement.
Review the A/C system pressure	Make sure the A/C system is properly charged and there are no leaks.