2000 Grand Limited 4.7L - P0645 A/C Clutch Relay Circuit Fault Diagnosis

You're dealing with a classic P0645 issue — only one code present, no others. Did you clear the code after replacing the relay? If so, did it return? The cycling behavior is not normal; P0645 specifically points to a wiring fault. Given your work history, I suspect a pinched or corroded wire in the A/C circuit — especially near the junction block or underhood harness.

Correct — only P0645 is present. After replacing the relay and clearing the code, it reappeared. The system worked flawlessly for months before this happened. Now, when the A/C cycles off, warm air comes through; when it turns back on, cold air returns. No engine work or wiring changes have occurred recently — so it's likely a component failure or intermittent connection.

How long does the cycle last? Usually just seconds, not minutes. Do you have conventional twist knobs or digital controls on your HVAC unit? Also, check for power at the Power Distribution Center (PDC) and junction block fuses — these are key in diagnosing P0645.

The cycle lasts about 10 minutes — it turns off and then comes back on after that period. I have traditional twist knobs with dual controls on the HVAC panel.

I'm now leaning toward a faulty HVAC control module, especially since the A/C started turning on by itself recently — which is not normal. The only possible fix might be cleaning or replacing the control unit if it's sending incorrect signals.

Are you using an OBD2 scanner? A key point: some scanners, especially older or basic ones, may not detect P0645 — this code is often hidden under 'Key Trick' limitations in certain vehicles.

I have a basic OBD2 scanner, but it doesn’t show the P0645 code. I only see it on the dashboard when the vehicle is running.

If your scanner isn't showing P0645, consider getting a free diagnostic scan at an auto parts store — many offer complimentary OBD2 checks. This can confirm whether the code is actually present. Also, be aware that the A/C relay circuit shares power with other systems — there could be a shared circuit issue causing intermittent faults.

I confirmed P0645 via the dashboard display. I’ll get a free scan at an auto parts store to verify it — thanks for the tip!

The code and issue persist. Yesterday, A/C worked fine, but today it’s completely off. I tested voltage at the relay with engine running — got 13 volts — but no power at the fuse. Any ideas on what to check next?

There are two fuses related to this circuit: one in the Power Distribution Center (PDC) and another in the junction block. The P0645 code specifically relates to the junction block circuit — fuse #21, a 10-amp fuse. However, note that there is also a 15-amp fuse #21 in the underhood fuse box (PDC), which may be part of a different but related circuit. To troubleshoot: check for power at the junction block fuse when the key is on and engine off (KOEO). This is critical to diagnosing the root cause.

Thanks for the clarification. I only know of a 15-amp fuse under the hood — that’s where the relay is located. I’m not sure what a shop would charge, but it seems like I might be overcomplicating this. I’ve inspected all accessible wires and don’t see any obvious damage or shorts.

The PDC fuse box is under the hood — fuse #21 there is a 15-amp. Is there power at that fuse when the key is on and engine off (KOEO)? There’s also a fuse #21 in the cab fuse box — this one is directly related to P0645. Both fuses have the same number but serve different circuits. This can be confusing, so double-check which circuit you're testing.

Thanks! Yes, it's a 15-amp fuse — I’ll check it again later. It’s currently too hot outside to work on it right now, but I’ll return and test when conditions improve.

Yes, it's a 15-amp fuse — but is there 12 volts at the KOEO position? It’s not hot outside — it’s actually unbearable heat right now!

I had to replace the battery recently and now the P0645 code is gone, and I have no voltage at the fuse with KOEO. I’ll drive the car a bit more and retest the fuse when I get back — thanks for the help!

The issue seems resolved temporarily — A/C is working normally again now. I’m going to keep an eye on it, but with summer still in full swing, I’ll let it ride for now and thank you all for the helpful input.