P0645 — Problem with A/C clutch relay electrical circuit

The code indicates an electrical issue in the air-conditioning clutch relay control circuit. It appears when the engine control module detects a fault in the wiring, relay, or control signal that engages the A/C compressor clutch.

Common causes

The cause is almost always one of a few things. Here they are — P0645 rarely comes from anything outside this list:

• Faulty A/C clutch relay
• Damaged or corroded wiring/connectors
• Blown fuse in A/C circuit
• Faulty A/C compressor clutch coil
• Engine control module fault

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Check and replace blown fuses
2. Inspect and repair wiring and connectors
3. Test and replace A/C clutch relay
4. Test A/C compressor clutch coil and replace if faulty
5. Check engine control module if all else fails

That is what holds for P0645 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.