P0645: A/C clutch relay control: circuit fault
The ECU detects a fault in the relay circuit that engages the air conditioning compressor. Usually the relay, a fuse or the wiring.
| Code | P0645 |
|---|---|
| System | Engine and transmission |
| Group | P06xx, Control unit and auxiliary outputs |
| Type | Generic: it means the same on every make |
| Can you keep driving? | Yes, no rush |
What P0645 means
For the air conditioning to cool, the ECU switches a relay that engages the compressor clutch. This code means that relay’s control circuit is not responding properly. The usual result is air conditioning that does not cool.
Most common causes
- Faulty or poorly seated compressor relay
- Blown air conditioning fuse
- Relay or compressor plug loose or corroded
- Control wire broken or chafed
- Faulty compressor clutch coil
Symptoms
- Engine warning light on
- Air conditioning not cooling
- Compressor not engaging, or engaging when it should not
Can you drive with P0645?
Yes, no rush It does not affect the engine or safety. You can drive normally, just without air conditioning.
How to check it
With the engine running and the air conditioning on, check whether the front plate of the compressor spins with the pulley. If not, check the compressor fuse and relay first.
How to read and clear P0645 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0645 with this same explanation and whether it’s confirmed, pending or permanent.
With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.
Similar codes
- P0638 Throttle actuator control: range or performance (bank 1)
- P0641 Sensor reference voltage A: circuit open or faulty
- P0642 Sensor reference voltage A: circuit low
- P0643 Sensor reference voltage A: circuit high
- P0650 Malfunction indicator lamp (MIL) control: circuit fault
- P0651 Sensor reference voltage B: circuit open or faulty
- P0652 Sensor reference voltage B: circuit low
- P0653 Sensor reference voltage B: circuit high
- P0600 Serial communication link fault
- P0601 Internal control module memory checksum error
- P0602 Control module programming error
- P0603 Internal control module keep alive memory (KAM) error
P0645 is a generic code, so it means the same on any make, for example on Alfa Romeo, Audi, Dacia, Kia, Opel / Vauxhall, Toyota. See what Final OBD does on your car in makes and models.
What does code P0645 mean?
For the air conditioning to cool, the ECU switches a relay that engages the compressor clutch. This code means that relay’s control circuit is not responding properly. The usual result is air conditioning that does not cool.
Can you drive with P0645?
It does not affect the engine or safety. You can drive normally, just without air conditioning.
How do you clear P0645?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0645 comes back.
Read it on your car
Final OBD reads the faults of every control unit for free, including each make’s own codes, and with Pro explains each one in plain English. Free clears engine faults at no cost.
Coming soon
Mid to late October 2026
If you want to try it in your car before anyone else, send us your email and we will add you as a tester.
info@finalobd.com