P0645 code meaning
A/C Clutch Relay Control Circuit
What P0645 means
The engine module switches the A/C compressor clutch relay through a low-side driver once the climate control requests cooling. P0645 is the general circuit fault of the relay coil control, usually an open circuit. GM Europe's 2011 Saab 9-5 sets P0645/P0646 for an open or short to ground and P0647 for a short to voltage, with the engine running and A/C requested; Ford's commercial trucks report it without the check engine light. Older Saab diesels (Bosch EDC16/PSG16) use a different assignment (P0645 short to voltage, 0.6 s). GM found a no-start with P0645 on the 2002–04 Alero/Grand Am 3.4 L because a shorted compressor clutch coil blew a fuse shared with the engine control; Ford traced warm air with P0645 on the 2020–23 Explorer to spread terminals at the body module. The compressor then does not engage or runs constantly.
P0645 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0645
- A/C not cooling (compressor does not engage)
- Compressor runs constantly
- Check engine light usually off, code stored only
- Rarely no start if a shared fuse has blown (GM 3.4 L)
Common causes of P0645
- Most likelyA/C clutch relay faulty (coil open)
- Most likelyControl wire or connector open, terminals spread
- PossibleRelay or clutch supply fuse blown (e.g. shorted clutch coil)
- Less likelyOutput driver in the module faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Ac clutch relayRelay control circuitEngine control module
What each part of P0645 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 6 | Area: computer (engine control module) and output circuits. |
| 45 | The exact fault in that area. |
What to do next
- Note P0645 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0645
What does P0645 mean?
P0645 means "A/C Clutch Relay Control Circuit". The engine module switches the A/C compressor clutch relay through a low-side driver once the climate control requests cooling. It is a generic code, so it means the same on every make.
Is it safe to drive with P0645?
P0645 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P0645 fail an emissions test?
P0645 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P0645 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0645 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.