Powertrain, generic · Computer and auxiliary outputs

P0645 A/C Clutch Relay Control Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

A/C clutch relay faulty (coil open)
Control wire or connector open, terminals spread
Relay or clutch supply fuse blown (e.g. shorted clutch coil)
Output driver in the module faulty

Symptoms

  • 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)

Repair estimate

DifficultyEasyDIY possible
Parts and labour€15–250
Time0.3–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

ac clutch relayrelay control circuitengine control module

Related codes

P0646P0647P0530P0532P0533P0534

References: law.cornell.edu, saabwisonline.com, roushcleantech.com, static.nhtsa.gov.