Climate and heating

P0645 — A/C Compressor Clutch Relay Control Circuit Fault

Severity: low Safe to drive

What P0645 means

The engine control module switches the A/C compressor clutch on and off through a relay by controlling its ground side. P0645 is set when the voltage in that control circuit doesn't match what the ECM commanded, such as an open, a short, or a failing driver inside the module. The compressor may then not engage at all or may behave erratically.

Symptoms

  • A/C blows warm air or won't turn on
  • No click from the compressor clutch when A/C is switched on
  • Check Engine light on
  • A/C works intermittently
  • A/C button lights up but there is no cooling

Possible causes

  1. Failed A/C clutch relay
  2. Open or shorted wiring in the relay control circuit
  3. Corroded or damaged relay socket or connector terminals
  4. Open or shorted compressor clutch coil
  5. Blown fuse in the relay or clutch circuit
  6. Faulty driver circuit inside the ECM
  7. Abnormal refrigerant pressure preventing clutch engagement

How to diagnose

  1. Scan for codes and freeze frame data, and look for other A/C or pressure sensor codes
  2. Inspect fuses and the relay socket for heat damage or corrosion
  3. Turn A/C on, listen for the relay click and measure the voltage at the control terminal
  4. Swap in a known good relay or an identical one from another circuit and retest
  5. Measure clutch coil resistance and compare it to specification
  6. Test the control wire from the relay to the ECM for opens and shorts to ground or power
  7. Check refrigerant pressure with gauges and test the pressure sensor
  8. If everything checks out, verify the control signal at the ECM connector and consider an ECM fault

Common fixes

  • Replace the A/C clutch relay
  • Repair damaged wiring or the connector
  • Replace the compressor clutch coil or the clutch assembly
  • Replace the fuse after fixing the cause of the short
  • Recharge or repair the A/C system
  • Repair or replace the ECM if its driver has failed

Questions and answers

Can I drive with a P0645 code?

Usually yes. The engine and brakes aren't affected, only comfort, since the A/C probably won't work. Still, fix it reasonably soon so a short doesn't damage the relay or ECM.

Will replacing the relay clear P0645?

Often it does, as the relay is the most common and cheapest cause. If the code returns, check the wiring, the clutch coil and the A/C system pressure.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.