The powertrain control module (PCM) controls the air conditioner (A/C) clutch by controlling an internal driver which pulls the A/C relay coil circuit to ground. The PCM monitors the relay coil circuit for low and high voltage faults. DTC P0646 sets when the A/C relay is being commanded OFF and the voltage at the PCM is low.

DTC P0646 will set if the A/C relay voltage is less than 1.25 volts when the A/C relay is being commanded OFF.

DTC P0646 diagnostic runs continuously with the ignition ON and the A/C relay commanded OFF.

DTC P0646 is a type D DTC.

To locate an intermittent problem, use a Scan tool to command the A/C RELAY OFF, while monitoring the A/C RELAY-LOW VOLT or A/C RELAY-OPEN test. When a fault, open or short to ground exists, the Scan tool will display TEST FAIL for both tests.

The test will fail only when the device is being commanded OFF and the voltage at the PCM is low. The tests update every 100 ms whenever the ignition is turned ON and the device is being commanded OFF via the Scan tool or the PCM.

The A/C relay coil resistance: 70-90 ohms.