DTC P0645: A/C Clutch Relay Circuit Open: Description

WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The A/C relay is located in the Underhood Fuse Block (UHFB). The ECM controls the A/C compressor clutch by supplying a ground to the A/C relay coil whenever the ECM receives an A/C request over the CAN link from the BCM with the A/C system pressure within operating range. When the A/C relay is commanded off (circuit not grounded), the voltage level at the ECM should be high (ignition voltage). When the A/C relay is commanded on (circuit grounded), the voltage level at the ECM should be low. The ECM A/C relay feedback circuit uses a pull-up voltage (2.6-4.6 volts), which allows the ECM to individually differentiate between an open, short to ground or short to voltage. DTC P0645 will set if the ECM detects an open A/C relay control circuit when A/C relay is commanded off with engine cranking/running.