The P0645 code in your 2021 Mazda CX-30 indicates a malfunction in the A/C compressor clutch relay control circuit. While the code description mentions "A/C fan 1," this typically refers to the A/C clutch control circuit in most applications. The ECM controls when the A/C compressor engages, and P0645 indicates a problem in this control pathway.
A/C Clutch Control System
When you press the A/C button, the climate control system requests compressor engagement from the ECM. The ECM then decides whether to allow engagement based on engine temperature, load, and refrigerant pressure. It commands the A/C clutch relay, which then powers the compressor clutch. P0645 indicates an issue in this relay control circuit.
Common Causes
A failed A/C clutch relay is the most common cause—relay contacts wear out or burn over time. Wiring issues between the ECM and relay affect control signals. Corroded connections at the relay or its connector create resistance that disrupts operation. In some cases, the ECM's relay driver circuit fails. Low refrigerant triggering safety cutouts can also set this code on some vehicles.
Symptoms in Your CX-30
The A/C may not engage at all when requested. You might hear clicking from the relay area without compressor engagement. The check engine light illuminates with P0645 stored. The A/C button indicator may behave abnormally—blinking or not illuminating. In some cases, the A/C works intermittently before failing completely.
Diagnostic Steps
Locate the A/C clutch relay in the fuse box. Test the relay by swapping with an identical relay from another circuit. Using Mazda IDS-II, command the A/C on and monitor relay control signal. Check refrigerant pressure to ensure the system isn't triggering low-pressure safety cutouts. Verify wiring from ECM to relay for continuity and proper voltage.