Safety: Turn off power to both indoor and outdoor units at the breakers before working on wiring.

Check thermostat wiring: At the thermostat backplate, verify the Y wire is firmly connected and not touching other terminals. Tighten the terminal screw.
Inspect at air handler/furnace: Locate the Y connection on the furnace/air handler control board. Ensure the Y wire to the outdoor unit is secure and not corroded.
Outdoor unit check: With power off, open the outdoor condenser’s control panel. Inspect the low-voltage connections and the small control fuse if present. Replace a blown fuse only after checking for shorts.
Test operation: Restore power, set the thermostat to Cool and lower the setpoint. Listen for the contactor click at the outdoor unit. If there is no click and E4 returns, the thermostat’s Y output or the outdoor control board may be faulty.
Resolution: If wiring and fuses are good, replace the thermostat. If problem persists, have a technician test the outdoor unit’s control board and contactor.

E4 error appears when calling for cooling; outdoor unit does not start

Possible Causes