Heil thermostat shows F3; system cycles erratically or not at all

Safety first: Low-voltage only at the thermostat, but turn off power to the system at the breaker before removing thermostat to avoid shorts.

• Step 1 – Reboot thermostat: Remove the thermostat from its wall plate for 30 seconds, then reinstall. Some models also have a reset option in the menu.
• Step 2 – Check location: Ensure the thermostat is not in direct sunlight, near supply vents, or heat sources that can skew readings.
• Step 3 – Verify wiring: Confirm R, C, Y1, Y2, G, W, and communication terminals are correctly wired per Heil installation instructions.
• Step 4 – Factory reset: Use the thermostat menu to perform a factory reset and reconfigure for your specific Heil air conditioner and indoor unit.
• Step 5 – Replace thermostat: If F3 persists, the thermostat’s internal sensor or electronics are likely defective and should be replaced with a compatible Heil communicating thermostat.