Thermostat displays E21 and indicates fan control fault

Safety first: Turn off power to the furnace/air handler at the breaker before working on wiring.

Steps to troubleshoot:

• 1. Observe fan behavior: Note whether the indoor fan runs constantly, never runs, or only runs in certain modes.
• 2. Inspect G wire at thermostat: Remove the thermostat and verify the G wire is properly inserted and not touching adjacent terminals.
• 3. Inspect G wire at furnace board: At the control board, confirm the G conductor is connected to the G terminal and that the terminal screw is tight.
• 4. Test fan relay (advanced): With power off, remove the G wire from the thermostat. Restore power and jumper R to G at the furnace board. If the fan runs, the board relay is likely good and the thermostat output may be faulty.
• 5. Check for shorts: Inspect the thermostat cable for damage where it passes through walls or near metal edges.
• 6. Replace thermostat or board as needed: If the fan relay does not respond to a direct jumper, the furnace control board may need replacement. If it does respond but E21 persists, replace the thermostat.

When to call a professional: Fan relay and control board replacement should typically be done by an HVAC technician.