Thermostat indicates 'Fan Error' or cannot control fan independently

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

Steps to troubleshoot:

• 1. Test fan from thermostat: Set fan mode to On. If the fan does not run, proceed with wiring checks.
• 2. Inspect G wire at thermostat: Remove the thermostat and ensure the G wire is firmly connected and not broken.
• 3. Inspect G wire at furnace: At the furnace control board, verify the same G conductor is connected to the G terminal.
• 4. Continuity test: With power off, test continuity of the G conductor between thermostat and furnace. Replace cable if open.
• 5. Check fan relay: If wiring is intact, the furnace's fan relay or control board may be defective and should be tested and replaced by a technician.

When to call a professional: If the fan relay or control board is suspected, an HVAC technician should perform the replacement and verify safe operation.