A blower fan relay that is stuck will often cause the indoor fan to run continuously even when the thermostat is not calling for heat or cooling.

When diagnosing a stuck relay, start by confirming there is no call from the thermostat while the fan runs. Check the thermostat wiring at the air handler and verify there is no voltage present at the green terminal when the fan is running and the thermostat is idle. If the motor remains powered with no green terminal voltage, that is a strong sign the relay or control board contact is welded closed. Listen for a faint electrical click when the system cycles off and on, because a functional relay will click; absence of that sound during control transitions points to a stuck relay. Inspect the control board visually for scorched contacts or melted plastic which indicate contact failure and imminent permanent closure. For safety, always cut power at the service disconnect before probing or replacing components and use a multimeter rated for the system voltage to confirm readings. When replacing parts, match the relay coil voltage and contact amperage exactly and prefer a relay with a mechanical separation between coil and contacts to reduce the chance of repeated welding.

Common mistakes include assuming the thermostat is at fault when the fan continues to run; verify wiring and voltage at the green terminal first, and if there is no voltage yet the motor runs, focus on the relay or control board. Another mistake is testing with power on while touching components without turning off the breaker; always shut power off at the disconnect before doing any hands on inspection, and if you must test live use insulated tools and keep one hand behind your back. A third issue is replacing only the relay when the control board has other damaged components; if you find scorched traces or multiple fried parts replace the entire control board or consult a technician so the underlying cause is addressed.

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