A continuously running fan when the system is set to “auto” usually means the control signal that tells the fan to stop is not being received or processed correctly.

Start by confirming the thermostat configuration and wiring at the thermostat and air handler. Many thermostats have a fan mode that can be set to “on” or “auto” so begin by verifying the device is set to “auto” and then remove the thermostat cover to inspect the fan control wire labeled G. Check that the G wire is fully seated at the thermostat terminal and at the air handler control board terminal and that there are no exposed strands touching other terminals. If the thermostat is battery powered, replace the batteries and perform a thermostat reset per the manufacturer instructions because corrupted settings can force the fan on. Use a noncontact voltage tester or multimeter to verify the G terminal is not shorted to the common or ground; a short can keep the fan energized even if the thermostat is not commanding it. If wiring and thermostat settings look correct, observe the air handler control board when cycling the thermostat from “auto” to “on” and back while watching for proper changes in voltage on the G terminal; if the control board continues to supply the fan relay coil voltage when the thermostat is not calling for fan, the board or a stuck relay is likely at fault. When replacing parts choose components that match the original control voltage and ratings and follow safety practice of killing power to the system at the breaker before touching wiring or replacing the relay or control board.

Common mistakes start with assuming the thermostat is faulty without checking wiring first so always inspect connections and test the G terminal with a meter before ordering parts. Another frequent issue is miswiring after a thermostat replacement where the green fan wire can get placed on the wrong terminal so correct the terminal assignments and retighten screws to stop intermittent contact. Homeowners often overlook a stuck mechanical relay inside the air handler which can weld closed and keep the fan running; tapping the relay cover to see if the fan changes state is a quick check but replacement is the reliable fix. Also do not forget that a shorted G wire to ground or to the common will energize the fan constantly and repairing or replacing the damaged section of wire fixes that. Finally a bad control board can mimic other faults so if every other test points to normal wiring and thermostat operation then swapping the board is appropriate.

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