If your blower runs with no thermostat call for fan, a stuck or shorted control board relay is a likely cause.

Start by checking for twenty four volts between the G terminal and the common terminal on the control board while the blower is powered by one hundred twenty volts. Use a reliable voltmeter and verify that the thermostat is not calling for fan while you measure. If you see no twenty four volts on G but the blower still has one hundred twenty volts and runs, that tells you the relay contact or driver may be welded closed or the relay driver circuit is shorted. Before replacing the board, remove the blower motor lead from the board and then energize the system briefly to confirm the motor stops when disconnected; if it does, the board is the path that was driving the motor. Inspect for obvious signs of damage on the board such as burnt components, swollen capacitors, or melted solder, and also check for loose or bridged connectors that could be feeding power to the motor outside the relay path. If the equipment manual provides a board diagnostic jumper or mode use that procedure to force relay actuation and observe the relay behavior while watching for correct twenty four volt presence and absence on G.

Three common mistakes are measuring voltages with the wrong reference and misinterpreting results, leaving the system powered and wiring still connected which can give false answers, and assuming a motor or external relay is at fault without isolating the control board. Always measure between the correct terminals, G to common, and confirm the thermostat is not calling so you are testing idle state. Power down and disconnect the motor lead to confirm the motor only runs when fed through the board and not through some alternate wiring or an auxiliary relay. If you are unsure about reading the schematic or finding the common terminal consult the product manual or a trained technician before replacing parts.

BigHomeProjects.com helps homeowners find local contractors by making it easy to post projects and see who is interested near them. We connect contractors with work and host a discussion community where we and other members share troubleshooting tips and real world experiences.

Recommended Links:

Contractor Directory Website: https://www.bighomeprojects.com/
List your Business with Us: https://bighomeprojects.com/add-listing/contractors/
Subscribe to our Newsletter: https://acumbamail.com/newform/web/YhV8H72kPdyzMrrdh8xtbtvyqRotvSPfUCXaRM8cf/53636/