A blower that will not shut off usually points to an electrical control or sensor fault that needs targeted troubleshooting.

Start by confirming power and control signals using a quality multimeter with leads and clips to measure voltage at the thermostat, control board and blower relay while the unit is running and while you attempt to turn it off. Inspect the thermostat wiring for stuck contacts or shorted wires by isolating the thermostat harness and checking continuity; remove the thermostat call for heat or cool to see if the blower still runs. Check the blower relay on the control board for welded contacts by listening for click when you toggle power and by measuring coil voltage and contact continuity; replace the relay if contacts are fused. Examine the fan limit switch or integrated fan control for a stuck actuator or failed temperature sensor by testing its continuity at room temperature and manually actuating it if accessible. Verify the control board outputs are responding correctly by comparing input command voltages to output voltages and swap in a known good board or module when diagnostics point to board failure. Always use safety gloves, lock out the power at the service disconnect, and follow manufacturer service procedures and recommended replacement parts when a specific diagnostic tool is suggested for a proprietary board.

Common mistakes include not isolating the thermostat so field wiring faults are missed, and the fix is to disconnect the thermostat harness and test the system independently. Another issue is assuming a fan motor failure when the relay or control board is the source, so measure relay coil and contact behavior before replacing the motor. A third frequent problem is working live without proper protective gear or without locking out power, which risks injury and component damage, so always de-energize the system, use insulated gloves and verify zero voltage before touching wiring.

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