A continuously running furnace blower that ignores thermostat commands usually signals a control system fault rather than a mechanical failure.

When the blower runs nonstop but will not respond to thermostat changes, start by isolating the control signals at the furnace control board. First turn power off and inspect the thermostat wiring terminals; a stuck fan call wire labeled G or a short to R can force the board to keep the fan energized, so gently tug or reseat the wire to remove corrosion and check for pinched insulation. Test the thermostat by disconnecting the G wire at the furnace and restoring power briefly; if the fan stops the thermostat or its wiring is at fault, and if the fan continues the control board or relay is likely stuck closed. Use a multimeter to verify that the board is not supplying a continuous five to twenty four volt control signal on the G terminal when there is no fan call from the thermostat. If the relay on the control board is electromechanical, listen for a click when you cycle the thermostat fan setting; lack of click suggests a welded or failed relay that needs replacement. If the control board uses a solid state driver, look for burned components, swollen capacitors, or visible damage and replace the board if you find them, because repairs are rarely reliable. Always verify after any repair that the thermostat and all wires are secure, and that the fan control behaves correctly in fan on, auto, heat, and cool modes before leaving the system powered.

Common mistakes include replacing the blower motor or capacitor first which wastes time and money when the issue is a control signal; avoid this by checking wiring and the G terminal first and only replace motor parts after ruling out control faults. Another frequent error is assuming the thermostat is fine; thermostats can fail or be misconfigured so test by disconnecting the G wire at the furnace and manually shorting G to R briefly to simulate a fan call which tells you whether the board reacts. Third, technicians sometimes replace the control board without checking for wiring shorts that will damage the new board; inspect wiring carefully for chafed insulation, rodent damage, or terminals contacting metal and correct wiring faults before installing a replacement.

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