Your blower fan keeps running and it is time to decide whether to call a professional.

A continuously running blower often stems from a thermostat wiring or configuration problem, a stuck relay in the air handler, a shorted fan limit switch, or an electronic control board fault, so start with safe, simple checks before touching anything live. First, set the thermostat fan setting to the off position and observe whether the blower stops after the thermostat cycles; if it does, the thermostat is likely sending constant power and you can replace or reprogram it following the manufacturer instructions. If the blower continues, turn off power at the service disconnect or breaker and look for obvious issues like a stuck relay or visible burn marks on the control board; if a relay appears physically welded closed you will need professional replacement. Test the fan limit or fan center switch by locating it in the furnace and verifying it is not mechanically stuck in the on position; replace the switch if it fails a basic continuity test done with the power off. For systems with variable speed motors, check that the control module is receiving proper signals from the thermostat and that the motor capacitor and wiring are intact; many variable speed systems use low voltage control circuits and require careful voltage measurement and wiring checks. Always use a multimeter and follow lockout tagout procedures when interacting with electrical components and document wire labels and terminal locations before disconnecting anything. If you are comfortable and safe with basic electrical diagnostics you can isolate the problem, but if you encounter live voltage you do not understand, suspect failed safety switches, or face repetitive trips of safety interlocks, stop and call a licensed heating, ventilation, and air conditioning technician.

Common mistakes include replacing the thermostat without verifying that the fan control setting or wiring is actually the cause, which wastes time and money; instead confirm that the thermostat is sending constant fan power by checking the fan terminal output before replacing the device. Another issue is attempting repairs with the power still connected, which risks injury and component damage; always shut off and lock out the breaker and verify zero voltage before touching wiring. A third frequent error is assuming a running blower is a fan motor failure when the true cause is a stuck relay or control board, so inspect relays and the control board visually and test for continuity before buying a motor.

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