Yes, a bad thermostat or faulty thermostat wiring can make your blower fan run continuously.

A persistent fan is often caused by a continuous twenty four volt call between the fan terminal and common, which tells the furnace or air handler to run the blower. Start by turning power off at the equipment and removing the thermostat faceplate to see if the fan continues to run when the stat is disconnected. Use a multimeter set to AC volts to measure between the fan wire and the system common at the thermostat and at the control board; if you read twenty four volts while everything is idle that indicates a continuous call. Inspect wiring for abrasion, pinching, or nail damage along the cable run where conductors can short together, especially between power and fan circuits. If the thermostat itself is old or has visible corrosion or loose terminals replace it with a basic replacement and recheck the voltage and function. When replacing wiring, pull new cable of the same conductor count and separate runs near metal edges, and secure both ends with proper connectors and strain relief so the signal remains clean.

Common mistakes include assuming the thermostat is the problem without checking for twenty four volt power at the control board first, which leads to unnecessary replacements; verify at the equipment to isolate wiring from the stat. Another error is swapping wires while the power is on which can create shorts and damage the control board; always kill power and label wires before disconnecting. People also overlook a short between the fan and power conductors inside junction boxes or at the air handler, so inspect splice points and replace brittle insulation where needed.

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