Thermostat fan won't turn on

Symptoms

• No air movement when Fan is set to On
• Heat or cool works but no separate fan-only mode
• No sound from the blower at all

Likely causes

• CommonBlown low-voltage fuseA small fuse on the control board can blow and disable fan calls specifically.
• CommonTripped door safety switch on the furnaceMost furnaces won't run the blower if the access panel isn't fully seated, tripping a safety switch.
• PossibleFailed fan relay or capacitorThese components can fail and prevent the blower motor from starting.
• PossibleWiring issue with the G (fan) wireA disconnected or miswired fan wire prevents the command from reaching the equipment.
• PossibleFailed blower motorMotors can burn out or seize after years of use.

What to do

1. Confirm the furnace or air handler access panel is fully closed and latched, since it often has a safety switch.
2. Check for a small blown fuse on the furnace control board and replace it with the same amperage rating if blown.
3. Check the breaker for the furnace or air handler circuit.
4. Remove the thermostat faceplate and verify the G wire is connected to its terminal.
5. Listen at the furnace or air handler for a hum with no movement, which suggests a failed capacitor or seized motor.
6. If none of these resolve it, the blower motor or relay likely needs professional repair.

Tools and parts

• Flashlight
• Replacement fuse (same rating)

When to call a pro

Call an HVAC technician if the panel is secure, fuse and breaker are fine, and the fan still won't run, since the motor or relay likely needs replacement.

Safety