Furnace Fan Won’t Turn Off? Causes & Fix

The usual suspects

The fan set to ON

At the thermostat, ON means always. AUTO means only with heat. The one-second fix everyone deserves to check first.

A stuck fan relay or board

The relay that switches the blower welds closed — the fan ignores everyone, including the OFF switch logic.

A limit switch doing overheat duty

After an overheat event the furnace runs the blower to cool itself. If overheats keep happening (dirty filter), so does the endless fan.

Thermostat wiring fault

A shorted G wire calls for fan forever — common after DIY thermostat swaps.

One switch or one relay away.

Fan-stuck-on calls are usually a settings fix or a single relay — fast visits, priced upfront.

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Worth trying first (5 minutes, free)

1
Thermostat fan to AUTO first, always.
2
Change the filter — overheat cool-downs come from starved airflow.
3
Flip the furnace switch off 60 seconds: if the fan restarts on its own afterward with everything off, a relay is stuck.

Still not working? Then it’s a technician job — and the sooner it’s booked, the sooner the house is comfortable again. Book the visit →

Stop and book a pro if…

A burning smell with the endless fan — switch it off.
The fan runs with the thermostat disconnected/off — relay or board; book it.
It started right after a thermostat install — likely wiring; easy professional fix.
Overheat cycles keep repeating even with a clean filter.

Any of these? Book priority service

Next Phase technician walking a homeowner through the repair

Why homeowners hand it to us

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Quick answers

Does a constantly running fan hurt anything?

It wears the motor, drags your electric bill, and in summer can re-evaporate coil moisture back into the house. Some people run ON on purpose for filtering — but it should be a choice, not a fault.

I flipped it to AUTO and it still runs. Now what?

Then a relay, control board, or limit circuit is holding it on — a straightforward meter diagnosis and usually a single-part fix, priced upfront.

Could this be the thermostat’s fault?

Yes — a shorted or miswired G terminal runs the fan forever. It’s one of the first things we test, because it’s the cheapest possible answer.

Done guessing?

A 24/7 blower costs real electricity and wears the motor. Book the fix.