Furnace fan won't turn off: why the blower runs constantly

The heat cycles normally but the fan never rests — or the fan runs with no heat at all.

Thermostat fan set to ON

ON means run continuously; AUTO means run only during heat cycles. This accounts for most "broken" blowers. Set AUTO.

Furnace stuck in cool-down

After the limit switch trips (usually a dirty filter overheating the unit), the board runs the fan to cool the heat exchanger and will not stop until it is satisfied. New filter, then power-cycle at the furnace switch for 60 seconds.

Stuck fan relay on the control board

If AUTO is set, the filter is clean, and a power-cycle does not stop the fan, the relay that switches the blower has likely welded closed. That is a control-board repair — technician, not DIY.

Miswired or failing thermostat

A thermostat calling for fan constantly (G terminal shorted) does the same thing. If pulling the thermostat off its base stops the fan, you found it.

Safety line: A blower that literally never stops is not dangerous by itself, but it can mask an overheating furnace that keeps cutting burners. If the constant fan started at the same time as weak or absent heat, treat it as an overheating symptom, not a fan problem.

Is the furnace flashing a light?

If the burners will not light, the control board is almost certainly flashing a diagnostic code behind the little window. Count the flashes and look the code up by brand here — walking into the phone call knowing the code saves diagnostic time.

Which code your brand shows for this

A fan that never rests usually is not a fan fault. After a limit switch opens, the control keeps the blower running to cool the heat exchanger, and it stores a limit code while it does it. Two control families go further and mark the limit trip as blower-caused, and on one brand a light that never changes is itself the code:

Brand	Code	What it reports
Carrier / Bryant	Status code 13	Limit circuit lockout — a limit or rollout switch stayed open more than three minutes
Goodman / Amana	E3	High limit switch is open — the furnace is overheating
Goodman GMEC96 / Amana AMEC96	11 red flashes	Limit switch open due to blower failure
York / Luxaire / Coleman	11 red flashes	Limit switch open more than five minutes — blower failure
Rheem / Ruud condensing	1 blink	Limit circuit is open
Trane / American Standard	LED continuously on	Replace the integrated furnace control

The GMEC96 and York rows are the interesting pair: those controls separate a limit trip they blame on airflow from one they blame on the blower itself — exactly the split a technician has to make on every constant-fan call, done for you by the board.

Fix it yourself, or call?

Homeowner territory: set the fan to AUTO, put in a fresh filter, and try one 60-second power cycle. If pulling the thermostat off its wall base stops the fan, the thermostat or a shorted G wire is your answer — a $25 replacement thermostat is a fair diagnostic tool.

Call a technician when the fan resumes with AUTO set, a clean filter, and no call for heat. That leaves a welded fan relay on the control board or a limit switch held open by genuine overheating, and both are meter diagnoses, not parts roulette. Do not keep power-cycling a furnace that is cutting its burners and running the fan to cool off — the fan is protecting the heat exchanger while something else is wrong.

What the fix typically costs

A welded fan relay means a board: $400–700 installed for a universal board, $500–900 for OEM, per the control board cost guide — which also argues that any board quote should include the $75–150 of diagnosis naming what killed it. If the trail instead ends at an overheating furnace, a limit switch is $150–350 in the repair cost guide, plus finding the airflow problem underneath it.

The opposite failure — a blower that will not start when the air conditioning calls for it — has its own page: furnace blower not running for AC.

Error codes behind this symptom

If your furnace is flashing a code while doing this, these code families are the usual suspects: blower motor codes, limit switch codes. Look your exact code up from the brand index, and see the blower motor replacement cost for what the fix typically runs.

Sources

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.