Furnace Blower Won't Shut Off: Fan Setting, Open Limit, Stuck Relay or a Shorted Wire
A blower that won't shut off is usually doing exactly what it was told. Something is calling for the fan, or the board is running it on purpose as a safety. The job is to find out which before you replace a board.
Safety: Some checks are made with power on and the blower door switch held closed. Line-voltage terminals on the board are live.
Check these in order
| Step | What you find | What it means |
|---|---|---|
| 1. Thermostat fan switch | Set to ON | Normal continuous fan. Set it to AUTO. |
| 2. Board LED | Limit or rollout code, burners off | The board is running the blower to cool the heat exchanger. Fix the cause of the trip. |
| 3. Pull R and G at the board | Blower stops | The call is coming from the thermostat or its wiring: a bad thermostat, a shorted G wire, or a wiring mistake. |
| 4. Still running with no 24 V on G, W or Y | Blower keeps running | The blower relay on the board has welded closed, or a fan control on an older furnace is stuck. |
| 5. Older furnace with a fan/limit control | Manual fan button pushed in, or the fan setting is wrong | Reset it to automatic, or replace the control. |
1. Thermostat fan set to ON
It sounds obvious, but check it first. With the fan switch at ON, the blower runs all the time by design. Some smart thermostats also have a circulate or fan-schedule mode that runs the blower on its own.
2. An open high limit or rollout
This is the one to take seriously. When the high limit opens, the board shuts the gas valve and keeps the blower running until the limit cools down and resets. Amana's installation instructions describe exactly that: when the limit trips, the gas valve closes, the inducer shuts off after its post-purge, and the circulator blower stays on until the limit resets.
On many Amana/Goodman controls, the LED flashes a limit code while the circulator blower runs continuously and the furnace won't fire. The causes their troubleshooting chart lists are a faulty primary limit or not enough air across the heat exchanger: a blocked filter, restrictive ductwork, the wrong blower speed or a failed blower.
So if the blower won't stop and the burners are off, read the code before you touch anything. Look for:
- A dirty filter or blocked return
- Closed or blocked supply registers
- A dirty evaporator coil on top of the furnace
- The heat speed tap set too low
- A weak blower motor or capacitor
See high limit switch tripping for the full airflow check, and rollout switch tripped if it's the rollout.
3. A call from the thermostat or a shorted wire
If the board shows no fault, find out whether something is calling for the fan:
- With the blower running, measure 24 VAC from G to C at the board. If G is energized and the thermostat isn't calling, the call is real.
- Disconnect the thermostat wires at the board (or just R). If the blower stops, the problem is upstream: the thermostat, or the thermostat cable.
- Look for a G wire touching R where the cable was stapled, chewed, pinched or spliced, and for a wiring mistake after a thermostat change. See thermostat wiring terminals and colors.
On a heat pump or AC, check Y too. A Y call also brings on the indoor blower.
4. A welded blower relay on the board
If there's no 24 V on G, W or Y and the blower still runs, the board is energizing the blower on its own. The usual cause is a blower relay with welded contacts. Before you replace the board:
- Confirm no fault code is showing and the limit is closed.
- Make sure the blower isn't just finishing its normal fan-off delay after a heat cycle; most boards run the blower for a set time after the burners shut off.
- Check the board's 24 V and line voltage, and look for burned traces or a blown fuse.
If the blower runs with all thermostat wires off and no fault, replace the control board with the correct part for the furnace model.
5. Older furnaces with a fan/limit control
Older furnaces without an integrated board use a combination fan/limit control on the plenum. A manual-fan button left pushed in will keep the blower running, and a fan control that sticks will too. If the furnace has one, check the button and the dial before anything else.
FAQ
Is it bad if the furnace blower runs all the time?
Running the fan on purpose doesn't hurt the furnace. A blower that won't stop because the limit is open is a warning: the furnace is overheating, and the airflow problem needs to be found.
Why does my blower keep running after the burners shut off?
A short run after the heat cycle is normal; the board runs the blower to pull leftover heat out of the heat exchanger. If it runs long after that, check for an open limit or a call on G.
Can a bad thermostat make the blower run constantly?
Yes. A thermostat with a failed fan relay, or a fan schedule turned on, can keep G energized. Disconnecting the thermostat wires at the board tells you quickly.
The blower runs with the thermostat disconnected. What now?
Check the board's LED for a limit or rollout code first. With no fault and no call, a welded blower relay on the board is the usual cause.
Sources
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