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.