Why won't my AC fan turn off?
Most of the time the fan switch is simply set to ON instead of AUTO, which keeps the indoor blower spinning nonstop whether the system is cooling or not. If it's already on AUTO and the fan still won't stop, the more likely cause is a stuck fan relay, a control board fault, or a thermostat sending a continuous run signal to the blower. In Florida's humidity, a fan that never rests can undo some of the drying the system just did.
The Switch Most Homeowners Never Touch
Most thermostats have a fan setting tucked below the temperature controls, usually labeled ON or AUTO, and it's easy to go years without ever touching it. AUTO is the setting almost every home runs on without thinking about it: the blower only spins while the system is actively heating or cooling, then shuts off once the thermostat is satisfied. ON keeps the blower running all the time, whether or not the compressor is working, which some homeowners choose on purpose for more even air circulation or constant filtering.
If your fan won't turn off, the first thing worth checking is which of those two it's actually set to. Which setting makes more sense for your home is its own separate question. This one is about the fan running when nobody meant for it to: what's actually wrong when AUTO is selected and the blower keeps going anyway.
When AUTO Still Doesn't Stop the Blower
A fan relay is a small, electrically controlled switch inside the air handler or control board that turns the blower motor on and off based on signals from the thermostat. Relays open and close that connection thousands of times over the years, and the contacts inside can weld together or stick in the closed position. Once that happens, power keeps flowing to the blower motor even after the thermostat has stopped calling for it, no matter what the display says.
A failing thermostat can cause the identical symptom from the other end of the wire. A control board fault, or a wiring issue where the fan wire ends up energized alongside the wire that should only carry power during an active heating or cooling call, sends the blower a constant run signal regardless of the ON or AUTO setting showing on the screen. From where you're standing, a stuck relay and a thermostat fault look exactly the same: the fan just keeps going.
The Humidity Cost of a Blower That Never Rests
When the compressor cycles off but the blower keeps running, air keeps moving across a coil that's still wet from cooling moments earlier, and that undoes some of the drying the system just finished doing, sending moisture back into the house instead of down the drain.
In a Southwest Florida home, where the air conditioner spends most of the year fighting humidity as hard as it fights heat, that's a real comfort cost. A house can feel clammy even while the thermostat reads the right number, and a fan that won't stop when it should is one of the more overlooked reasons why. There's a simpler cost too: a blower motor that never gets a break logs far more running hours every month than one cycling normally, and that shows up on the electric bill and shortens how long the motor lasts.
Ruling It Out Yourself, Then Calling It In
Checking the fan switch costs nothing and takes ten seconds, so it's always worth doing first. Set it to AUTO, walk away, and see whether the blower actually stops within a few minutes once the system isn't calling for cooling. If it does, that was the whole story. If it's already set to AUTO, or moving it there makes no difference, stop at the thermostat. What's happening next is inside the control wiring or behind the air handler's panel, and neither is a place to start testing on your own.
A relay or thermostat fault isn't something to diagnose by trial and error, and it's not worth letting run for weeks either, since the extra runtime costs you comfort and electricity every day it continues. A licensed AC repair visit starts with an actual diagnosis of the wiring and controls, so you know whether it's the relay, the thermostat, or something else before anything gets replaced.
A Fan That Won't Shut Off Is Telling You Something
Call or text (239) 350-5234. Tyler, Guiseppi, or someone from their team can trace it to the relay, the thermostat, or the wiring, with an upfront quote before any repair begins.