AC or Furnace Won’t Shut Off: What Is Still Running

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the sources at the end

AC or Furnace Won’t Shut Off: What Is Still Running

Short answer

Set the thermostat to Off, wait three minutes and see what carries on. The indoor fan still going is usually the fan switch left on ON. The outdoor unit still going is a contactor welded shut or a shorted wire. Everything stopping and then starting again means something is asking for it, which is a different fault entirely.

A system that will not shut off is easy to sort out because the answer is standing in front of you. Turn the thermostat to Off, wait three minutes, and whatever is still running has told you where to look.

There are three possible answers and they lead to three different places, and one of them is not a fault at all.

Before you start

What to carry: a torch and a screwdriver. Ten minutes.

Gas or electric? Indoors the blower sits in a gas furnace or an electric air handler and both behave the same here. Outdoors it is the same box either way, and on a heat pump it is the same box again.

One thing that is not a fault. On the hottest or coldest days of the year a correctly sized system runs almost continuously, because it is only just keeping up. That is the machine working, not failing, and it stops once the weather eases.

Do not leave a system running with nothing controlling it. A unit that keeps running because a contact has welded shut is no longer being controlled by anything. It will run until somebody stops it, and on a cooling system that means an iced coil and eventually liquid refrigerant going back to the compressor.

Switch it off at the breaker rather than letting it run until somebody can come out. There is no harm in a house being warm for a day and real harm in a compressor being destroyed.

Turn it off and see what carries on

Three answers, three different places to look, and the test costs nothing.

Check 1Free, three minutes

Set the thermostat to Off and wait

Off means Off for both heating and cooling, not just a temperature you think it will never reach.

Give it three minutes. Some systems run the blower on for a while after a cycle on purpose, to get the last of the heat or cold out of the coil, and that delay is normal. Carrier and Lennox furnaces can be set to run it for up to 180 seconds after heating.

Then listen indoors and walk outside. What you can still hear decides which section below is yours.

The indoor fan will not stop

Check 2

The fan switch, then the relay, then the wire

The fan switch on the thermostat. On ON the blower runs day and night by design. This is a common answer, and it also explains air from the vents that never feels warm or cold.

The fan delay setting. Some systems are set to run the blower for several minutes after each cycle, and a few are set to run it continuously for air circulation. Both are settings rather than faults.

A stuck fan relay. If the fan switch is on Auto and the blower still never stops, the relay on the control board may have stuck closed.

A shorted G wire. The green wire calls the fan, and if it is touching the red one anywhere in the run the fan is being asked to run constantly. A staple through a cable in a wall does exactly this: what each letter and colour does.

To tell the last two apart, take the thermostat off its plate and separate every bare wire end. If the fan still runs, the thermostat is innocent and the fault is the relay or the wiring.

The outdoor unit will not stop

The contactor is a switch held closed by a magnet. Once its contacts weld together, nothing can open them.

Check 3

Suspect the contactor first

Inside the outdoor unit is a contactor, a relay that switches the mains to the compressor when the thermostat calls for cooling.

Its contacts carry a lot of current, and every time they open they draw a small arc. Over years they pit, and eventually they weld together. Then the thermostat can stop calling all it likes and the unit carries on.

The giveaway is that the unit keeps running with the thermostat off and even with the thermostat taken off the wall completely. Nothing upstream is in control any more.

Switch it off at the breaker and at the pull out block on the wall until it is changed. The part is cheap and it lives in a box with mains in it, so most people stop there: the power path to the outdoor unit.

The other possibility is the yellow wire shorted to red somewhere, which asks for cooling permanently. Disconnecting the wires at the thermostat tells you which of the two it is.

It stops, then starts again straight away

Then nothing is stuck. Something is asking for it, and the question is what.

The setpoint. A thermostat set to 68 in a house that is 76 will run all day and that is correct behaviour.

The schedule. A programme that drops the temperature at a certain hour looks exactly like a system switching itself on.

A thermostat that is reading wrong. Sunlight on it, a lamp near it, or a draught up the wall cavity behind it makes it believe the room is hotter or colder than it is, so it keeps calling. Stuffing insulation into that wall opening is free.

A system that cannot keep up. A dirty filter, a blocked coil or low refrigerant all mean it never reaches the number, so it never stops. Measure the temperature drop across the coil before assuming anything: how to measure it and what the number means.

Questions people ask

Is it bad for the system to run constantly?

In a heatwave, no, and it is actually more efficient than short bursts. Running with nothing controlling it is a different matter, because there is no limit on where it ends up.

The outdoor unit runs even with the thermostat removed.

That usually points to the contactor welded shut. Switch it off at the breaker and get the part changed.

The blower runs but nothing is heating or cooling.

Fan switch on ON. It pushes room temperature air round the house, which feels like a system that has stopped working and is not.

My water heater seems to run all the time.

A tank that reheats constantly usually has a stuck thermostat or hot water leaving somewhere, and the reset button on it is where that shows up: why a reset button keeps tripping.

It started after somebody worked on it.

Then a wire is almost certainly the answer. A pinched or crossed thermostat wire gives exactly this, and it did not do it on its own.

The furnace blower keeps running after the heat stops.

Up to three minutes is normal, and it is clearing the heat out of the exchanger. Several minutes every cycle with short burner runs is a different fault: why a furnace short cycles.

Sources: Goodman service instructions RS6200006 (Comfort Alert code 8, welded contactor: compressor always runs), Carrier Residential Air Conditioners and Heat Pumps Application Guideline and Service Manual (burned or pitted contacts), Honeywell T6 Pro installation instructions (fan On: fan is always on), and the blower off delays in Carrier 58SC0A/58SC1A, Goodman *MSS9 IOG-2009E and Lennox SL280UHV 508245-01 furnace instructions