Furnace Not Working: Find the Fault Before You Call

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the sources at the end

Furnace Not Working: Find the Fault Before You Call

Short answer

Do the five free checks first: the thermostat and its batteries, the switch on the side of the furnace, the breaker, the front panel clicked properly shut, and the filter. Then read the flashing light through the little window and match it to the label inside your own furnace door, not to a code list from another make.

A furnace not working in the first cold week of the year is usually one of five things that cost nothing to check. A flat thermostat battery and a front panel that was not clicked properly shut are both on that list, and both are worth ruling out before anybody charges you for a call out.

This page does the free ones in order, then shows you how to read the light the furnace is flashing at you.

Before you start

What to carry: a torch, a screwdriver and a new filter if yours is more than a few months old. Twenty minutes.

Is it a furnace at all? A gas furnace has a gas pipe into it and a flue out of the top. An electric one has a thick cable and no flue, and it usually needs two or three breakers rather than one. If the box outside your house runs in winter and the indoor unit has no gas pipe, you have a heat pump instead, and air that feels lukewarm rather than hot is normal for one.

Does it do anything at all? Completely dead, or a fan runs and then stops, or it lights and drops out again. They lead to three different places, and the checks below sort them.

Two things that are not a reset job. If you smell gas, leave it. Out of the house first, no switches and no lights, then the gas company from outside.

If you open the burner box and find a switch that has tripped near the front of the burners, called a rollout switch, that one has tripped because flame came out where it should not. Soot, a yellow lazy flame or a scorched look around the burners means the same thing. Pressing it back in and walking away puts carbon monoxide in the house. That one needs somebody with a combustion analyser.

The five that cost nothing

Five minutes, no tools and no parts. Do all five before anybody is called out.

Check 1

The thermostat, and its batteries

Fresh batteries, set to Heat rather than Cool or Off, and pushed at least five degrees above the room temperature so there is a real call for heat.

Check the fan setting as well. On ON the blower runs day and night and pushes room temperature air through the vents, which feels like a broken furnace and is not one.

If somebody changed the thermostat recently, the wiring is worth a second look: what each letter and colour does.

Check 2Free, one minute

The switch on the side of the furnace, then the breaker

There is a switch on the furnace cabinet or on a joist beside it that looks exactly like a light switch, because it is one. Decorators, people carrying boxes past it and anybody tidying a basement turn it off.

Then the breaker. A tripped one often does not look off, it sits limply in the middle, so push it firmly all the way off and back on.

Check 3

The front panel

Behind the blower door there is a plunger switch that cuts the furnace dead when the panel is off. The panel has to be seated and clicked properly shut, not leaned into place.

If the furnace died right after somebody changed the filter, this is usually it.

Check 4

The filter, and everything else that feeds it air

Pull the filter and hold it up to a light. If you cannot see light through it, that is your answer.

A furnace starved of air overheats, and its limit switch shuts the gas off to protect the heat exchanger. Do that often enough and the exchanger cracks, which is a new furnace. So open every vent in the house and pull the furniture off the return grille while you are there.

Check 5

Let it out of lockout, once

After a few failed attempts a furnace locks itself out and sits there doing nothing for an hour, or three hours on some Carrier furnaces.

Switch it off at the cabinet switch, wait thirty seconds, switch it back on and watch what it tries to do. Once is a test. Doing it every evening all winter is hiding a fault that is still there.

Read the light it is flashing at you

Count the flashes with the room light off, then read them on the label inside your own door. The same count means a different fault on another make.

Many furnaces have a small window in the front panel with a light behind it. That light is the control board telling you which safety stopped it.

Turn the room light off, watch through the window and count. A pattern that repeats after a pause is the code. Some boards flash a colour instead, and some use two lights.

Then open the door and read the label glued to the inside of it. Do not use a code list from another brand. Four flashes is an open limit on a Goodman or a Lennox and a pressure switch on a Honeywell S9200 board, and that is how people end up buying the wrong part.

What the common ones usually mean

What it is called	What is actually happening
Pressure switch fault	The draught fan cannot prove it is moving air. A blocked flue, a bird nest or snow over the pipes outside, or a blocked condensate drain on a high efficiency furnace
Limit switch open	It got too hot. Nearly always airflow, which means the filter, closed vents or a blocked return
Ignition lockout, or flame sense	It lit and the board could not feel the flame. Usually a coated sensor rod
Rollout switch	Flame came forward out of the burner box. Not a reset job
Low flame signal	The same sensor, still working but on its way out
Reversed polarity, or no ground	The furnace needs the live and neutral the right way round at the outlet. An electrician, and a quick job

If it lights and then quits

The burners fire properly, run about five seconds and the gas shuts off again while the blower keeps going. That is the control board saying it cannot feel the flame, and often it is a rod that has grown a coating: cleaning a furnace flame sensor costs nothing.

If it runs for a few minutes and then blows cold, over and over, that is the overheating one and it starts at the filter. Furnace blowing cold air sorts it by when the air goes cold.

If there is a knob marked OFF, PILOT and ON and no little flame behind the window, nothing else will happen until it is relit: lighting a furnace pilot safely.

The high efficiency ones have one more

If white plastic pipes run out through the wall rather than a metal flue, your furnace makes water while it runs, and that water has to get away.

When the condensate trap or the little pump blocks, a switch stops the burners lighting and you get a pressure or drain code. Pouring a cup of warm water through a blocked trap can be the whole repair, and a pump that has died can be replaced.

Look at the two pipes outside as well. Snow drifted over them, or a nest in the intake, stops the furnace the same way and is free to fix.

Questions people ask

The fan runs but there is no heat at all.

Something stopped the gas side while the blower carried on. Read the code, and check the fan is not simply set to ON at the thermostat.

It worked yesterday and today it is completely dead.

Batteries, the cabinet switch, the breaker, the panel. In that order, and one of them is usually it.

Is it worth resetting the breaker more than once?

No. A breaker that trips again straight away is telling you about a short or a seized motor, and flicking it repeatedly is how a small fault becomes a fire.

My furnace is oil, not gas.

Check the tank first, then the single reset button on the burner. Press it once only. Pressing it repeatedly pumps unburnt oil into the chamber, and that is genuinely dangerous.

How old is too old to repair?

Fifteen to twenty years for a gas furnace. At that age a sensor, an igniter or a switch is still worth doing, and a heat exchanger or a control board usually is not.

Should I have a carbon monoxide alarm?

Yes, one on every floor, EPA says, and it matters more than any check on this page. They are cheap, and they are the only warning you get.

Sources: Trane "Furnace Troubleshooting: 7 Gas Furnace Solutions", "Furnace Light Blinking" and "How Long Should a Furnace Last?", Goodman installation instructions IOG-2009E (*MSS9, *CSS9), Lennox installation instructions 507323-01 (ML180UH), Resideo (Honeywell) S9200U1000 instructions 69-2075EF-09, Carrier 58SC0A/58SC1A installation, start-up, operating and service instructions, Resideo (Honeywell) R7184 oil primary instructions 69-1459-06, and EPA "Where should I place a carbon monoxide detector?"