Furnace Not Igniting: Which Igniter You Have and Why

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

Furnace Not Igniting: Which Igniter You Have and Why

Short answer

Three different things light a furnace and they fail in three different ways. Find out whether yours has a standing pilot, a ceramic bar that glows orange, or electrodes that click and spark. Then watch one cycle and note how far it gets, because where it stops names the part.

A furnace not igniting is a fault in one particular part of the sequence, and which part depends on how your furnace lights in the first place. There are three ways, and they fail differently.

So before anything else, find out which one you have. It takes one look through the sight window.

Before you start

What to carry: a torch, a screwdriver and a multimeter if you have one. Twenty minutes to find the fault, another fifteen to change an igniter.

Gas or electric? An electric furnace has no igniter at all. It brings heating elements in one at a time with sequencers, and if yours has a thick cable and no flue, nothing on this page applies. A gas furnace has a gas pipe into it and a flue out of the top, and that is the one that has to light something.

Does the house have gas at all? Light a ring on the gas stove. If that lights, gas is reaching the house, and that rules out the meter, the supply and an empty propane tank in one second.

Three rules before the panel comes off. If you smell gas, leave it. Out of the house first, no switches and no lights, then the gas company from outside.

Power off at the switch on the furnace and at the breaker before you touch anything inside. There is 120 volts and a gas valve behind that door.

And never handle the glowing part of a hot surface igniter with bare fingers, even a brand new one. The oil off your skin bakes into a hot spot and takes years off it. Hold it by the metal bracket.

Which igniter do you have?

Three ways a furnace lights. Only the first one has anything for you to light by hand.

Check 1Free, ten seconds

Look through the sight window with the room light off

A small flame burning all the time, and a knob marked OFF, PILOT and ON. That is a standing pilot, and it is an older furnace. If the flame is out, nothing else will happen until it is relit: lighting a furnace pilot safely.

A small bar that glows bright orange for about twenty seconds before the gas comes on. That is a hot surface igniter, and it is what most furnaces built since the nineties use. It is also a part that wears out.

A rapid clicking and a spark you can see. That is spark ignition, either straight onto the burner or onto a small pilot that then lights the burners.

Watch one cycle and note where it stops

Turn the thermostat up five degrees and stand at the furnace. A healthy sequence goes: a fan hums for fifteen to thirty seconds, something glows or sparks, the burners light with a soft whump, and the big blower starts about half a minute later.

Check 2

Find the last thing that happened

Nothing at all, not even a fan. The furnace is not being asked, or it has no power. That is a different page: the five free checks, then the blinking code.

The fan runs and then everything stops. The furnace could not prove it is moving exhaust. A blocked flue, a nest or snow over the pipes outside, or a blocked condensate drain on a high efficiency furnace.

The igniter never glows or never sparks. The igniter itself, its plug, or the board output that feeds it. The next section is yours.

It glows properly and no gas arrives. The igniter is fine. That is the gas valve, the gas supply, or the board.

It lights and dies after about five seconds. That is not ignition at all, it is the board failing to feel the flame: cleaning a furnace flame sensor.

The hot surface igniter

Three checks on one part. Two of them are free and the third needs a meter.

Check 3

Look at it, then read it with a meter

Take it out, or at least get a torch on it. A hairline crack across the element, a white blistered patch or a broken corner all mean it is finished, and a cracked one often still looks intact until you turn it to the light.

With the power off, unplug it and put a meter across the two leads on the igniter side. The older thick ones read roughly 40 to 90 ohms. No reading at all means it has gone open circuit and will never glow again.

The newer thin flat ones do not read the same way, so if yours is one of those, go by the figure on the part or in the manual rather than a number from a forum.

Check 4

Changing it, which is a fifteen minute job

Power off at the switch and the breaker. The igniter sits in a bracket beside the burners held by one or two screws, with a plug on the end of its wires.

Take a photo before anything moves so the new one goes back at the same angle and the same depth. That position matters, because it has to sit in the gas stream.

Unplug it, unscrew the bracket, fit the new one holding it only by the bracket, plug it in, put the panel back and clicked properly shut. Then run a full cycle and watch it.

Spark ignition

If yours clicks and sparks, the part to look at is a thin electrode sitting beside the burner with a white ceramic insulator on it.

Look for a cracked or sooty insulator, which lets the spark go to ground where it should not, and for a bent electrode that has drifted away from where the gas comes out. Clean it gently with emery cloth if it is dirty.

The gap between the electrode and the burner matters and it is set by the maker, so check the figure in its installation manual rather than guessing at it. Too wide and there is no spark, too close and it will not light the gas.

A loud healthy click with no visible spark usually means the electrode or its wire. No click at all points at the board.

When the igniter is fine and it still will not light

The gas valve at the furnace. There is a lever or a small handle on the gas pipe, and in line with the pipe means open. People turn these off for work in the summer and forget.

Lockout. After three to five failed attempts the board gives up and sits there for an hour, or three hours on some Carrier furnaces. Switch it off at the furnace switch for thirty seconds and back on to release it. Once is a test. Doing it every evening is hiding a fault.

The condensate trap on a furnace with white plastic pipes going outside. When it blocks, a switch stops the burners lighting and the furnace behaves exactly like an ignition fault. A cup of warm water through it is often the whole repair.

The gas valve itself, or the board. These are the expensive ends of it, and both are worth a second opinion before spending. On a furnace past fifteen years, the age at which ENERGY STAR says to consider replacing one, a control board is often the moment to think about the whole machine.

Questions people ask

How long does a hot surface igniter last?

It varies a great deal. They wear out from being switched on and off, so a furnace that short cycles gets through them faster: why a furnace short cycles.

It glows but the glow looks weak.

A dull or patchy glow is an igniter on its way out. It will still light the gas for a while and then start missing, usually on the coldest night of the year.

Can I clean a hot surface igniter?

No. It is not a sensor, it is a heating element, and rubbing it is how it gets cracked. The flame sensor is the one that gets cleaned, and they sit close together, which is why people mix them up.

Is it worth carrying a spare?

If your furnace is more than ten years old, yes. They are cheap, they fail without warning, and the call out on a Sunday in January costs a great deal more.

The furnace lights sometimes and not others.

That pattern is usually an igniter that is almost gone, or a flame sensor that is coated. Both are cheap, and the igniter is the one to suspect if the failures happen before any gas arrives.

Does a new igniter need the burners cleaned as well?

Not necessarily, but if the old one failed early it is worth a look. Rust or dust across the burner ports makes the gas slow to reach the igniter and shortens the life of the next one.

Sources: Carrier 58SC0A/58SC1A installation, start-up, operating and service instructions and Carrier troubleshooting guide 58-3T (igniter reading 40 to 90 ohms), Lennox installation instructions 507323-01 for the ML180UH, Goodman installation instructions for the *M9S80 and IOG-2009E (*MSS9, *CSS9), Resideo (Honeywell) S9200U1000 instructions 69-2075EF-09, Rheem installation instructions 92-24161-124-12 (spark gap), Trane "Furnace Not Igniting" and the ENERGY STAR "Repair or Replace?" guide