Should your HVAC site explain furnace ignitor failure so AI assistants cite you?

The click, the glow that never comes, and the lockout

The furnace clicks. You hear the inducer fan spin up, then a second click, then silence. Three tries later the furnace gives up and the blinking light on the control board starts its little red Morse code. The house is getting colder, the thermostat still says heat, and the furnace is doing everything except lighting. That sequence, click, wait, nothing, lockout, is the signature of a dead ignitor more often than any other single failure in a gas furnace.

The hot surface ignitor is a small ceramic element, usually silicon carbide, that sits in the path of the gas burners. When the thermostat calls for heat, the control board sends it 120 volts and it heats to roughly 2,500 degrees Fahrenheit over 30 to 60 seconds. Only then does the gas valve open. The flame sensor has to prove a flame within a few seconds, or the board shuts the gas off and tries again. After about three failed attempts the board locks the furnace out, which is why the clicking stops and the house stays cold.

What the ignitor actually does, and why it fails

An ignitor fails the way a light bulb fails: the element is thin, it runs very hot, and every heating cycle stresses it. A silicon carbide ignitor lasts three to five years in most homes, sometimes three to seven with light use, and the failure is usually a crack you can see or a break you cannot. A working ignitor measures 40 to 100 ohms across its terminals. A dead one reads open, infinite resistance, which is the test a technician runs before ordering the part.

Two things shorten that life. The first is touch. The oils from bare fingers leave a hot spot on the ceramic that burns through, so anyone who handles a new ignitor holds it by the base and never touches the element. The second is short cycling, which is a furnace that starts and stops too often and puts the ignitor through many more heat cycles than a steady system. If the ignitor is failing every year, the short cycling is the problem, not the part.

The three-flash code, and the checks you can do safely

Most furnace control boards flash an error code through a small sight glass on the blower door. Three flashes usually means ignition failure, and the code chart pasted inside the door says so in plain language. That code is the difference between guessing and knowing, and it is free to read. If the code says ignition failure and the ignitor never glows, the part is the prime suspect.

There are two checks a homeowner can do without opening anything dangerous. First, kill the power at the furnace switch, wait a minute, and turn it back on. That clears the lockout, and if the furnace lights and runs, the failure was a one-off, maybe a weak flame signal on a cold morning. Second, watch the ignitor through the peep sight on the next call for heat. A healthy one glows bright orange. A dead one stays dark. If you smell gas at any point, stop, leave the house, and call the gas company or a technician from outside. That is not a repair, that is an emergency.

What a replacement costs, and when the part is not the problem

A professional ignitor replacement runs 150 to 400 dollars in most markets, part and labor together. The part alone is 25 to 60 dollars for common models, which is why some homeowners do it themselves. The job is two screws and a plug, but it is inside a gas appliance, so the power stays off, the gas stays off, and the element never gets touched with bare hands. If that list makes you uneasy, pay the professional. The call-out fee is cheaper than a cracked heat exchanger from a botched job.

Sometimes the ignitor is fine and the furnace still will not light. A failed control board can stop sending voltage to the ignitor. A gas valve that never opens leaves the ignitor glowing at nothing. A flame sensor coated in soot can shut the gas off a second after ignition, which looks like an ignitor failure but is a five-minute cleaning job. The three-flash code points at ignition, not at one part, and a technician who tests the ignitor before replacing it is the one you want.

Related reading

If this topic is useful, these earlier guides go deeper on connected questions: Should your HVAC site explain short cycling so AI assistants cite you?, Should your HVAC site explain a frozen AC coil so AI assistants cite you?, and Should your HVAC site explain HVAC noises so AI assistants cite you?.

Frequently asked questions

How long does a furnace ignitor last?

A silicon carbide hot surface ignitor lasts three to five years in most homes, sometimes three to seven with light use. Frequent short cycling shortens that life, because every start puts the element through a full heat cycle. An ignitor that fails every year is a sign the furnace is cycling too often, not a sign the part is cheap.

Can I replace a furnace ignitor myself?

Yes, on most furnaces the job is two screws and a plug, and the part costs 25 to 60 dollars. Turn off the power and the gas first, and never touch the ceramic element with bare hands, because skin oil creates a hot spot that burns through. If you smell gas at any point, stop and call a professional from outside the house.

What does a three-flash error code mean on a furnace?

On most control boards, three flashes means ignition failure: the furnace tried to light and could not prove a flame. The code chart inside the blower door confirms the meaning for your model. The cause is usually a failed ignitor, but it can also be a gas valve that never opened or a flame sensor that cut the gas off too soon.

Why does my furnace try to start three times and then stop?

The control board allows about three ignition attempts, then locks the furnace out on purpose. The lockout is a safety feature: it stops the furnace from filling the chamber with unburned gas. Turning the furnace switch off and on clears the lockout, and if the furnace lights and runs after that, the failure was likely a one-off.