Furnace clicks but won't ignite? Here is how to find the cause

What should happen when a furnace starts up

Knowing the normal sequence is what lets you turn what you hear into a diagnosis, because a furnace that will not light has stopped at one of these steps. In order, a modern gas furnace does this on a call for heat:

• The thermostat calls for heat and signals the control board.
• The draft inducer, a small fan, spins up first and runs for a short purge to clear the heat exchanger and prove the vent is open.
• The pressure switch senses that draft and closes. The board will not move on until it does.
• The igniter energizes. On most furnaces a ceramic hot surface igniter glows bright orange; on older spark units it clicks rapidly instead.
• The gas valve opens and the burners light.
• The flame sensor, a thin metal rod in the flame, confirms a real flame within a couple of seconds, or the board shuts the gas right back off.
• Only after flame is proven does the blower start and push warm air.

The click you hear is usually the gas valve or its relay on the board trying to open the gas. So a click followed by no flame means the furnace got all the way to the gas step and combustion failed. That narrows it to three things: a bad igniter, no gas reaching the burners, or a flame the sensor cannot confirm. The rest of this page walks each one.

Furnace clicks but won't ignite: is it the igniter?

A cracked or worn hot surface igniter is the single most likely cause of a furnace that will not light, because it is the part that does physical work on every heat call and it is fragile. It is a ceramic element that current heats until it glows bright orange to light the gas. Over years of heating and cooling it develops hairline cracks or simply burns out, and once it does, the gas has nothing to light it.

The check is visual and free. With a heat call active, look through the burner inspection window in the first several seconds of the sequence and watch for the igniter to glow bright orange. No glow at all points at the igniter or its circuit. A normal bright glow shifts suspicion downstream to the gas. Common signs on a bad element are visible cracks, white marks, or black spots, though the crack is often too fine to see.

The igniter is an inexpensive part, and for a handy owner it is a quick swap once power is off, but there is one rule that matters more than any other: never touch the element with your bare fingers. The oil from your skin creates a hot spot that cracks the new igniter, sometimes within days. Handle it only by the ceramic base. If your furnace is still under warranty, check before you do it yourself, since a DIY part swap can void coverage. If you are not comfortable, this is a cheap tech repair.

Furnace lights then shuts off after a few seconds: the flame sensor

If the burners light, run for a few seconds, then go out, and the furnace retries and does it again, the flame sensor is the most likely culprit, and this is the cheapest real fix on the list. The flame sensor is a thin metal rod that sits in the flame and proves to the board that a real fire is burning. When a film of soot or oxidation builds up on the rod, it can no longer pass the tiny current the board reads, so the board assumes there is no flame and shuts the gas off as a safety. The burner lights, dies in about three to ten seconds, retries, and eventually locks out.

The fix is usually cleaning, not replacing, and it solves this the large majority of the time. Cut power to the furnace first at the switch or breaker, this is non-negotiable. Find the thin metal rod near the burners, held by a single screw. Gently scuff it with fine emery cloth or fine steel wool until the metal is bright and bare, then wipe it with a dry cloth and reinstall it. The goal is to remove the gray-black film, not to grind the rod down, so go light. It takes about ten to fifteen minutes. If the burner now stays lit past ten seconds, you fixed it. If you are cleaning it more than once a season, or the rod is cracked or pitted, replace it, which is also a cheap part.

Igniter glows but the burners never light: check the gas

If the igniter glows bright and full but no flame ever appears, the gas is not reaching or not lighting at the burners. Some of this you can check safely, and some of it is a pro repair. The look-only checks:

• The gas valve at the furnace. The handle on the supply pipe near the unit is open when it runs parallel to the pipe and closed when it sits crosswise. This is the one gas item worth checking yourself, by looking, not by taking anything apart.
• Other gas appliances. If the stove and the gas water heater also will not run, the problem is the gas supply or the utility, not the furnace. If only the furnace is out, gas to the house is probably fine.
• Propane tank. On a propane home, an empty or near-empty tank is a very common winter no-heat cause, and a regulator can ice over in hard cold. Check the tank gauge.

If gas is on at the house and the igniter glows but the burners still will not light, the gas valve itself may not be opening. That is a pro repair, because the valve is the gas-control safety component and diagnosing or replacing it is not a homeowner job. Do not take the valve apart or try to adjust gas pressure.

The inducer runs but nothing else happens: pressure switch and vent

If the small inducer fan spins but the furnace never advances to the igniter, the pressure switch is not closing, and the most common reason a homeowner can fix is a blocked vent. The pressure switch is a safety that proves the inducer is actually moving exhaust before any gas flows. If it stays open, the board blocks ignition entirely.

On a high-efficiency furnace, the intake and exhaust are usually two white PVC pipes that end on an outside wall or the roof, and they get plugged by snow drifts, ice, leaves, or a bird or insect nest. Clearing snow and debris off the outside vent terminations is the one venting fix you should do yourself, and it is a real seasonal cause worth checking on the first cold snap. An older furnace vents through a metal flue instead; check it is not crushed or blocked by a nest. The inside causes, a cracked or disconnected pressure hose or a failed switch, are pro territory, and one rule is absolute: never bypass or jumper a pressure switch. Defeating it can let the furnace fire while it is venting exhaust into your house, which is a carbon monoxide risk.

Check the filter and the easy stuff first

Before you suspect a part, rule out the free causes, because a couple of them masquerade as ignition failure. Set the thermostat to heat and a few degrees above room temperature, and put fresh batteries in if it has them. Confirm the furnace power switch, the unlabeled switch that looks like a light switch on or beside the unit, is on, and that the breaker has not tripped. Make sure the front panel is fully closed, because a door safety switch behind it has to be pressed in or the furnace will not run at all, which trips people right after a filter change.

Then check the filter itself. A badly clogged filter starves airflow, the heat exchanger overheats, and a safety limit shuts the burners off, which reads like an ignition fault but is really airflow. Hold the filter up to a light, and if none passes through, replace it with the same size, arrow pointing toward the furnace. If the furnace keeps overheating and shutting off, do not just keep restarting it; fix the airflow first, because repeated overheat cycling stresses the heat exchanger. For the full schedule on this, see the guide on how often to change a furnace filter. On a high-efficiency furnace, also glance for standing water around the base, since a clogged condensate drain can trip a float switch and block ignition the same way.

How to reset a furnace in ignition lockout

Most modern furnaces try to light about three times, then go into a hard lockout for roughly an hour to keep unburned gas from building up. During lockout the furnace may look dead or just run the inducer. You can force a reset two ways: set the thermostat to off, or well below room temperature, for about a minute and then back to heat; or flip the furnace power switch off, wait a minute, and turn it back on.

One reset is fine to confirm the diagnosis. But if the furnace lights and runs after a reset, you have not necessarily fixed anything, and if it locks out again right away, stop resetting it. A lockout that returns immediately means the board is catching a real fault, not a glitch, and repeatedly forcing ignition can dump raw gas into the chamber. Note any blinking code on the small light behind the access panel, which usually identifies the fault, and call a tech.

Is this the same as a furnace that won't turn on?

Not quite, and matching your symptom to the right page saves time. This page is the furnace that clearly has power and is actively trying to light but cannot. A few close cousins have their own pages:

• If the furnace is totally dead, no display, no hum, no inducer, that is a power-side problem covered in furnace not turning on.
• If it lights and runs but keeps cycling off and on repeatedly, that is furnace short cycling.
• If it has an older standing pilot flame that will not stay lit, that is the thermocouple, covered in furnace pilot light keeps going out.

What furnace ignition repairs cost

Two of the common causes are cheap, and the rest climb with the part. These are installed ranges, parts plus labor, and regional labor moves them.

The flame sensor and igniter are cheap enough that age rarely changes the call; fix those even on an old furnace. But if a tech quotes an expensive part like a board, gas valve, or inducer on a furnace that is fifteen to twenty years old, get a replacement quote before you commit, because you can spend a third of a new furnace patching an old one that likely has more failures coming. The furnace replacement guide walks that repair-versus-replace math.

When to stop and call for help

Because this is a gas appliance, a few lines are not worth crossing. The most important one: if you smell gas, a rotten-egg or sulfur odor, do not flip any switch, do not search for the source, and do not use a phone inside the house, since any spark can ignite gas. Get everyone out and call your gas company's emergency line or 911 from outside. Never keep hitting reset or forcing ignition if you smell gas, because each failed attempt dumps more gas into the chamber.

Call a pro, without the drama, for anything involving the gas valve or gas line, a pressure switch that will not close after you have cleared the vent and condensate, a control board fault, a breaker that re-trips the instant you reset it, or repeated lockouts after your one reset. And never bypass any safety switch, the pressure switch, the high-limit, or the door switch, because they exist to stop the burners firing without proper draft, and defeating one risks carbon monoxide and fire. A cracked heat exchanger can leak carbon monoxide, which is odorless, so a working CO detector on every floor with a bedroom is non-negotiable in a gas-heated home. If a CO alarm sounds, get out and call from outside.