Why Is My Furnace Not Igniting? No Heat, Diagnosed in Order

Why is my furnace running but not igniting?

It means the blower or the draft inducer is spinning, so you hear the furnace running, but the burners never light and no heat is produced. A modern gas furnace lights in a fixed sequence: the thermostat calls for heat, the draft inducer motor starts, the pressure switch proves the furnace is venting, the hot surface ignitor glows red, the gas valve opens, the burners light, and only then does the flame sensor confirm a flame and the main blower kick on. If anything in that chain fails before the burners catch, you get the sound of a running motor with cold air or no air, and the furnace shuts down on safety.

Knowing the sequence tells you where to look. If you never hear the inducer spin up, the problem is early: thermostat, power, or a safety switch stopping the whole cycle. If you hear the inducer run and the ignitor glow but the burners never catch, the problem is gas, the ignitor, or the flame sensor. Almost no residential furnace built since the 1990s has a standing pilot light, so if yours has electronic ignition there is no pilot to relight; a furnace old enough to have a standing pilot should be relit only by its printed instructions, and if you are unsure, call a pro.

What causes a furnace to fail to ignite?

Start with the free checks, because they solve a large share of no-ignition calls. Confirm the thermostat is set to HEAT and a few degrees above room temperature, and swap in fresh batteries, since a thermostat with dead batteries or a loose wire simply never sends the call for heat. Then check power: most furnaces have a switch that looks like a light switch on or near the unit, and it gets flipped off by accident or by someone who forgot they turned it off. Reset the furnace breaker if it is tripped, and make sure the blower access door is fully seated, because a door safety switch will keep the furnace from running at all if the panel is loose.

If power and the thermostat check out, airflow and the ignition parts are next. A dirty air filter is a leading cause: it starves the blower, the furnace overheats, and the high-limit switch locks the burners out on safety. Replace a clogged filter and confirm it faces the right way (our guide on which way a furnace filter goes covers reading the arrow, and the filter-change frequency guide gives the interval). After that come the two most common failed parts: a hot surface ignitor that has cracked or aged so it no longer glows hot enough to light the gas (see furnace ignitor replacement), and a dirty flame sensor that makes the furnace light and then shut off within seconds because it cannot prove the flame (see the furnace flame sensor guide).

The remaining causes sit on the gas and combustion side and are a pro's job to diagnose. No gas is a common and simple one: a closed gas shutoff, an empty LP tank, or a utility outage will stop ignition, so check that other gas appliances in the house still work. Beyond that, a blocked flue or a weak draft inducer motor trips the pressure switch and halts the cycle; on a high-efficiency furnace a clogged condensate drain trips a float switch and does the same (the same blockage that shows up in our furnace leaking water guide); a bad gas valve means the ignitor glows but the burners never light; and a failing furnace control board can scramble the whole sequence. Read the flashing fault code on the board through the sight glass and write it down before you call anyone.

Why is my furnace fan running but blowing cold air or no heat?

If the blower runs but the air stays cold, either the fan is set to run on its own or the burners are not staying lit while the blower keeps circulating room-temperature air. The single most common and harmless cause is the thermostat fan setting left on ON instead of AUTO: on ON the blower runs constantly and pushes unheated air between heating cycles, which feels like cold air from the vents. Switch the fan to AUTO and it will only run when the furnace is actually making heat. That one setting explains a large share of furnace fan will not stop running complaints.

The other bucket is a failed ignition that keeps trying. When the furnace lights and then loses flame, or never lights at all, the control board still runs the blower to purge the heat exchanger, so you get a stretch of cold air before the furnace locks out. That points back to a dirty flame sensor or a weak ignitor from the section above. A separate case is a tripped high-limit switch: when a dirty filter overheats the furnace, the burners shut off but the blower keeps running continuously to cool the metal, which is why a fan that will not shut off so often traces back to airflow. If the fan runs nonstop with the thermostat on AUTO and the filter is clean, that is a limit or control issue for a tech, and it can shade into short cycling if the burners fire and quit in a loop.

How do I reset a furnace that will not start?

The safe reset is a power cycle plus clearing the obvious blockers, not forcing the gas. Turn the thermostat all the way down, flip the furnace switch (or its breaker) off, wait 30 to 60 seconds, turn it back on, then set the thermostat to HEAT a few degrees above the room. A clean power cycle clears a soft lockout, which is the furnace's own safety timer, and gets a healthy furnace to run through its ignition sequence again. While the power is off, replace a dirty filter, confirm the blower access door is fully closed, and check that the gas shutoff at the furnace is open (the handle runs in line with the pipe when open).

There is a hard stop built into every modern furnace, and you should respect it. After about three failed ignition attempts the control board locks the furnace out for roughly an hour on purpose, and repeatedly resetting power to force another try pushes unburned gas into the heat exchanger, which is exactly what the lockout exists to prevent. Give it one clean reset. If it fires and holds, you are done. If it lights and quits, or never lights, or locks out again, stop resetting and book a diagnostic, because the cause is now one of the gas or combustion faults that need a meter and a trained eye. If you smell gas at any point, do not touch a switch: leave the house and call your gas utility or 911.

Furnace won't ignite: what is DIY, and what does the repair cost?

The owner-safe checks are all free or nearly free. Setting the thermostat, replacing the batteries, changing the filter, resetting the switch or breaker, seating the blower door, and confirming the gas is on cost between nothing and about $20, and they fix most no-ignition calls. Everything past that needs a technician, and a standard diagnostic service call runs $75 to $200, usually credited toward the repair if you go ahead.

The common parts repairs, from cheapest to most: cleaning or replacing a flame sensor runs about $80 to $250, a hot surface ignitor $150 to $400, a pressure switch $100 to $400, and a control board $300 to $700. A gas valve is several hundred dollars installed and is licensed-pro work, as covered in our gas valve guide. Prices vary by furnace brand, part access, and your local labor rate, so treat these as planning ranges, not quotes.

If the furnace is old and the repair is a large fraction of a replacement, weigh fixing it against replacing it. A furnace past 15 to 20 years facing a control board or repeated combustion faults is often not worth another expensive repair. Before you replace it, compare a modern furnace against a heat pump, and size the replacement by the home's real heating load with the BTU calculator rather than copying the old nameplate, because an oversized furnace short cycles and wears out early.

When should you call a pro for a furnace that won't ignite?

Call a pro the moment the free checks do not fire the furnace, when it lights and quits over and over, or any time gas or carbon monoxide is involved. The line is simple: thermostat, filter, power, blower door, and gas shutoff are yours; the ignitor, flame sensor, gas valve, pressure switch, inducer, control board, and heat exchanger are a technician's. A tech reads the board's flashing fault code, meters the ignitor and flame sensor, checks gas pressure, and inspects the heat exchanger in one visit, and handing them the exact symptom (never lights, versus lights then quits after a few seconds, versus locks out with a specific blink code) shortens the job and sometimes changes the answer.

Two situations are safety, not convenience. If you smell gas, leave the house and call your gas utility or 911 before you call an HVAC company, and if a carbon monoxide detector alarms or anyone feels headachy, dizzy, or nauseous when the furnace runs, get out and get air first. A furnace that keeps failing to light and locking out is telling you a part has failed, and the cheapest outcome is catching it as an $80 flame sensor or a $150 ignitor at the annual tune-up, not as a no-heat emergency on the coldest night of the year.

Frequently asked questions