Why won't my furnace ignite?
A furnace won't ignite because one startup step has failed, and where it stops points to the part: no inducer means power or the thermostat, inducer without igniter glow means a blocked vent or condensate trap, glow without flame means gas or the gas valve, and flame that drops out means the flame sensor.
Key points
- A gas furnace starts in the same order every time. Find the step where it stops and you've narrowed the cause.
- No inducer at all: check the thermostat, the service switch, the breaker and that the blower door is seated.
- Inducer runs, no igniter glow: blocked PVC vents, a frozen or plugged condensate trap, or a bad pressure switch.
- Igniter glows, no flame: gas turned off, propane tank empty, a failed gas valve or a weak igniter.
- Reset once. If it locks out again, stop and call, and bring the blink code.
When a furnace won't ignite, one step of its startup sequence has failed, and the step where it stops points to the part. No inducer hum means power, the thermostat or the door switch. An inducer that runs without the igniter glowing usually means a blocked vent or a plugged condensate trap. A glowing igniter with no flame means gas isn't arriving. Flame that lights and drops out is almost always the flame sensor. Watch one startup, note where it quits, reset once, and call if it locks out again.
The startup sequence, in order
A modern gas furnace lights the same way every time:
- The thermostat calls for heat.
- The control board starts the inducer motor, a small fan that pulls air through the heat exchanger and out the vent.
- The pressure switch senses that draft and closes.
- The hot surface igniter heats until it glows bright orange. (Some furnaces use a spark or pilot instead.)
- The gas valve opens and the burners light.
- The flame sensor proves there's a flame.
- After a short delay, the blower starts sending heat through the house.
If any step fails, the board stops the sequence and usually flashes a code. So troubleshooting comes down to watching the startup and noting where it quits. Set the thermostat a few degrees above room temperature and watch through the small window in the furnace door, or listen closely from beside it. Leave the doors on. Many furnaces won't run with them off, and the doors are part of how the furnace draws combustion air safely.
| Where it stops | Likely causes | What you can check | Typical national repair cost (HomeGuide) |
|---|---|---|---|
| Nothing: no inducer, no board light | Thermostat, furnace switch, breaker, door switch or a board fuse | Settings and batteries, the switch, the breaker, the lower door | Often no part; a control board is $300–$650 if it failed |
| Inducer runs, igniter never glows | Blocked PVC vent, plugged or frozen condensate trap, pressure switch or hose, weak inducer | Snow on the vents, the drain line | Pressure switch $150–$400; inducer motor $400–$1,100 |
| Igniter glows, no flame | Gas off, empty propane tank, weak igniter or gas valve | The tank gauge and the gas handle | Igniter $100–$250; gas valve $200–$800 |
| Lights, then drops out | Dirty flame sensor, poor ground or low gas pressure | One reset | Flame sensor $80–$250 |
What does it mean when the furnace does nothing at all?
No inducer hum and no board light means the furnace isn't getting a call or isn't getting power.
- Thermostat. Mode on Heat, setpoint above room temperature, fresh batteries. A blank screen is your answer.
- Service switch. Every furnace has a light-switch-style disconnect on or near it, often on a joist or the stairway wall. It gets bumped off more than you'd think.
- Breaker. Check the furnace circuit in the panel. If it trips again right away, leave it off and call.
- Door switch. A plunger switch behind the lower door cuts power when the panel is off or not seated. Push the door firmly into place.
- Board fuse. Many control boards carry a small automotive-style fuse, commonly 3 or 5 amps, that blows when a thermostat wire shorts. It often happens right after a new thermostat goes in. If you've just installed a smart thermostat, our post on the thermostat C-wire covers the common wiring mistakes.
Why does the furnace inducer run but the igniter never glow?
This is the most common no-heat pattern on high-efficiency furnaces in a Minnesota cold snap. The inducer spins, but the pressure switch never closes, so the board won't go further.
- Blocked vent pipes. The white PVC intake and exhaust outside can drift shut with snow or grow a frost cap on the exhaust in deep cold. Clear snow by hand, not with a shovel, which can crack the pipe. Our guide to clearing a frozen furnace vent covers the safe steps.
- Condensate trap. A plugged or frozen trap backs water up into the inducer housing, and the pressure switch won't close. A gurgle at startup is a hint. If the drain line runs through a garage or out a wall, our post on a frozen furnace condensate line shows how to thaw it without cracking the pipe.
- Hose or switch. The small rubber hose to the pressure switch can crack or fill with water. The switch itself can fail too, but it's often replaced when the real problem was the vent or trap.
- Weak inducer. A worn inducer can spin without pulling enough draft.
Why does the furnace igniter glow but the burners never light?
If the igniter lights up and the burners never catch, the gas isn't arriving or the igniter isn't hot enough.
- Gas turned off. Look at the handle on the gas line to the furnace. Parallel to the pipe is on, across the pipe is off. If it's off, don't reopen it yourself; someone may have shut it for a reason, so call.
- Empty propane tank. Check the gauge. After a tank runs dry, the supplier normally leak-checks the system before relighting, and our post on propane heating costs covers tank basics.
- Weak or cracked igniter. An aging igniter can glow dull orange without reaching lighting temperature. Look for a hairline crack or a white spot.
- Gas valve. If gas is on and the igniter is strong, the valve may not be opening.
Don't touch a hot surface igniter, even cold. Skin oil shortens its life.
Why does a furnace light and then shut off after a few seconds?
Burners that light for a few seconds and shut off, then try again, almost always point to the flame sensor. It's a thin metal rod in the flame, and a coating on it keeps the board from reading the flame. Our guide on cleaning a flame sensor walks through it. If a clean sensor doesn't fix it, the cause is usually a poor ground, low gas pressure or the board.
Why fall startups and cold snaps bring these calls
Two stretches of the year produce most no-ignition calls here. The first is the first cold week of fall, when an igniter or flame sensor that sat idle since spring gets its first real test. A weak igniter that lit fine in April can fail on the first 20°F night. Our guide to turning the furnace on for the first time this fall shows how to find that out on a mild afternoon instead.
The second is a deep cold snap. A high-efficiency furnace runs long cycles and makes a lot of condensate, so a drain line that crosses cold space can freeze and back water into the pressure switch. Drifting snow and frost close off PVC vents, usually on the windward side of the house. Propane use climbs with the cold, and a tank that ran lower than expected leaves the furnace nothing to light. All three look like a failed part and aren't.
Read the code, reset once, then call
Count the blinks on the control board light (usually visible through a small window in the lower door) and match them to the chart inside the door. Then turn the furnace off at the service switch for about a minute and back on. One reset is fine. If it locks out again, leave it off. Repeated resets hide the fault, and a furnace tripping a rollout or limit switch shouldn't keep firing.
If you smell gas at any point, leave the house and call your gas utility from outside. If a carbon monoxide alarm sounds, get everyone outside and call 911.
When you call, tell us where the sequence stopped and what the code was, and we'll bring the likely parts. A furnace repair visit is a $79 trip charge, waived when you approve the repair, and furnace repair in Saint Paul is priced the same as everywhere else we work. Our furnace repair cost guide covers what the common fixes run, and if the furnace is old enough that repair doesn't make sense, our furnace replacement cost guide explains the numbers. If the house is cooling off while you wait, our no-heat checklist covers keeping the pipes safe.
Questions
Why does my furnace click but not light?
The clicking is usually relays on the control board or the gas valve trying to open during the startup sequence. If you hear clicks but never see the igniter glow and the burners light, the sequence is stopping at the pressure switch or the igniter. If it lights and then goes out, the flame sensor is the usual cause.
How many times should I reset a furnace that won't light?
Once. Turn it off at the service switch for about a minute and turn it back on. Many control boards try to light a few times, then lock out, and some retry on their own after a while. If it fails again after one reset, leave it off and call. Repeated resets hide the fault instead of fixing it.
What does it cost to fix a furnace that won't ignite?
Most no-ignition repairs come down to one part. National cost guides such as HomeGuide put an igniter at $100–$250, a flame sensor at $80–$250 and a pressure switch at $150–$400, with an inducer motor or gas valve costing more. With us, a service call is a $79 trip charge, waived when you approve the repair, and the price is written down before work starts.
What should I tell the technician when I call?
Tell them where the startup stops: nothing happens, the inducer runs with no glow, it glows with no flame, or it lights then goes out. Add the blink code from the control board light, your fuel (natural gas or propane) and the furnace brand. That helps the technician arrive with the right parts.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com