When a gas furnace won't ignite, it has usually shut itself down on purpose. Before it opens the gas, the furnace checks for proper venting, a hot or sparking igniter, and a flame it can detect. If any step fails, it stops and tries again. A few checks are safe for you. The igniter, burners and gas parts are not.
Safety first: If a CO alarm goes off, leave the home now and call 911 from outside. If you smell gas, leave and, from outside, call 911 and Enbridge Gas's emergency line at 1-866-763-5427. On propane, call 911 and your propane supplier. Don't try to restart the furnace.
What does it look and sound like when a furnace won't ignite?
You'll usually hear the furnace try to start, but no heat follows. A small fan may hum, then nothing. You may hear repeated clicking, or see a flame that lights for a few seconds and goes out. The main blower may then push cool air, and a status light may start flashing. Each of these points to a different step in the startup.
What to watch and listen for
- A hum from the small fan (the inducer) with no flame after it
- Repeated clicking with no flame
- No orange glow through the window in the furnace door, on furnaces that use a glowing igniter
- A flame that lights, then goes out within a few seconds
- Cool air from the vents after a failed start
- A flashing status light
Write down what you notice, in order. It helps whoever looks at the furnace.
How does a gas furnace light, and where can it fail?
A modern gas furnace lights in steps. A small fan starts first to clear the venting, and a switch confirms that air is moving. Then the igniter glows red-hot or sparks, the gas valve opens, and a sensor has a few seconds to detect the flame. If the flame isn't confirmed, the gas shuts off.
Where the startup usually stops
- Fan runs, nothing else happens: the furnace may not be confirming its venting, or it's waiting to try again.
- No glow or spark: the igniter or its power may be the problem.
- Flame lights, then goes out: the furnace may not be detecting the flame.
Many older furnaces use a pilot flame instead of a glowing or sparking igniter. The idea is the same: no confirmed flame, no gas.
Why does the flame light and then go out after a few seconds?
That usually means the furnace lit, but couldn't confirm the flame, so it closed the gas valve as a safety step. A dirty or worn flame sensor, a weak flame or a control problem can all cause it. The furnace will often try again on its own. Fixing the cause means working inside the burner area, which isn't a homeowner job.
What you can do
- Watch one full attempt through the window. Don't open the burner door.
- Note how many seconds the flame stays lit.
- Note whether the blower comes on afterward.
- Leave the furnace off if it keeps repeating, and call.
What can I safely check before I call?
Start with the outside-the-furnace basics. Make sure the thermostat is set to heat and above room temperature, the furnace switch and breaker are on, and the furnace door is closed tight. Many furnaces won't start with the door open. Then check the outside vent pipes and, on propane, the tank gauge.
Checks that matter for ignition
- Vent pipes outside: high-efficiency furnaces vent through plastic pipes on an outside wall. Snow or ice can block them, and in the snowbelt north and west of Barrie that happens after heavy snow or drifting. Brush snow away gently, by hand or with a soft brush. Our guide to furnace vents blocked by snow explains how to find and clear them.
- Propane tank level: many rural homes in Springwater and Oro-Medonte heat with propane. If the gauge reads empty, leave the furnace off and call your supplier. Don't turn the tank valve back on yourself. The Canadian Propane Association recommends having a technician inspect the system before propane is turned back on (industry advice can change, so check the current page).
- Status light: count the flashes before you touch anything. Our furnace blinking light codes guide explains how to read them without erasing them.
For the full thermostat, power and filter checklist, use our furnace not working checklist. If it quit after dark, see what to do when your furnace won't turn on at night.
What should I not do when the furnace won't light?
Don't keep resetting it, don't touch the igniter, and don't try to light the burners yourself. Each reset sends the furnace through another try. That can hide the real problem or make it worse. A hot surface igniter is fragile, and even skin oil can shorten its life. If you smell gas at any point, stop and leave.
Hard no's
- Don't flip the power off and on again and again to force a restart.
- Don't remove the burner door or touch the igniter, flame sensor, gas valve or wiring.
- Don't light a burner with a match or lighter.
- Don't open or adjust the gas valve or gas line.
- Don't bypass or unplug any switch to make the furnace run.
How many times will a furnace try to light before it gives up?
Many furnaces try to light a few times, with a short pause between tries. If none work, the control locks out and stops trying. Some controls try again on their own after a set time, often an hour or more, depending on the furnace. A lockout is a safety feature, not a breakdown in itself. It means the furnace couldn't light safely.
One reset, then stop
- Fix anything obvious first, like a closed furnace door or a snow-blocked vent.
- Reset once by turning the furnace switch off, waiting a minute, then turning it back on.
- If it locks out again, leave it off and call.
When should I call for 24/7 help?
Call when the furnace won't light after your basic checks, keeps locking out, or lights and drops out again and again. Call sooner if it's below freezing and the house is getting cold, or someone in the home is vulnerable to cold. If you smell gas, leave first and call 911 and Enbridge from outside.
Call now if
- The house keeps getting colder in a Barrie cold snap
- The furnace locks out again after one reset
- You hear a bang or boom when it tries to light
- The propane tank ran empty
Our emergency furnace repair page explains what happens when you call. Furnace Repair Barrie answers 24/7 at (249) 209-2216.