Furnace Not Working? 8 Fixes to Try First (2026)
Furnace Not Working? 8 Fixes to Try First (2026)
Aire One East Heating & Cooling, 1910 Dundas St E #117, Whitby
A furnace not working in the middle of a Durham winter is stressful, but the cause is often something simple you can check yourself before calling for service. This guide walks through the free checks to try first, the faults that need a technician, and the gas-safety rules that always come first. If the checks do not solve it, see our furnace repair page.
- Before calling about a furnace not working, check the thermostat, the breaker, the furnace switch, the air filter, and the gas supply.
- A dirty filter is one of the most common causes of a furnace that short cycles or shuts down.
- A furnace that starts then quickly shuts off is often a dirty flame sensor, a job for a technician.
- If you smell gas, leave the house first and call the gas utility from outside; never troubleshoot a gas smell.
- Any suspected cracked heat exchanger or carbon monoxide alarm is an emergency, not a repair to delay.
Short answer: when your furnace is not working, check that the thermostat is set to heat, the breaker and furnace switch are on, the filter is clean, and the gas is on, before calling for service.
If those do not fix it, or you smell gas, call a licensed technician. Aire One East runs 24/7 across Durham Region. Call 905-576-7600.
Furnace not working: the free checks to try first
Most of the time, a furnace not working comes down to one of a few simple things, and all of these are safe to check yourself. Run through them in order before you book a service call.
- Thermostat: confirm it is set to heat, the target is above room temperature, and the batteries are not dead. Wait a few minutes for the furnace to respond.
- Breaker and furnace switch: check the breaker has not tripped, and confirm the furnace power switch, which looks like a light switch near the unit, is on.
- Air filter: a clogged filter starves airflow and can shut a furnace down; if light does not pass through it, replace it.
- Gas supply: confirm the gas valve near the furnace is open, with the handle parallel to the pipe, and that other gas appliances are working.
- Furnace door: the panel must be fully closed, because a safety switch stops the furnace if the door is off.
If none of these fixes a furnace not working, the cause of the furnace not working is likely internal and needs a licensed technician.
Why is my furnace not heating?
When the free checks do not solve a furnace not working, one of these faults is usually the reason. The first three you can handle; the rest need a professional.
| Fault | Typical symptom | Who fixes it |
|---|---|---|
| Wrong thermostat setting or dead batteries | Nothing runs, or the fan runs with no heat | You |
| Tripped breaker or furnace switch off | Completely dead furnace | You once; a pro if it trips again |
| Clogged air filter | Weak heat, short cycling, shutdown | You |
| Pilot light out or faulty igniter | No flame, no heat | You for a pilot; a pro for an igniter |
| Dirty flame sensor | Furnace starts then shuts off in seconds | Pro |
| Blower motor fault | No air movement, humming | Pro |
| Blocked condensate drain (high-efficiency) | Shutdown, water near the unit | Pro |
| Cracked heat exchanger | Shutdown, possible CO risk | Pro, urgent |
Ranked by how often we see them on Durham no-heat calls, verified August 2026.
Why is my furnace blowing cold air?
A furnace running but blowing cold air is a specific version of a furnace not working, and it usually means the burners are not firing while the blower runs. Common causes are the thermostat fan set to on instead of auto, a pilot or igniter problem, a tripped safety from a dirty filter, or an out gas supply. Set the fan to auto, replace the filter, and confirm the gas is on. If it still blows cold, the ignition system likely needs a technician.
Furnace not working: when to call a technician
Some signs mean stop and call rather than keep trying. A furnace not working after the free checks, a unit that starts then shuts off repeatedly, a breaker that trips more than once, grinding or booming noises, a burning or gas smell, or any carbon monoxide concern all point to professional work. Our TSSA-licensed team covers Whitby, Oshawa, Ajax, Pickering, Courtice, Bowmanville, Port Perry, and Uxbridge with 24/7 emergency service, and a seasonal tune-up through our maintenance and protection plans prevents most no-heat calls. If the unit is old and failing often, our guide to how long a furnace lasts in Ontario covers replacement.
Key terms in this article
- Flame sensor: a rod that confirms the burners have lit; when dirty, it shuts the furnace off within seconds of starting.
- Igniter: the part that lights the gas in modern furnaces, replacing the old standing pilot light.
- Heat exchanger: the component that separates combustion gases from your home air; a crack can leak carbon monoxide.
- Short cycling: a furnace that turns on and off rapidly without properly heating the home.
- Furnace switch: a light-switch-style power switch near the furnace that is easy to knock off by accident.
Frequently asked questions
| Your furnace questions, answered |
|---|
| Why is my furnace not working all of a sudden? A furnace not working suddenly is most often a tripped breaker, an off furnace switch, a dead thermostat battery, a clogged filter, or an out gas supply. Check those five first; they solve a large share of no-heat calls without a service visit. |
| Why does my furnace start then shut off? That short-cycling pattern is often a dirty flame sensor, which stops the furnace seconds after the burners light. A clogged filter can also cause it by overheating the unit. The filter you can change; the flame sensor needs a technician. |
| Is a furnace not working an emergency? A no-heat furnace in a deep freeze is urgent, and any gas smell or carbon monoxide alarm is a true emergency: leave and call the gas utility or 911. We run a 24/7 line for no-heat calls across Durham Region. |
| Can I fix a furnace not working myself? You can safely check the thermostat, breaker, furnace switch, filter, and gas valve. Anything involving the burners, igniter, flame sensor, blower, or heat exchanger requires a licensed technician, because gas and combustion work is not a do-it-yourself job. |
| Why is my furnace blowing cold air? Usually the fan is set to on instead of auto, or the burners are not firing due to an ignition fault, a dirty filter, or an out gas supply. Set the fan to auto and change the filter; if it still blows cold, the ignition system needs a professional. |
Do not take our word for it alone. Read our 364 verified reviews on Google, and see the Heating, Refrigeration and Air Conditioning Institute of Canada for what accredited contractors are expected to deliver.
