When a furnace is not turning on, check five things first: the thermostat, the furnace power switch and blower door, the breaker, the filter and the error light. If those don’t solve it, the fault is inside the furnace, and in Ontario and Quebec only a certified gas technician may repair or service it.
Get Your Furnace FixedOne liner: Do the five safe checks, record the error code, then call a certified gas technician for anything inside the furnace.
| Stat | What it measures | Source |
| G2 | Minimum gas technician certificate for working on gas-fired appliances without supervision in Ontario | TSSA |
| 1 reset | Breaker resets to try before calling for service | Bryant Canada |
| 1 to 3 hours | How long some furnace lockouts last before the control tries again on its own | Bryant installation instructions |
| Monthly | How often to inspect the furnace filter during the heating season | Carrier user’s manual |
| Once a year | Furnace and vent inspection by a qualified technician | Carrier user’s manual |
| January 1, 2026 | Date Ontario began requiring CO alarms on every storey of homes with a fuel-burning appliance | Government of Ontario |
| 1-866-763-5427 | Enbridge Gas 24-hour emergency line in Ontario | Enbridge Gas |
| 1 800 361-8003 | Énergir emergency line in Quebec where 911 isn’t available | Énergir |
| 149$ | Superior Plumbing & Heating service call, included in the total if you go ahead with the repair | Superior Plumbing & Heating |
In short: You can check the thermostat, switch, blower door, breaker, filter and error light. Repairing or servicing the gas furnace itself takes a certified gas technician in Ontario and Quebec.
In Ontario, the Gaseous Fuels regulation says no one may repair or service a gas appliance, your furnace included, without a certificate for that work [1]. The Technical Standards and Safety Authority (TSSA) puts it plainly: only registered fuel contractors and people with a valid gas technician certificate are legally permitted to work on gas appliances [2]. Quebec has a similar rule. Anyone working on a natural gas or propane appliance there must hold a gas qualification certificate [4].
That leaves you a clear lane, and it covers more than you might think.
| You can check | Certified gas technician only |
| Thermostat setting and batteries | Igniter and flame sensor |
| Furnace power switch | Gas valve and burners |
| Blower door panel | Inducer motor and pressure switch |
| Breaker or fuse | Limit and rollout switches |
| Air filter | Control board |
| Error light (reading the code only) | Venting and heat exchanger |
The rule is about safety oversight. Under Ontario law, furnaces must be maintained by TSSA-registered contractors, and TSSA notes that work by uncertified workers isn’t subject to its safety audits [5].
So check who you’re letting near the furnace. In Ontario, search the company in the TSSA fuels contractor lookup [2] and ask whether the technician holds at least a Gas Technician 2 (G2) certificate, the minimum for working on gas-fired appliances without supervision [3]. In Quebec, look the company up in the Régie du bâtiment du Québec (RBQ) licence register [6].
Superior Plumbing & Heating holds TSSA licence 000394817 in Ontario and RBQ licence 5876-1974-01 in Quebec.
In short: Match what you see to a likely cause, then start at Step 1 either way.
A furnace that’s not working usually leaves a clue. Match what you see to the row below, then note who handles it.
| What you notice | Likely cause | Who handles it |
| Thermostat screen is blank | Dead batteries or no power to the thermostat | You (Step 1) |
| Thermostat calls for heat, but the furnace won’t kick on and shows no lights | Power switch off, blower door not seated, or a tripped breaker | You (Steps 2 and 3) |
| Furnace starts, then shuts down after a few minutes | Clogged filter causing overheating that trips the high limit switch [7] | You for the filter (Step 4), a technician if it keeps happening |
| Status light is flashing a code | The control board has logged a fault or a lockout | You read it (Step 5), a technician repairs it |
| Clicking or a glowing igniter, but no flame | Ignition or gas supply fault | Certified gas technician |
| Breaker trips again after one reset | Electrical fault inside the furnace | Technician |
Even if a row looks like a perfect match, start at Step 1. Several of these symptoms share a cause, and the simple checks take minutes.
In short: The thermostat must be on Heat, set above room temperature, powered, and not held low by a schedule.
The thermostat tells the furnace when to start. If it isn’t calling for heat, the furnace stays off, however cold the house gets.
Start with the mode. It should read Heat, not Off or Cool. Then set the temperature a few degrees above what the room feels like now. If the screen is blank or dim, replace the batteries and watch for it to power back up.
Programmable and smart thermostats add one more layer. A schedule, a hold or an away setting can keep the target low without you noticing. A common case: away mode left on after a weekend trip, and the house drifts down to 16°C on Sunday night. Clear the schedule or hold, then set the temperature by hand.
If the thermostat shows that heat is on but the furnace still won’t kick on, look for an error message on the thermostat screen and check its owner’s manual [8]. A hardwired thermostat with a blank screen points to a power problem, so move on to Steps 2 and 3.
Give it a few minutes after any change. If the furnace starts, the problem was a setting.
In short: The furnace switch must be on and the blower door fully seated, because an open door cuts power to the furnace.
Most furnaces have their own power switch, and it looks like an ordinary light switch on the wall or on the side of the unit [8]. In a finished basement, it’s easy to flip it off while reaching for the stairway light. Find it, make sure it’s in the On position, and give the furnace a few minutes to respond.
Next, look at the front panel that covers the blower, called the blower door. On most basement furnaces, it’s the lower one. Behind it sits a safety switch. When the door isn’t fully in place, that switch cuts power to the furnace control and nothing can run [10]. It often happens after a filter change, when the panel goes back on slightly crooked. Press it firmly into place until it sits flat and its clips or screws are secure. Manufacturers ask this same question in their checklist before a service call: is the blower access door firmly in place [9]?
Never tape, hold or bypass that door switch. Running a furnace with the blower door off can pull combustion products into the air circulating through your home [11].
If the switch is on, the door is seated and the furnace still shows no lights, check the breaker.
In short: Reset a tripped breaker once. If it trips again, leave it off and call for service.
A furnace with no lights at all and a working power switch may have lost power at the electrical panel. Open the panel and find the breaker labelled for the furnace. A tripped breaker may sit between On and Off rather than lining up with the others.
Push it fully to Off, then back to On, once [12]. Give the furnace a few minutes. If it starts, watch it through a full heating cycle.
If the breaker trips again, leave it off and call for service. A repeat trip means there’s an electrical problem in the furnace [8], and resetting it over and over won’t fix that. One reset is a check. Two is a symptom.
Older homes with a fuse panel work the same way, so look for a blown fuse on the furnace circuit. Many furnaces also carry a small fuse on the control board inside the cabinet [9]. That one sits behind the panels, so leave it to a technician.
Power confirmed? Next, pull the filter.
In short: A clogged filter can overheat the furnace and trip its high limit switch, so replace it if it’s grey and caked.
A clogged filter can stop a furnace that has nothing else wrong with it. When air can’t get through, heat builds up inside the cabinet and a safety switch called the high limit shuts the burners down [7]. That’s the furnace that starts, runs a few minutes, then quits.
If the status light is flashing, write the code down first (see Step 5). Then turn the furnace power switch off and slide the filter out of its slot. Hold it up to a light. A clean filter lets light through. A clogged one looks grey, often with a visible layer of dust, and blocks most of the light [7].
If it needs replacing, match the size printed on the frame and point the airflow arrow toward the furnace. Put back any panel you moved, make sure it sits flat, then switch the power back on.
No spare in the house? Don’t run the furnace without a filter while you wait [14]. Pick one up first, then restart. Through the heating season, inspect the filter once a month and replace it when it’s dirty [13].
If the old filter was badly clogged, the furnace may already have logged a fault and locked itself out for a while. The error light in Step 5 will tell you.
In short: Write down the flashing code before you cut power, then allow one reset at most.
Many furnaces diagnose themselves. A small status light on the control board shows through a window, called a sight glass, in the blower door [15]. Read it with the door in place. Taking the door off cuts power to the control, and the code can disappear with it.
Count the flashes and watch for the pause before the pattern repeats. Many furnaces pair short and long flashes into a two-digit code, for example two short and three long on a furnace that uses that system [17]. Match what you see to the chart on the service label, usually on the blower door [17]. The same flash count can mean different things on different brands, so your own label is the one that counts.
Write the code down before you turn off the power [15], along with the model number from the furnace’s rating plate. Give both to the technician when you call [16].
A flashing code often means a lockout. After repeated failed starts or overheating, the control shuts the furnace down for a set time. On some models, that lasts one to three hours before the furnace tries again on its own, depending on the fault [18]. Once the code is recorded, one reset at the furnace power switch is reasonable. If the same code comes back, the cause is still there.
Some codes point to a manual-reset safety switch in the burner area, such as a flame rollout switch [18]. That switch trips when heat or flame escapes where it shouldn’t, and it sits on the technician side of the line.
In short: The igniter, flame sensor, gas valve, inducer, safety switches, blower motor and control board are technician work.
When a furnace is not starting after all five checks pass, the fault usually sits in one of the parts below. A technician tests these with instruments rather than by trial and error.
| Part | What it does | What you might notice |
| Hot surface igniter | Heats up to light the gas | The furnace tries to start but never lights, then shows a lockout code |
| Flame sensor | Confirms the burners lit, or the control shuts the gas off | Burners light, go out after a few seconds, and the cycle repeats [18] |
| Gas valve | Opens to send gas to the burners | The start sequence runs, but no flame appears |
| Inducer motor and pressure switch | Clear exhaust from the furnace and confirm the vent is drawing before ignition | A hum or whir at start-up, then nothing, often with a pressure switch code [17] |
| Limit and rollout switches | Shut the burners off when the furnace overheats or flame escapes the burner area | Short runs that end early, with a limit or rollout code [18] |
| Blower motor | Moves warm air through the ducts | The furnace starts, then shuts down quickly because air isn’t moving [12] |
| Control board | Runs the start-up sequence and shows the codes | No response with power present, or the same control code returning [18] |
One gas check needs no tools and no touching. Find the manual shut-off valve on the gas pipe beside the furnace, which is separate from the gas valve inside it. If the lever runs along the pipe, it’s open. If it sits at a right angle to the pipe, the gas is off [9]. Don’t turn it back on yourself, since it may have been shut for a safety reason [9]. If your gas stove or water heater is also out, call your gas utility.
Every part in the table sits behind the furnace panels. The code and model number you wrote down in Step 5 give the technician a starting point.
In short: A gas smell, a CO alarm, or deep cold with vulnerable people at home means call now, not troubleshoot.
Some situations skip the checklist entirely.
You smell gas. Natural gas carries a rotten-egg odour. If you notice it, leave the house now, don’t flip switches or use electronics near the smell, and don’t try to restart the furnace. Call from outside or from a neighbour’s home. In Ontario, that’s Enbridge Gas at 1-866-763-5427 or 911 [19]. In Quebec, call 911, or Énergir at 1 800 361-8003 where 911 isn’t available [23].
A carbon monoxide alarm sounds, or people feel sick near the furnace. Get everyone out and call 911 from outside [20]. Carbon monoxide (CO) has no smell, and symptoms include headaches, nausea, dizziness, confusion and drowsiness [20]. In Ontario, any home with a fuel-burning appliance such as a gas furnace needs a CO alarm next to each sleeping area, and since January 1, 2026, on every storey as well [20][21].
It’s deep cold outside and the heat is out. Pipes are the first thing at risk, especially those against exterior walls and where the water line enters through the foundation [22]. People are at risk too. Cold is harder on older adults, infants, young children and anyone with a heart or lung condition [24]. If someone like that lives with you, don’t wait for morning. Book 24/7 emergency service.
In short: An annual inspection, monthly filter checks and clear vent pipes prevent most no-start calls.
Most of the causes above are easier to prevent in the fall than to fix on a January night. A few habits cover the bulk of them:
For the full list, including vents, humidifiers and detectors, use our winter furnace checklist. If you’d rather hand the inspection off, Superior’s fall Furnace + A/C Maintenance visit is $240.
In short: If the five checks pass and the furnace still won’t start, book a licensed technician.
If the thermostat, switch, breaker and filter all check out and the furnace still won’t start, what’s left belongs to a certified gas technician. Superior Plumbing & Heating’s furnace repair team works on gas and electric furnaces of any brand, with same-day and 24/7 service across Ontario and Quebec. The 149$ service call is included in the total if you go ahead with the repair.
Have your error code and model number from Step 5 ready, then call (866) 932-2818.
| Term | Meaning |
|---|---|
| Blower door | The front panel that covers the furnace blower, usually the lower panel on a basement furnace |
| Blower door safety switch | A switch behind the blower door that cuts power to the furnace control when the door is off or not seated |
| Carbon monoxide (CO) | An odourless gas from fuel burning that can only be detected with a CO alarm |
| Control board | The circuit board that runs the furnace’s start-up sequence and flashes its status codes |
| Flame rollout switch | A manual-reset safety switch that shuts the burners off when flame or heat escapes the burner area |
| Flame sensor | A rod that confirms the burners lit, so the control can shut the gas off if they didn’t |
| Gas Technician 2 (G2) | The Ontario gas technician certificate that is the minimum for working on gas-fired appliances without supervision |
| High limit switch | A safety switch that shuts the burners off when the furnace overheats |
| Hot surface igniter | A part that heats until it glows to light the gas |
| Inducer motor | A small fan that clears exhaust from the furnace before and during burning |
| Lockout | A timed safety shutdown after repeated failed starts or overheating |
| Pressure switch | A switch that confirms the vent is drawing before the furnace lights |
| Sight glass | A small window in the blower door for viewing the status light |
| Status light | The light on the control board that flashes a code to show the furnace’s condition |
| TSSA | The Technical Standards and Safety Authority, Ontario’s regulator for fuel-burning equipment and gas work |
| RBQ | The Régie du bâtiment du Québec, which licenses construction and gas contractors in Quebec |
| Property | Value |
|---|---|
| Role | Regulates fuel-burning appliances and gas work in Ontario |
| Rule that matters here | Only registered fuel contractors and certified gas technicians may legally work on gas appliances |
| How to check a contractor | TSSA fuels contractor lookup |
| Superior Plumbing & Heating licence | TSSA 000394817 |
| Property | Value |
|---|---|
| Made under | Technical Standards and Safety Act, 2000 |
| Key section | Section 6(1) |
| What it says | No person may install, alter, purge, activate, repair, service or remove a gas appliance without a certificate for that purpose |
| Property | Value |
|---|---|
| Applies to | Homes with a fuel-burning appliance, a fireplace or an attached garage |
| Requirement | A CO alarm next to each sleeping area and on every storey |
| Every-storey rule in effect | January 1, 2026 |
| Where it lives | Ontario Fire Code (O. Reg. 213/07), Division B, Section 2.16 |
| Property | Value |
|---|---|
| Service area | Ontario and Quebec |
| Furnace service | Repair of gas and electric furnaces, any brand, same day and 24/7 |
| Licences | TSSA 000394817 (Ontario), RBQ 5876-1974-01 (Quebec) |
| Service call | 149$, included in the total if you go ahead with the repair |
| Phone | (866) 932-2818 |
