The Short Answer
When a furnace won’t turn on, the cause is usually one of nine things: the thermostat, the power switch or breaker, the blower door interlock, a clogged filter, a closed gas valve, an ignition lockout, a blocked vent pipe or condensate drain, or, on older furnaces, a pilot light that has gone out. A technician checks them in roughly that order because the first five take minutes and need no tools.
This guide walks through each check the same way we do on a no-heat call. About a third of the “dead furnace” calls we get on the first cold morning of fall turn out to be something on this list that a homeowner can fix safely. The rest need a licensed gas fitter, and the section on when to stop tells you exactly where that line is.
Safety first: if you smell rotten eggs, do not touch the thermostat, light switches or the furnace. Leave the house and call the FortisBC Emergency Line at 1-800-663-9911 (24 hours) or 911 from outside.
Why Furnaces Fail on the First Cold Morning
A furnace that sat idle from May to October has had five months for small problems to develop quietly: a flame sensor coated in residue, a filter that loaded up with summer dust and wildfire smoke, a condensate line that grew algae, a wasp nest in the intake pipe. None of them show up until the furnace is asked to fire.
That is why no-heat calls spike in the first week the overnight low drops below about 5°C. In the Lower Mainland that is usually mid-October. If you get a pre-season tune-up, most of these get caught in September instead.
The 9 Checks, In Order
1. Thermostat: Mode, Setpoint, Batteries, Schedule
Start here. It is the most common cause and the fastest to rule out.
- Mode is set to Heat (not Cool, Off, Auto with a cooling setpoint, or Emergency Heat on a heat pump system).
- Setpoint is at least 2-3°C above the current room temperature. A furnace will not fire if the thermostat thinks the room is already warm enough.
- Batteries: a blank or dim screen usually means dead batteries. Many thermostats lose their heating call before the screen goes fully blank.
- Schedule or hold: a smart thermostat running a summer “away” schedule, or a hold someone set in July, will override the setpoint you just entered.
- Smart thermostat delays: ecobee, Nest and Honeywell units often show “waiting”, “equipment protection” or a countdown for up to five minutes after a change. Give it the full delay before moving on.
If you have a dual-fuel system (heat pump plus gas furnace), check the balance point or “aux heat” setting too. A thermostat configured to use the heat pump down to -5°C will not call the furnace on a 4°C morning, and that is working as designed.
2. Power: The Service Switch and the Breaker
Every furnace has a service switch that looks exactly like a light switch. It is usually on the side of the furnace, on a nearby wall, or at the top of the basement stairs. It gets flipped off by accident all the time, often by someone looking for the basement light.
- Make sure the service switch is on.
- Check the furnace breaker in your electrical panel. If it has tripped, reset it once.
- If the breaker trips again, stop. A breaker that trips twice is telling you there is an electrical fault, usually a shorted motor or a failing control board, and resetting it again can damage components or create a fire risk.
3. The Blower Door Safety Switch
The front panel of the furnace presses a small door interlock switch when it is fully closed. If the panel is crooked or sitting a few millimetres proud (common after a filter change), the switch stays open and the furnace will not start at all, with no lights, no fan, nothing.
Take the panel off and put it back on, making sure it seats flat on all sides. Many “dead” furnaces come back to life at this step.
4. The Filter
A clogged filter is the most common cause of a furnace that starts, runs for a few minutes, then shuts off, and of a furnace that eventually stops starting altogether.
When airflow is restricted, the heat exchanger overheats and the high-limit switch shuts the burners off to protect it. After several limit trips in a row, many furnaces go into a hard lockout.
- Slide the filter out and hold it up to a light. If you cannot see light through it, replace it.
- Use the size printed on the filter frame. A filter that is too small lets air bypass around the edges.
- If you upgraded to a MERV 13 filter for wildfire smoke this summer, know that it loads up faster than a MERV 8. Check it monthly through the heating season. (Our MERV 11 vs 13 vs HEPA guide covers the trade-offs.)
5. The Gas Supply
- Find the gas shut-off valve on the pipe leading into the furnace. The handle should be parallel to the pipe (open). Perpendicular means closed.
- Check whether your other gas appliances work. If the stove and water heater are also out, the problem is upstream (the meter or supply) and FortisBC needs to know.
- On propane systems, check the tank gauge. A tank below about 20% can deliver inconsistent pressure in cold weather.
Never try to adjust the gas valve on the furnace itself, the burners, or anything inside the burner compartment. That is licensed gas-fitter work in BC under Technical Safety BC regulations.
6. Read the Error Code Before You Reset Anything
Modern furnaces tell you why they stopped. Look through the small window in the blower door for a blinking LED on the control board.
- Count the flashes. Most codes are a number of short flashes followed by a pause, repeating.
- Match the count to the legend printed on the inside of the furnace door.
- Write it down or take a photo.
Common codes point to a pressure switch problem, flame failure, a limit trip or an ignition lockout. Recording the code before you reset the furnace saves real diagnostic time if you do end up calling a technician, because a reset clears it.
7. Ignition Lockout and the Flame Sensor
Most furnaces built since the late 1990s use a hot surface igniter and a flame sensor instead of a standing pilot. If the furnace tries to ignite three times without the flame sensor confirming a flame, the control board locks out for about an hour as a safety measure.
To clear a lockout: turn the service switch off for 30 seconds, then back on. Do this once.
If it fires and keeps running, you are probably fine for now, but book a service visit. A furnace that locks out once usually does it again. If it locks out again right away, the cause is still there. In fall, the usual suspects are:
- A dirty flame sensor. A thin coating on the sensor rod stops it from detecting the flame, so the furnace lights, runs for 3-10 seconds, then shuts the gas off. This is the single most common no-heat cause we find in October.
- A cracked or worn-out hot surface igniter. If you watch through the window and never see the igniter glow orange, it has likely failed. Igniters typically last 5-10 years.
Both are inexpensive repairs, but they sit right next to the burners and gas valve. Leave them to a technician.
8. High-Efficiency Furnaces: Vent Pipes and the Condensate Drain
If your furnace has two white PVC pipes running out the side of the house or through the roof, it is a high-efficiency (90%+ AFUE) condensing furnace. These have two extra failure points that stop the furnace from starting:
- Blocked intake or exhaust terminations. Fall leaves, wasp or bird nests, and in winter drifted snow or ice can block the pipe ends outside. The pressure switch senses the blockage and will not let the furnace ignite. Check both pipe ends outside and clear anything you can reach by hand.
- A clogged condensate trap or drain line. These furnaces produce several litres of water a day in heating mode. If the drain clogs, water backs up and trips a float switch or the pressure switch. Look for water pooling under the furnace or in the drain tubing.
You will often hear the inducer motor (a small fan) start and run without the burners ever lighting. That points to a venting or pressure switch problem.
9. Older Furnaces: The Pilot Light
If your furnace is older than about 25 years, it may have a standing pilot, a small flame that burns constantly. If the pilot is out, the furnace cannot light.
The relighting procedure is printed on a label inside the furnace door. Follow it exactly. If the pilot will not stay lit after you release the button, the thermocouple has likely failed. That is a technician repair, and on a furnace that old it is also a good moment to start planning a replacement. (See our furnace brands comparison for what is worth buying in BC.)
When to Stop and Call a Technician
Stop troubleshooting and call a licensed HVAC technician if any of these happen:
| Sign | What it usually means |
|---|---|
| Rotten egg smell | Possible gas leak: leave first, then call FortisBC at 1-800-663-9911 or 911 |
| CO alarm sounds | Possible combustion problem or cracked heat exchanger: get everyone outside, call 911 |
| Breaker trips a second time | Electrical fault in a motor or control board |
| Furnace locks out again after one reset | Flame sensor, igniter, gas valve or venting fault |
| Burning, scorching or electrical smell | Overheating motor or wiring |
| Soot around the burners or flue | Incomplete combustion |
| Water pooling around the furnace | Condensate leak or secondary heat exchanger issue |
| Inducer runs but the furnace never lights | Pressure switch, venting or inducer fault |
A small burning-dust smell for the first 10-20 minutes of the season is normal. That is a summer’s worth of dust burning off the heat exchanger. A smell that lingers, or smells electrical, is not.
What No-Start Repairs Typically Cost in BC (2026)
| Repair | Typical cost (Lower Mainland) |
|---|---|
| Diagnostic / service call | $150 - $250 |
| Flame sensor cleaning or replacement | $100 - $250 |
| Hot surface igniter replacement | $200 - $400 |
| High-limit switch replacement | $200 - $400 |
| Pressure switch replacement | $250 - $450 |
| Condensate trap / drain clearing | $150 - $300 |
| Inducer motor replacement | $500 - $1,200 |
| Control board replacement | $650 - $1,400 |
Ranges include parts and labour. After-hours and holiday calls cost more. If your furnace is more than 15-20 years old and needs one of the larger repairs (inducer motor, control board or anything involving the heat exchanger), it is worth getting a replacement quote at the same visit. Repair money spent on a furnace near the end of its life rarely pays back.
No-Start Patterns in Older Vancouver and Burnaby Homes and Fraser Valley Propane Systems
The checks above apply everywhere, but the most likely culprit shifts depending on where you live in the Lower Mainland.
Older homes in Vancouver, Burnaby and New Westminster. Many pre-1980 houses still run mid-efficiency furnaces vented through a masonry chimney, sometimes shared with a gas water heater. Chimney liners deteriorate, and debris falling down the flue can trip the furnace’s flue safety (spill) switch. Flame sensors in these older units also tend to be original and heavily coated. If you are in a character home in East Vancouver, Kitsilano or Burnaby Heights and your furnace locks out on the first cold day, the flame sensor and venting are the first places we look.
Newer homes in Surrey, Langley and Coquitlam. These are mostly high-efficiency furnaces with side-wall PVC venting. The most common October no-start in these subdivisions is a blocked termination: a leaf drift against the vent, a nest, or a plant that grew over the pipe ends during summer. Check outside before you call.
Fraser Valley propane systems in Abbotsford, Chilliwack, Mission and Agassiz. Rural properties east of Langley often run on propane. Low tank levels during a cold snap, regulator icing and longer supply lines all show up as ignition failures. When Fraser Valley outflow winds hit Abbotsford and Chilliwack in December and January, side-wall vents on the windward side can drift shut with snow overnight. Keep a metre clear around both pipe ends all winter.
Bottom Line
Before you call, work through the quick checks in order: thermostat, service switch and breaker, blower door, filter, gas valve. Then read and record the error code, and try one reset. On a high-efficiency furnace, check the vent pipes outside. That sequence solves a meaningful share of first-cold-morning no-heat calls in minutes.
If the furnace still won’t start, locks out again, or shows any warning sign from the table above, stop there. Call 604-991-4894 or send us a message with the error code you recorded. Our licensed technicians handle furnace repair and installation across the Lower Mainland and Fraser Valley, from Vancouver and Burnaby to Surrey, Langley, Abbotsford and Chilliwack.
Frequently Asked Questions
The most common causes, in order, are a thermostat problem (wrong mode, dead batteries, a schedule override), the furnace service switch turned off, the blower door not fully closed, a clogged filter tripping the high-limit switch, a closed gas valve, and an ignition lockout after three failed ignition attempts. On high-efficiency furnaces, a blocked intake or exhaust pipe or a clogged condensate drain will also stop the furnace from starting.
Turn the furnace service switch (it looks like a light switch on or near the furnace) off, wait 30 seconds, then turn it back on. This clears an ignition lockout on most modern furnaces. Only do it once. If the furnace locks out again, the cause is still there, usually a dirty flame sensor, a failing igniter or a venting problem, and repeated resets will not fix it.
The status LED on the control board flashes a code that tells you why the furnace stopped. Count the flashes through the small window in the blower door, then match the count to the legend printed on the inside of the furnace door. Write the code down before you reset anything, because a reset clears it and the code saves the technician diagnostic time.
Yes. A clogged filter restricts airflow, the heat exchanger overheats, and the high-limit switch shuts the burners off. Many furnaces lock out completely after several limit trips in a row. Replacing a filter that is grey and caked with dust often restores heat within the hour.
Stop immediately and leave the house if you smell rotten eggs (natural gas), then call the FortisBC Emergency Line at 1-800-663-9911 or 911 from outside. Call a licensed HVAC technician if the breaker trips again after one reset, the furnace locks out again after one reset, your CO alarm sounds, you smell burning or scorching, or you see soot, water pooling or a cracked heat exchanger.
A diagnostic visit typically runs $150 to $250 in the Lower Mainland. Common no-start repairs range from about $100 to $250 for a flame sensor cleaning or replacement, $200 to $400 for a hot surface igniter, $250 to $450 for a pressure switch, $500 to $1,200 for an inducer motor and $650 to $1,400 for a control board.
