Safety First: When Not to Troubleshoot
If you smell gas at any point — a rotten-egg or sulphur odour near the furnace — stop immediately. Do not flip switches, do not relight anything, and do not look for the cause. Get everyone out of the home and, from outside, call FortisBC at 1-800-663-9911 or 911. Only after the utility or fire department has cleared the home should you call us at 604-359-1081.
Likewise, if a carbon monoxide alarm is sounding, treat it as an emergency: leave, ventilate by leaving doors open as you go, and call for help from outside. Ignition problems are usually mechanical and not dangerous to inspect, but gas odour and CO alarms always override troubleshooting.
How Modern Furnace Ignition Works
Understanding the start sequence helps you describe the problem. When the thermostat calls for heat, the furnace first runs its inducer motor to clear the flue and prove draft via a pressure switch. Then the hot surface igniter glows (or, on some units, a spark lights a pilot). The gas valve opens, the burners light, and a flame sensor confirms the flame within a few seconds. If the sensor does not detect flame, the valve closes for safety and the furnace tries again.
After several failed attempts — usually three — the control board enters lockout and stops trying, often flashing a diagnostic code. This is the furnace protecting your home from releasing unburned gas. Knowing this sequence tells you where it is failing: no inducer hum points one way, a glowing igniter with no flame points another, and a flame that lights then dies points to the sensor.
Common Reasons a Furnace Won't Light
Dirty flame sensor. The most frequent cause. A thin coating of oxidation on the sensor rod prevents it from "seeing" the flame, so the furnace lights for a second or two then shuts off and retries. A technician cleans or replaces it quickly.
Failed hot surface igniter. These brittle elements crack with age. If the igniter never glows orange, it likely needs replacement.
Tripped pressure switch or blocked flue/intake. Leaves, snow, ice, or a bird's nest in the intake or exhaust termination can stop the furnace from proving draft. Greater Vancouver's wet weather makes blocked terminations a real seasonal issue.
Clogged condensate line. High-efficiency furnaces shut down when the condensate trap or drain backs up. Closed gas supply or pilot/valve fault and a control-board lockout round out the usual suspects. Many of these require a meter, combustion analyzer, or replacement parts to resolve safely.
Safe Steps Before You Call
You can safely try a few things. Set the thermostat to Heat and well above room temperature. Confirm the furnace wall switch is on and the breaker has not tripped. Replace a heavily clogged filter, since poor airflow can cause limit shutdowns that mimic ignition failure. Make sure the front access panel is fully closed so the door switch engages.
Then try one reset: turn the furnace off at its switch for 30 seconds and back on to clear a lockout. If it lights and runs, great — but a lockout that returns means a real fault that needs diagnosis. If you can safely note the LED blink-code pattern through the sight glass, write it down for the technician. Do not remove burner assemblies, sand the flame sensor without proper tools, or run the furnace if it repeatedly fails — call 604-359-1081.
Why It Needs a Certified Technician
Ignition repairs touch the gas valve, igniter circuit, flame-proving safety, and venting — all regulated work in BC. A Red Seal gas fitter can test the igniter resistance, measure flame current on the sensor, verify the pressure switch and inducer, and confirm safe combustion with an analyzer. Guessing at parts or bypassing the flame-proving circuit is how unburned gas ends up in a home.
Furnaces are serviced by our parent company, CanroHeat, with the same certified team that handles boilers and water heaters across the Lower Mainland. We carry common igniters, sensors, and pressure switches on the truck, so many ignition faults are fixed in one visit. We will also confirm the furnace is venting and burning cleanly before we leave, not just that it lights. For an exact diagnosis and quote, call 604-359-1081.