A furnace that lights, runs a few seconds, then cuts out is telling you the control board can't confirm the flame is real — and it's usually the sensor.
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Quick answer
The flame sensor's whole job is to prove combustion is genuinely happening, not to help start it. When it can't send that confirmation, the control board shuts the gas valve as a precaution, which is why a fouled sensor produces a furnace that lights briefly and then cuts out, sometimes repeatedly. Cleaning restores many sensors in a Vancouver furnace; a sensor that's pitted, cracked, or fouling again quickly needs replacing instead.
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It's easy to assume the flame sensor helps start the furnace, but its actual job runs the opposite direction — it's a safety check, sitting in the flame after the ignitor has already lit it, confirming to the control board that combustion is genuinely happening. That confirmation runs on a tiny electrical signal, and if it doesn't arrive within a few seconds of ignition, the board assumes something is wrong and cuts the gas rather than let it flow unconfirmed. That's a deliberate design choice, not a flaw, and it's exactly why a furnace with a failing sensor doesn't run badly — it shuts itself down completely, sometimes over and over.
Every burn cycle leaves an almost invisible residue on the sensor rod, a byproduct of combustion settling on the metal. Individually it's nothing, but across enough cycles that residue accumulates into a thin insulating film that weakens the flame-confirmation signal even though the burner is genuinely lit and burning normally. It's a gradual process, which is why this symptom tends to creep in slowly — a furnace that used to light every single time starts needing a second or third attempt, and eventually won't hold a flame at all without repeated tries.
A sensor that needs replacing more than once in a couple of years is usually a symptom of something else worth checking — flame quality affected by burner alignment or gas pressure, or airflow issues that change how the flame burns near the rod. Older character-home furnace closets in Vancouver, often doing double duty for storage, can also mean a combustion-air supply that's marginally restricted, which subtly changes how clean the flame burns even when nothing else seems obviously wrong.
The technician watches the actual shutdown pattern rather than assuming, inspects the sensor with the panel safely open, and only recommends replacement once cleaning has already been ruled out or has already failed to hold. Pricing depends on the sensor style and how your specific burner compartment is laid out, and the office confirms it before any work starts. Many Vancouver homeowners have this checked at the same visit as their annual furnace service, since a sensor that's starting to foul is often caught before it causes a full lockout.
Safety note
The flame sensor sits directly in the burner's flame path, so working near it means working around live gas combustion components — not a homeowner repair. If lockout attempts are ever paired with a gas smell, leave the home and call FortisBC's emergency line at 1-800-663-9911 from outside.
This is the signature flame-sensor pattern — the sensor isn't confirming the flame fast enough, so the control board shuts the gas valve as a precaution. It's one of the most common single-repair furnace complaints.
It means opening the furnace cabinet and working next to live burner and gas components, so it isn't something we'd recommend as a DIY task even though the part is small.
It varies a lot by usage, so there's no fixed interval. A sensor check during annual maintenance often catches it building up before it ever causes a lockout.
Not in itself — it's the sensor doing exactly its job by shutting off gas flow it can't confirm is burning safely. The inconvenience is lost heat, not a safety risk, though repeated lockouts should still be diagnosed properly.
If the sensor was the actual cause, yes, in most cases. If the problem returns quickly, that points to a secondary cause like flame quality or gas pressure, which the technician would check at the same visit.
Coating builds up gradually across cycles, so a furnace mid-way through a cold stretch with more cycles behind it is more likely to reach the point where the signal finally drops below what the control board needs.
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