A flame sensor reports to the control board that the burner has actually lit, using a small electrical signal, and if that signal reads too weak the board shuts the gas valve as a safety measure, even if the flame is genuinely burning.
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On a modern condensing gas boiler, the flame sensor's job is proving flame, not lighting it. Once the igniter starts the burner, the sensor confirms combustion is actually happening by reading a small electrical signal through the flame itself. If that signal is too weak, the control board shuts the gas valve as a safety measure, since it can't confirm the flame is real. In an Abbotsford home, this most often traces back to a sensor coated in combustion residue rather than a failed part.
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The flame sensor is a small metal probe positioned in the flame itself. When flame is present, it conducts a tiny electrical current, in the microamp range, back to the control board. The board reads that signal as confirmation the burner lit successfully and allows the boiler to continue its normal cycle.
If the signal comes back too weak or absent, the board treats that as an unproven flame condition, closes the gas valve, and locks out rather than risk gas flowing without confirmed combustion. This is a safety design, not a flaw, but it means a boiler can shut down even when the flame is visibly burning fine, if the sensor itself just can't read it properly anymore.
| Cause | Effect on signal |
|---|---|
| Combustion byproduct coating on the probe surface | Insulates the sensor, weakening the signal it can read |
| Corrosion on the sensor tip | Reduces conductivity, similar effect to coating |
| Sensor positioned incorrectly in the flame | Weak or inconsistent signal even if the probe itself is clean |
| Failed sensor or broken wiring | No usable signal at all |
A coated or lightly corroded sensor is the most common cause by a wide margin, and it's the reason a technician cleans the sensor and checks its microamp reading before recommending replacement.
Cleaning the sensor surface and confirming its positioning resolves most weak-signal cases and costs nothing beyond the service visit. Replacement makes sense once the sensor's underlying metal is degraded enough that a clean reading can't be restored, or if the probe itself has physical damage.
A boiler that goes into lockout repeatedly over a short period is worth having checked properly rather than just reset each time, since repeated lockouts from a flame-proving fault are the system doing exactly what it's supposed to do when it can't confirm safe combustion.
Replacing a flame sensor means removing the old probe from its mounting position near the burner, installing the new one at the correct depth and angle for that boiler model, and running the boiler through a start-up cycle while checking the microamp signal to confirm it's reading well above the minimum threshold, not just enough to pass.
Safety note
If you smell gas, hear hissing near a fitting, or notice soot, a yellow flickering flame, or a headache that eases when you leave the house, get everyone out first and call FortisBC's emergency line at 1-800-663-9911 from outside. Don't try to relight anything or search for the leak yourself.
A weak or absent flame sensor signal is a common cause. The control board can't confirm flame is present, so it shuts the gas valve as a safety measure even if the burner is actually lit.
Yes, and it's the first thing a technician tries, since a coated or lightly corroded probe often just needs cleaning and repositioning rather than replacement.
They serve a similar proving function but are associated with different ignition system designs; a technician identifies which type is on your specific boiler before servicing it.
A technician measures the microamp signal during a start-up cycle rather than guessing from symptoms alone, which confirms whether the sensor is reading properly.
The lockout itself is the safety system working correctly. It's worth having checked promptly so the underlying cause is addressed rather than repeatedly resetting the boiler.
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