The flame sensor is a small safety device that confirms real combustion is happening, and a fouled sensor is one of the most common causes of nuisance lockouts.
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Quick answer
A gas boiler's flame sensor is a thin metal rod positioned in the burner flame that proves to the control board that real combustion is occurring after ignition, using a tiny electrical signal called flame rectification. If the board does not receive that signal within a few seconds of opening the gas valve, it shuts the valve immediately as a safety measure rather than letting gas flow unburned. The most common failure is a sensor fouled with a thin layer of carbon or oxidation buildup that weakens its signal, which a technician typically cleans, and less often the sensor itself or its wiring fails and needs replacement.
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Every time a boiler's burner ignites, the control board needs proof within seconds that a real, stable flame is present, not just that the igniter fired. The flame sensor rod sits directly in the flame and carries a microampere-level signal back to the board using flame rectification, a property of flame that allows current to pass more easily in one direction than the other. If that signal is missing or too weak, the board interprets it as no flame and immediately closes the gas valve, preventing unburned gas from continuing to flow into the combustion chamber.
This fail-safe design means a fouled or failed flame sensor does not create a gas hazard on its own; instead it typically causes the boiler to shut down and lock out, which is the system doing exactly what it is supposed to do when it cannot confirm safe combustion.
A boiler that ignites briefly, runs for a few seconds, then shuts off and tries to reignite repeatedly before locking out is a classic pattern associated with a weak flame sensor signal. This is different from a boiler that never attempts ignition at all, which points toward a different category of fault such as gas supply or the igniter itself. Flame sensor cleaning is not a homeowner task, since it requires accessing the burner compartment on a gas-fired appliance.
| Situation | Typical outcome |
|---|---|
| Light carbon buildup on sensor rod | Cleaning restores normal operation, low cost |
| Cracked rod or failed ceramic insulator | Sensor replacement, inexpensive part |
| Weak signal persists after cleaning | Replacement, and check for a related grounding issue |
| Sensor and wiring both intact but fault continues | Broaden diagnosis to gas valve or control board |
Safety note
Flame sensor service is performed inside the burner compartment of gas-fired equipment and must be done by a licensed gas fitter. If you ever smell gas near the boiler or water heater, leave the home immediately and call FortisBC at 1-800-663-9911 before doing anything else.
This short-cycling-on-ignition pattern is a common symptom of a fouled or weak flame sensor that cannot confirm stable combustion to the control board.
No, the boiler's safety design shuts the gas valve when it cannot confirm flame, so a fouled sensor causes a lockout rather than a hazard.
There is no fixed universal schedule, but it is a normal item checked during annual maintenance, since buildup accumulates gradually with normal use.
This requires accessing the burner compartment of gas-fired equipment safely and should be done by a licensed technician.
It is one of the less expensive boiler parts; the greater cost is usually diagnostic time to confirm the sensor, rather than something else, is the actual cause.
An intermittent weak signal, such as from a sensor with light buildup or a slightly loose connector, can produce inconsistent lockouts rather than a fault every single cycle.
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