Flame sensor replacement is usually a small, quick repair, but diagnostic time and access to the burner compartment are what actually shape the cost.
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Quick answer
A boiler flame sensor generally needs replacing, rather than just cleaning, when the rod is visibly cracked, the ceramic insulator has failed, or a properly cleaned sensor still cannot hold an adequate signal during a live ignition test. It is one of the least expensive individual boiler parts, so most of the cost in a service call goes toward correctly diagnosing that the sensor, and not the gas valve, igniter, or control board, is the actual cause. A licensed technician confirms this with a meter before recommending replacement and can give a firm number after that diagnosis.
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The flame sensor part itself is inexpensive relative to labour, and this repair is not comparable to the ballpark ranges used for a full boiler replacement (roughly $7,500 to $15,000-plus) or a new installation (roughly $6,500 to $14,000-plus). Your technician gives a firm number once the sensor is confirmed as the cause.
| Situation | Typical service |
|---|---|
| Light buildup, no physical damage | Cleaning only, often no part needed |
| Cracked rod or failed insulator | Replacement |
| Clean sensor, still weak signal | Replacement plus check of grounding path |
| Repeated failures within months of a previous cleaning | Replacement rather than repeat cleaning |
Safety note
Flame sensor replacement takes place inside the burner compartment of gas-fired equipment and should be performed by a licensed gas fitter only. 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.
Yes, on most residential boilers it is a quick, same-visit job once the sensor is confirmed as the cause, since the part is small and generally accessible.
Most of the cost reflects the technician's time to safely access the burner compartment and properly diagnose that the sensor, rather than a costlier component, is the actual cause.
Often yes, if the sensor is intact and only fouled with buildup; replacement is reserved for sensors that are physically damaged or still weak after cleaning.
Manufacturers do not publish a guaranteed lifespan for this part, and how quickly buildup returns depends on flame characteristics and maintenance frequency for that specific boiler.
A flame sensor issue typically shows up after ignition, when the boiler briefly fires then cuts out; a boiler that never attempts ignition points to a different category of fault, worth mentioning to your technician.
Warranty coverage on individual parts is set by the manufacturer and varies by model and installation date, so it is worth checking your specific certificate.
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