When a gas water heater's pilot lights successfully but the main burner will not stay lit, the cause is usually the thermocouple, gas control valve, or a draft issue.
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Quick answer
If the pilot light on a gas water heater lights but the main burner fails to stay lit, or the whole flame goes out shortly after, the most common causes are a worn or misaligned thermocouple that is not sensing the flame correctly, a failing gas control valve, or a draft blowing across the pilot or burner area. This is a distinct pattern from a pilot that will not light at all, and it points a technician toward a more specific set of components.
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A gas water heater's ignition sequence has more than one stage: the pilot must light, a safety sensor (the thermocouple or, on newer models, a thermopile or flame sensor) must confirm that flame is present, and only then does the gas control valve allow gas to the main burner and keep it flowing. When the pilot lights normally but the main burner will not stay on, or the whole system shuts down again shortly after, the fault usually sits in that safety-sensing step or in the valve responding to it, rather than in whatever ignites the pilot in the first place.
| Symptom | Likely cause | Category |
|---|---|---|
| Pilot lights, burner won't stay on, must hold button | Worn or misaligned thermocouple | Safety sensor |
| Pilot and burner both go out after a few minutes | Thermocouple or thermopile weak signal | Safety sensor |
| Pilot flickers or leans to one side | Draft affecting the pilot area | Draft / airflow |
| Burner lights briefly then valve closes | Gas control valve fault | Control valve |
A thermocouple is a small safety sensor positioned in the pilot flame that generates a tiny electrical signal as long as it senses heat, and that signal tells the gas control valve it is safe to keep the main gas supply open. If the thermocouple has worn out, shifted out of the flame's direct path, or become coated with residue, it can generate a weak or intermittent signal that is just strong enough to let the pilot stay lit but not strong enough to keep the main burner supplied, or that drops out shortly after the burner starts, closing the safety valve again. This is one of the most common, and most straightforwardly repaired, causes of this exact symptom pattern.
A pilot or burner flame that is disturbed by a draft, whether from a nearby exterior door, a strong exhaust fan elsewhere in the home, or the water heater's own combustion air intake being partially restricted, can flicker or momentarily lift away from the thermocouple enough to interrupt its signal, even if the thermocouple itself is in good condition. This is worth ruling out particularly in utility rooms or basements that have direct exterior access or share space with a laundry area where exhaust appliances run, both common configurations in Metro Vancouver homes with the mechanical room near an exterior wall or garage.
If the thermocouple and draft conditions both check out and the problem persists, the gas control valve itself may not be responding correctly to the thermocouple's signal, which is a less common but real possibility, particularly on an older water heater where the valve assembly has simply reached the end of its service life. A gas control valve is a specific, code-regulated part that a licensed technician sources and installs, and it is not something to attempt to bypass or work around, since the valve's role in cutting off gas safely if flame is lost is central to the appliance's safe operation.
You can safely observe the pilot flame's colour and shape through the sight glass, following your owner's manual, noting whether it looks like a small, steady blue flame or something weaker, larger, or unsteady, and whether it seems to sit directly on the thermocouple. Note any nearby drafts, such as a door often left open near the mechanical room. Relighting the pilot following the manufacturer's exact instructions printed on the appliance is generally safe for a homeowner to attempt once; repeated relighting attempts without success, or any hesitation about the process, should be handed to a licensed technician rather than repeated indefinitely.
Safety note
The thermocouple and gas control valve are safety components that prevent gas from flowing without a confirmed flame present. Do not attempt to bypass, hold open, or modify the gas control valve to keep the burner running. If you smell gas at any point, leave the home and call FortisBC at 1-800-663-9911.
This usually points to the thermocouple not sending a strong enough signal to the gas control valve to keep the main gas supply open, though a draft or valve issue can also cause it.
It is a relatively small, inexpensive part, but working on gas appliance components should be left to a licensed gas fitter to ensure it is installed and tested correctly.
No, if the burner requires you to continuously hold the pilot button to stay lit, that is a sign the safety sensing system is not working correctly and should be repaired, not worked around.
Yes, a draft that disturbs the pilot flame's contact with the thermocouple can interrupt its signal even if the thermocouple itself is fine, so airflow around the unit is worth checking.
A technician tests the thermocouple's electrical output directly; if that output is within specification and the valve still won't hold, the valve itself is the more likely cause.
It should be addressed reasonably soon since it typically means no hot water until resolved, but it is a controlled shutdown situation, not an active gas leak, unless you also detect a gas odour.
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