What should I do if my boiler pilot light will not stay lit?

What this symptom usually means

A pilot light that ignites but will not stay lit points to a failure in the pilot safety circuit, not just a general no-heat condition. On standing-pilot boilers, the flame has to stay wrapped around a thermocouple or thermopile long enough for the gas valve to sense a stable flame; if that proof is lost, the valve closes and the pilot goes out. That is why this symptom is different from a boiler that runs and then shuts off later, leaks water, loses pressure, or makes noise.

What you can notice before a technician arrives

What the pilot flame looks like matters. If it is tiny, yellow, flickering, or seems to lift away from the sensor, a dirty pilot or air disturbance is more likely than a bad circulator, pressure issue, or thermostat problem. If the pilot lights normally while you hold the knob down but dies the instant you release it, a weak thermocouple, weak thermopile, or faulty gas valve climbs to the top of the list. If the problem began suddenly after years of normal operation, a failed sensor is common; if it worsened gradually with more relight attempts needed over weeks or months, buildup in the pilot assembly is often involved.

When to treat it as more than a routine repair

Do not keep relighting the pilot over and over. Repeated failed lighting attempts can flood the combustion area with gas. If you smell gas, notice fumes, or your carbon monoxide alarm sounds, leave the building immediately and call the gas utility or emergency services from outside. If there is no gas smell and the pilot simply will not remain lit, this is usually a schedule-soon boiler repair rather than the kind of symptom that points to a water leak or a pressure-only issue.

What a service visit usually includes

A boiler technician will typically inspect the pilot flame pattern, test the output of the thermocouple or thermopile, verify gas pressure, and check whether the gas control valve is holding the pilot circuit open. They may clean the pilot assembly, confirm proper flame contact on the sensor, inspect for venting or draft interference, and then relight and cycle the boiler to see whether the pilot stays stable. On older boilers, the visit often also includes checking whether replacement pilot parts are still available or whether the gas valve has become obsolete.

What affects repair cost

Cost depends heavily on which pilot-specific part has failed. Cleaning a pilot assembly or replacing a thermocouple is often at the lower end, while replacing a thermopile or gas control valve is usually more involved and more expensive because the technician has to verify safe burner operation afterward. Typical 2025-2026 U.S. repair ranges are about $150-$350 for pilot cleaning or thermocouple replacement, roughly $250-$500 for many thermopile-related repairs, and about $400-$900 or more for a gas valve replacement, with higher totals possible on older boilers or when exact parts are hard to source.