Direct answer: When a boiler burner lights and then shuts off, the usual causes are a lost call for heat, a blocked vent or condensate path, a dirty flame sensor, or a safety limit opening because the boiler is overheating or not moving water properly.
Most likely: Start with the easy tells: confirm the thermostat is still calling, check for any fault light or display message, look for a full condensate trap or drain issue on a condensing boiler, and note whether the flame dies smoothly after a few seconds or the whole boiler cuts out on limit.
First pin down the pattern. A burner that lights for 2 to 10 seconds and drops out points you one way. A boiler that runs for a minute or two, gets hot fast, then shuts down points you another way. Reality check: many boilers are doing exactly what they were built to do when a safety sees bad draft, poor water flow, or no proven flame. Common wrong move: cleaning random parts or cycling power over and over without watching what happens right before shutdown.
Don’t start with: Do not start by replacing gas valves, ignition parts, or control boards. On boilers, short burner run time is often a safety shutdown, not a failed major part.
You hear ignition, see flame briefly, then the burner stops while the thermostat is still calling.
Start here: Start with flame-sensing clues, vent intake or exhaust blockage, and condensate drainage if the boiler is a condensing type.
The burner stays on long enough to heat the boiler, then cuts off and may restart again soon.
Start here: Start with water flow and high-limit clues: zone valves, circulator operation, trapped air, or closed service valves.
It lights and drops out repeatedly, then shows a fault light or error code.
Start here: Read the display or indicator first. Repeated failed ignition or flame-loss lockout is a pro-service clue, not a parts-guessing job.
The boiler fires, then stops, but the real complaint is that one area never warms up.
Start here: That is usually a distribution problem, not a burner problem. Check the cold-zone path instead of chasing combustion parts.
If the burner lights and the boiler stops without a fault, the control may simply be losing the call for heat from the thermostat or zone control.
Quick check: Raise the setpoint several degrees and watch whether the thermostat still shows a heat call when the burner shuts off.
A boiler that lights cleanly and then drops flame within a few seconds often is not proving flame reliably.
Quick check: Watch the flame through the sight area if visible. If ignition is smooth but the burner quits almost immediately, flame proving is high on the list.
Condensing boilers and direct-vent units will shut the burner down fast if they cannot move flue gases or drain condensate properly.
Quick check: Look outside for blocked intake or exhaust terminations and check for water backing up at the condensate tubing or trap area.
If the boiler heats up quickly and shuts off hot, the burner may be doing its job while the water is not moving through the system well enough.
Quick check: Feel for very hot supply piping near the boiler with cooler return piping, listen for air noise, and confirm any accessible isolation valves are open.
Boilers cycle on and off normally once water temperature is satisfied. You want to separate a true fault from a thermostat or zone demand ending.
Next move: If the burner now stays on longer and the home starts heating, the issue may have been an intermittent thermostat or zone call rather than the boiler itself. If the thermostat is still calling and the burner still drops out early, move on to the shutdown pattern and visible clues.
What to conclude: A steady call for heat with an early burner shutdown means the boiler is stopping itself for a reason, usually flame proving, venting, condensate, or limit protection.
The timing tells you more than the symptom name. A 3-second dropout is a different problem than a 2-minute hot shutdown.
Next move: If you catch a clear fault code or a very consistent timing pattern, you have narrowed the cause without taking anything apart. If the behavior is random or changes from cycle to cycle, treat it as a higher-risk control or combustion issue and do not keep forcing restarts.
What to conclude: Very short flame time usually points to flame proving, venting, or condensate. Shutdown after the boiler gets hot points more toward limit or circulation trouble.
Blocked vent terminations, frozen intake screens, and condensate backups are common field problems and safer to inspect than internal gas components.
Next move: If clearing a blocked vent area or restoring condensate drainage lets the burner stay on normally, monitor the next few cycles and stop there. If venting and condensate look normal and the burner still drops out, the remaining causes are less DIY-friendly.
These two failures look similar from across the room, but the physical clues are different and the next action is different.
Next move: If the clues clearly point to one side, you can stop chasing the wrong half of the system. If you cannot tell whether it is losing flame or tripping on limit, do not keep cycling the boiler. Get a heating pro to test it live.
Boilers are high-risk equipment. Once the easy outside checks are done, the safe move is a targeted repair path or a service call with good notes.
A good result: A good service call starts faster when you can describe the exact pattern instead of saying only that the boiler shuts off.
If not: If the boiler will not run safely enough to heat the home, use backup heat safely and arrange prompt professional service.
What to conclude: At this point you have ruled out the common homeowner-level causes and narrowed the problem to the right side of the boiler system.
That usually points to flame proving, venting, or condensate trouble rather than a simple thermostat issue. If the thermostat is still calling and the flame drops out almost immediately, the boiler is not happy with what it is seeing and is shutting itself down.
Yes, on boilers that use flame rectification, a dirty or weakly proving flame sensor can cause a brief light-off followed by shutdown. But it is not the only cause, so it should not be treated as an automatic parts swap.
That pattern often means the boiler is hitting a limit because heat is not moving away fast enough. Air in the loop, a circulation problem, or a closed valve can make the boiler get hot quickly and cycle off even though the house is still cold.
No. One reset may be allowed by your manual, but repeated resets are a bad idea. Lockout is a warning that the boiler is not operating normally, and forcing it to retry can make diagnosis harder or create a safety problem.
No. Those are expensive, high-risk guesses on a boiler. Start with the heat call, fault code, venting, condensate, and shutdown timing. If those checks do not reveal a simple outside cause, this is a service-call problem.