A boiler that starts and then stops may be doing exactly what a safety control was designed to do. The display, indicator light, or control history can preserve the reason. Repeatedly resetting the boiler can erase useful evidence and force another unsafe attempt.
Write down the sequence
Note what happens after the thermostat calls for heat. Does a pump or fan start? Do you hear ignition? Does flame appear and then drop out? Does the unit run until it reaches temperature? Which code appears, and does domestic hot water fail too? A phone photo of the display can be more useful than a broad statement that it “will not stay lit.”
Older standing-pilot equipment and modern electronic-ignition boilers use different controls. A thermocouple explanation does not apply to every boiler. Use the model and fault information rather than naming a part before testing.
Common fault categories
Ignition or flame proving
The appliance must establish and prove flame within its programmed sequence. An electrode, wiring, grounding, gas-pressure, burner, control, or combustion problem can interrupt that sequence. The code narrows the test but does not prove which component failed.
Airflow, venting, or condensate
A blocked intake or exhaust, failed fan, pressure-switch condition, frozen drain, full condensate pump, or obstructed trap can stop a condensing boiler. Homeowners can look at accessible outdoor terminals for snow or debris, but should not dismantle venting or bypass a switch.
Water flow, pressure, or temperature
Low system pressure, a failed pump, closed valve, air problem, blocked heat exchanger, or control fault can cause a limit shutdown. Do not add water or change a fill valve unless you understand the system and manufacturer procedure.
Power and control
Loose connections, low voltage, a tripped breaker, thermostat wiring, zone controls, and communication errors can create intermittent operation. Check the service switch and breaker once. A breaker that trips again needs diagnosis.
Lockout and short cycling are not the same symptom
A lockout normally leaves a code or indicator and requires a defined reset or service action. Short cycling describes repeated brief runs that may end without a hard lockout. The latter can come from control settings, low water flow, an oversized appliance, a small active zone, rapid temperature rise, or a thermostat and zoning problem. Timing each run and noting which zones are active gives the technician better evidence than counting burner sounds from another room.
Also note whether the shutdown occurs only during space heating, only while making domestic hot water, or during both. On a combi or indirect-water-heater system, that distinction changes the control path being tested. Do not raise limits, force a zone open, or defeat a domestic-hot-water priority setting to keep the burner running. The correct target is the condition causing the shutdown, not a longer run at any cost.
Stop immediately for these conditions
- A gas odor, soot, scorch mark, or visible flame outside the burner area
- A carbon-monoxide alarm
- A loose, blocked, or damaged vent
- Water at electrical parts or active relief discharge
- A rollout, blocked-vent, low-water, or other safety that will not stay reset
If you smell gas, leave and call 911 from outside. If a CO alarm sounds, move outside to fresh air and call emergency services. The Consumer Product Safety Commission says not to ignore a CO alarm or try to locate the source yourself. Its carbon-monoxide guidance also recommends annual professional heating-system inspection.
How the technician uses the code
The code identifies the part of the sequence that failed. The next step is the manufacturer's test procedure: verify inputs, observe operation, take measurements, and rule out wiring or system causes before replacing a component. After repair, the boiler should complete a heat call and the relevant safeties and combustion should be checked.
Prepare for a cold-weather shutdown
If the house is losing heat, protect occupants first and follow safe temporary-heating instructions. Do not use an oven or range to heat the home. Identify vulnerable plumbing and arrange service before indoor temperatures approach freezing. The repair arrival time depends on weather, call volume, parts, and access, so no article can promise a specific response.
See our heating-system service page for boiler diagnostics. For broader maintenance and replacement questions, read the boiler repair and maintenance guide.