Overview
On a Keston S30 system boiler the L2 fault code means the boiler has detected flame loss during the ignition sequence and has gone into ignition lockout. The boiler will normally try to light several times and, if the flame fails to establish or is lost, it will display L2 to protect against unburned gas. The official manual describes L2 as a flame loss/ignition lockout condition and notes the boiler will show this code after repeated failed ignition attempts. Severity ranges from relatively minor and easily fixed (for example a blocked or frozen condensate pipe, a blocked flue or temporary low gas supply) to more serious component faults (faulty flame sensor/electrode, gas valve, fan, or control electronics). Because L2 relates to gas ignition and flame detection it is a safety shutdown; you can carry out a few safe checks and resets yourself, but any internal checks, component replacements or persistent/repeating L2 faults must be diagnosed and repaired by a Gas Safe registered engineer (or equivalent in your country). If you smell gas or suspect a leak, treat it as an emergency and follow the gas-safety steps below rather than attempting troubleshooting.
Possible Cause: Flame loss
Troubleshooting Steps
Safety precautions:
1) If you smell gas: stop using gas appliances, do not switch electrical items on or off, ventilate the property, evacuate occupants, and call the gas emergency number immediately. Do not attempt any repairs.
2) Do not remove the boiler casing, attempt to adjust the gas valve, or carry out ignition/flame electrode work unless you are a registered gas engineer. Working on gas appliances is dangerous and illegal without appropriate certification.
3) If the boiler is repeatedly locking out and you cannot restore heating/hot water, switch the boiler to OFF and isolate the electricity if advised in the manual, then call a Gas Safe engineer.
Initial homeowner checks (safe, simple checks you can do):
1) Note the exact code (L2), when it happened and any noises (e.g. gurgling) or circumstances (very cold weather). This helps the engineer diagnose the cause.
2) Check other gas appliances (hob, gas fire) to confirm the property has a gas supply. If nothing works, check the gas meter and any isolation cock or the prepayment meter credit.
3) Check the boiler display and controls: make sure external controls (programmer/timers, room thermostat, cylinder thermostat) are calling for heat and are set correctly. Set mode to BOILER ON and try a restart.
4) Attempt a single reset using the boiler RESTART button as described in the manual. Watch and listen during the ignition sequence. If it lights and runs, monitor for recurrence. Do not repeatedly reset more than the manual allows.
5) Check the system pressure gauge (if fitted) and ensure it is within the recommended range (typically about 1.0–1.5 bar for sealed systems). If the pressure is low you can top up using the filling loop only if you know how; consult the manual or wait for an engineer if unsure.
6) Inspect the external flue outlet and immediate surroundings for obvious blockages (bird nests, leaves, debris) and remove safely. A blocked flue can prevent proper combustion and cause ignition failure.
7) In cold weather check for a frozen condensate pipe or frozen condensate discharge outside the property. A gurgling noise before lockout and very cold conditions are common signs. If you suspect freezing, carefully pour warm (not boiling) water along the external condensate run to thaw it, or temporarily insulate the pipe and call an engineer.
Specific diagnostics and steps to consider (what the engineer will check if the simple fixes don’t work):
1) If the boiler makes ignition noises but no flame is detected the likely causes include faulty flame detection electrode/sensor, faulty ignition electrode, gas valve failure, weak/absent gas supply or a problem with the combustion fan or air supply. These are not DIY fixes.
2) Recurrent gurgling before L2 points to condensate drainage issues (blocked condensate trap, pump or pipe) which may require removing the boiler cover to inspect/clear the condensate path—an engineer job.
3) If the boiler tries and then locks out quickly, the control electronics and wiring (PCB, connectors, flame sensing circuit) may be at fault. Engineers will check flame signal levels with test equipment and inspect the control box.
4) Low mains voltage or fan faults can also prevent proper ignition on some S30 faults; an engineer will check voltages, fan signals and motor operation.
When to call a professional and what to tell them:
1) Call a Gas Safe registered engineer if the L2 code does not clear after the safe checks and a single reset, if the fault recurs, or if you are not confident performing the safe checks listed.
2) Provide the engineer with: the exact fault code (L2), frequency and timing of lockouts, any noises (gurgling), whether other gas appliances were working, recent servicing history, system pressure readings, and whether the condensate pipe was frozen or the flue was obstructed.
3) The engineer will have the skills and tools to test gas pressure, inspect/replace flame electrodes or sensors, check and replace gas valves, inspect the combustion fan and PCB, clear condensate traps, and fully recommission the boiler safely.
Final notes:
- L2 is a safety lockout for flame loss; do not ignore repeated lockouts.
- Simple homeowner actions: check gas supply, reset once, check pressure, clear external condensate freeze or flue obstruction.
- Any internal inspection or component replacement must be done by a Gas Safe registered engineer. If at any point you smell gas or suspect a leak, follow the gas emergency procedures immediately.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston S30 System Gas Boiler.