Error L5

Safety first

1) If you smell gas, do not operate electrical switches or the boiler. Ventilate the area, turn off the gas at the meter or appliance isolation if you can do so safely, leave the property and call the gas emergency number immediately. 2) If you have any doubt about gas or water leaks, or if the boiler is visibly damaged, call a Gas Safe engineer immediately. 3) Do not remove the boiler case or attempt to access internal components unless you are a registered gas engineer; isolation of gas/electricity and internal work is dangerous and illegal for unqualified people.

Initial simple checks a homeowner can do

1) Turn the boiler off at the fused spur or isolating switch, wait 30 seconds, then turn it back on and use the boiler Restart button once. Note: avoid repeatedly pressing Restart — allow the boiler a few minutes between attempts. 2) Check the system pressure on the pressure gauge; for most S30 system boilers the cold pressure should be around 1.0–1.5 bar. If the pressure is low, follow your manual to re-pressurise the system using the filling loop, then try a single restart. 3) Check the condensate/drain route for freezing or blockage: follow the condensate pipe from the boiler to its outside termination; if it is external and frozen, thaw it with warm (not boiling) water or a warm towel; do not use a naked flame or excessive heat. 4) Check that external flue termination and air intake are not blocked by debris, bird nests or snow. 5) Check whether other gas appliances are working — if none are working, the issue may be the gas supply rather than the boiler itself. 6) Check the programmer/room thermostat is not rapidly switching the boiler on and off or causing very frequent demands.

Specific diagnostic and next steps

1) After doing the simple checks above, power-cycle the boiler fully at the fused spur for 30 seconds and then switch back on. Press the Restart button once and observe. 2) If the boiler starts and operates normally, monitor for recurrence. If L5 returns after repeated restarts, stop attempting further resets — the boiler will lock again. 3) Make a note of what happens when the fault occurs (for example: ignition sequence starts but fails, gurgling noise before lockout, fan runs then stops, pressure drops, or burner lights then flames out). These details are useful to an engineer. 4) If you found low pressure and topping up fixed the problem but pressure drops again, there may be a leak or filling loop issue — an engineer should investigate. 5) If condensate thawing or clearing a blockage allowed normal operation, consider insulating the condensate pipe or re-routing/protecting it — an engineer can advise a permanent fix. 6) If you suspect intermittent mains supply (lights dimming, other electrical faults) contact a qualified electrician or your electricity supplier; unstable mains voltage can cause repeated lockouts.

When to call a professional and what to tell them

1) Call a Gas Safe registered heating engineer if L5 returns after the basic checks and a single proper restart, or if the boiler locked out repeatedly and you cannot resolve it with the safe steps above. 2) Give the engineer the exact error code (L5), how many resets were attempted, what you observed (noisy, gurgling, ignition attempts, pressure readings), whether other gas appliances were working, and whether condensate or external blockages were found. 3) Do not attempt to swap or replace PCBs, ignition leads, gas valves, thermistors or other internal parts yourself — these require specialist training, calibration and gas/combustion checks.

Final notes

L5 is a protective response indicating a persistent problem. Temporary steps such as checking pressure, clearing condensate blockages and a single controlled restart are appropriate for a homeowner; any recurring lockout or signs of ignition/flame instability, gas supply problems or electrical issues should be investigated and repaired by a Gas Safe registered engineer to ensure safety and correct operation.