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Safety precautions:

- If you smell gas: do not operate electrical switches. Turn off the gas at the meter if it is safe to do so, get everyone out of the building and call the gas emergency service immediately.

- If in doubt, isolate electricity to the boiler at the fused spur or consumer unit before touching the boiler or controls.

- Never attempt to access or work on internal gas, valve, burner, or PCB components unless you are a Gas Safe registered engineer.

Initial checks a homeowner can do:

1. Check the obvious: make sure the boiler has power (display or neon lights), and that the mains fuse or isolator hasn’t tripped. Check other electricals in the house to confirm there wasn’t a recent power cut.

2. Check the gas supply: confirm other gas appliances (hob, oven) work. If none work, check your gas meter and contact your gas supplier. Do not attempt internal repairs to the gas supply.

3. Check room thermostat, programmer and room stat batteries. Replace batteries if low and ensure the boiler is actually being called for heat/hot water.

4. Check the pressure gauge on the boiler: if pressure is below about 0.5 bar the boiler may not ignite. If it is low, you can repressurise the system using the filling loop (consult your boiler manual) but only if you are comfortable doing so.

5. Look at the outside flue terminal and condensate discharge: remove any visible obstruction (leaves, bird nest) and check for frost/ice on the condensate pipe if temperatures are low.

Step-by-step diagnostic and simple fixes:

1. Reset the boiler (one attempt only):

- If your unit has a RESET button: press and hold until the lockout light goes out and the boiler attempts to relight.

- If it uses a control knob: turn the central heating temperature knob fully anticlockwise, hold until the lockout light goes out, then return to your setting.

- If it has an ON/OFF/SELECTOR switch: open the front panel, turn the selector to R (Reset) and hold for at least 5 seconds then release. Watch to see if the burner lights.

2. Observe the outcome: if the boiler lights and runs normally, monitor for repeat lockouts. A single lockout after a temporary issue (power cut / frozen condensate thawed) can be resolved by reset. If it locks out immediately or repeatedly, further action is needed.

3. Condensate pipe freeze (common in cold weather): if the condensate pipe is frozen it can cause ignition timeout. Turn the boiler off, thaw the frozen external condensate pipe with warm (not boiling) water or a hot water bottle and clear any ice. Insulate/lag the pipe afterwards to prevent recurrence.

4. Check for blocked flue: clear any obvious debris from the flue terminal outside. If you suspect the fan or flue components have failed, leave it for an engineer.

5. Check system pressure again after reset and adjust to the normal running pressure (check the manual for correct pressure). If pressure drops repeatedly you will need an engineer to find the leak or failing component.

6. Gas supply check: if the gas has been interrupted or the gas valve is not supplying fuel the boiler will fail to ignite. If other gas appliances work and you still see lockout, do not try to repair gas components yourself; call a Gas Safe engineer.

7. Flame detection/ignition faults: these often cause three failed ignition attempts then a lockout. Cleaning a flame sensor or ignition lead can sometimes help, but this requires isolating power and gas and accessing internal components. If you are not trained or your manual does not explicitly instruct you, do not open the boiler – call a Gas Safe engineer. Incorrect work here is dangerous.

When to call a professional (and why):

- If the boiler continues to lock out after a reset or re-locks frequently. Repeated lockouts indicate a persistent fault that needs diagnostic testing and potentially replacement parts (ignition electrode, flame sensor, PCB, fan, gas valve).

- If you smell gas, experience gas supply issues, or see signs of combustion problems (unusual noises, persistent fault lights) you must contact the gas emergency service or a Gas Safe registered engineer.

- Any internal repairs, testing of ignition or gas valves, replacement of safety devices, fan replacement, PCB work or pump/electrical component replacement must be carried out by a Gas Safe registered heating engineer.

Additional notes:

- If your boiler manual shows an E133/E120/E168 etc alongside neon indicators, cross-check the complete code sequence in the manual – some codes are multi-digit and further detail may appear after the initial light pattern.

- If the fault only appeared after a power cut or storm, a single reset may cure it; if it returns call an engineer.

- Keep a record of when lockouts occur and any actions taken – this helps the engineer diagnose intermittent faults.

Remember: resetting the boiler is a temporary step to restore service. Persistent ignition lockouts are a sign of a component failure or system fault and should be investigated and repaired by a Gas Safe registered engineer for safety and to ensure reliable operation.