Boiler failing to ignite is one of the most common heating faults in London. Lockout codes, ignition failures, and when to call a Gas Safe engineer — a diagnostic guide.
Why a Boiler Won't Ignite
A boiler that fails to ignite will typically show a fault code or lockout light on the display. The burner fires (or attempts to fire) and goes out within a few seconds — this is the ignition sequence failing. Common causes in London properties:
Condensate Pipe Blockage
London winters cause the external condensate pipe (the white plastic pipe running from the boiler to a drain) to freeze — this is the most common cause of boiler lockout in cold weather. The boiler trips on a pressure fault and will not ignite until the condensate pipe is thawed. Pour warm (not boiling) water along the pipe and reset the boiler.
Ignition Electrode Fault
The ignition electrode produces the spark that lights the burner. If the electrode is cracked, dirty, or positioned incorrectly, it will not produce a reliable spark. This requires a Gas Safe engineer to diagnose and replace.
Flame Sensor (Ionisation Probe) Fault
The flame sensor confirms to the boiler control board that a flame is present. If the probe is dirty or failed, the boiler ignites but immediately shuts down (thinking there is no flame). Cleaning or replacing the probe is a common repair on London boiler callouts.
Gas Valve Fault
If the gas valve fails to open when the burner fires, there is no fuel — the ignition sparks but nothing lights. Gas valve faults require a Gas Safe engineer and may mean the boiler needs a new valve.
Low Gas Pressure
If your London property has low gas pressure at the meter (check by lighting another gas appliance), the boiler may not receive enough gas to ignite reliably. Report low gas pressure to Cadent (National Gas network) on 0800 111 999.
Frequently asked questions
How do I reset my boiler after an ignition failure in London?
Locate the reset button on your boiler — usually a button or symbol that looks like a flame with an arrow. Hold it for 3–5 seconds, then release. Wait for the boiler to complete its ignition sequence. If it lights and stays on, the fault was transient (e.g. a gas supply interruption or temporary pressure drop). If it locks out again immediately or within a few minutes, there is an underlying fault that needs a Gas Safe engineer to diagnose.
Is it safe to keep resetting a boiler that won't ignite in London?
You can safely reset a boiler up to 3 times. If it locks out 3 consecutive times, stop and call a Gas Safe engineer. Repeatedly resetting a boiler with an ignition fault can damage the ignition electrode and control board (each attempted ignition cycle stresses components). More importantly, if the boiler is failing due to a gas valve problem, repeated reset attempts will not resolve the issue and may indicate a safety concern. Three strikes, call an engineer.
Why does my London boiler ignite but then immediately cut out?
A boiler that fires briefly then shuts down (within 1–5 seconds of the burner lighting) is typically suffering from a flame sensor fault — the ionisation probe detects the flame correctly but the control board thinks it has gone out. This can be caused by: a dirty probe (carbon deposits preventing electrical conductivity), a cracked or failed probe, incorrect probe positioning (moved during a previous service), or a faulty control board. A Gas Safe engineer can diagnose this on the first visit in most cases.