BoilerFaultCodesThe 6 most common faults
Your boiler is failing to start up because the small ignition flame is not lighting properly.
Why it happens: This fault occurs when the boiler's safety system cannot detect a steady pilot flame, preventing it from safely firing the main burner. The most common cause is a worn-out thermocouple that no longer senses heat, or a tiny blockage in the pilot injector that stops gas from reaching the ignition point.
Full guide for 49Your boiler is failing to keep its small starter flame lit, which prevents the main heating system from firing up.
Why it happens: This fault occurs when the boiler's safety mechanism fails to detect a constant flame, causing it to shut off the gas supply for safety. The most common reason is a worn-out sensor called a thermocouple that can no longer signal that the pilot light is safely lit, or a slight blockage in the pilot burner preventing a strong flame. In some cases, it may also be triggered by an internal safety switch that has tripped to prevent the unit from overheating.
Full guide for 50Your boiler's small pilot light has started, but the main burner is failing to ignite to provide heating and hot water.
Why it happens: This fault occurs when the boiler successfully lights its small pilot flame, but the main burner fails to catch light to start heating your home. This is usually caused by a blockage in the pilot injector that prevents the flame from reaching the sensor, or a mechanical failure within the gas valve that stops the main flow of fuel.
Full guide for 51Your boiler has failed to ignite the gas flame after three attempts and has locked itself out for safety reasons.
Full guide for L FYour boiler's electronic brain is experiencing an internal communication error and has safely shut itself down to prevent unpredictable operation.
Full guide for L EYour boiler has detected a failure to ignite or a loss of flame during operation, and it has safely locked itself out to prevent gas buildup.
Full guide for L Aon the Ideal Mexico Super 100FF / 125FF
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Ideal shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue