Technical description
Ignition sequence: the spark starts and the gas valve opens.
What causes this fault?
This fault occurs when the boiler's control system fails to confirm that a flame has successfully lit after opening the gas valve. The most common reasons are a worn-out sensor that can no longer 'see' the flame, or a fault within the gas valve preventing fuel from reaching the burner. Over time, soot buildup or moisture can also interfere with the electrical signal required to keep the boiler running.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.