Technical description
Ionisation occurs when heat is no longer required, the gas valve does not close.
What causes this fault?
This fault occurs when the boiler's sensors detect heat or a flame after the heating cycle has finished. The most common reason is a mechanical failure in the gas valve that prevents it from closing fully, or a faulty sensor that is incorrectly reporting a flame is still present. Because the boiler believes gas may still be burning when it should be off, it will shut down immediately for your safety.
Engineer required
This is a emergency severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.