In message , obclare
writes
I have a problem with an Ariston a/23 combi boiler that has beaten
British gas and a handful of gas engineers.
The system fires up and immediately fires down again and fires up
again immediately and so on. Anybody any ideas? Various bits have been
replaced but to no avail. Very occasionally the system works.

Is the high gas coming on blowing the flame out?

Is the high gas coming on blowing the flame away from the flame sense
electrode?

Dicky relay on the control board ?

You don't say what bits have been replaced

--
geoff