Gas boiler intermittent ignition fault

[Sumar]

I have a gas condensing boiler - Potterton F2 50 (10 years old).
The electronic ignition fires a pilot light, the boiler lights, runs for a few minutes then clicks and drops back to pilot light for about 1 min then clicks and restarts. This process will be repeated several times, the boiler may then run normally for a while or switch off. The fault appears to be intermittent and occurs when running either HW or CH or both. PCB was changed about 10 months ago as boiler stopped working altogether. Boiler has been serviced, engineer thinks this may be the gas control valve but does not seem to be confident; he left saying he would source a valve but I have heard nothing more. He also told me that the fan is looking the worse for wear. I am reluctant to chase him up without a better understanding of the possible cause – can anyone advise me over this.

[Ronald0]

Does seem to point to a gas valve fault. Main burner solonoid is dropping out; not holding main burner open.

Sounds like boiler still has demand from a remote stat.

If it was me I would verify switched live present, then quickly check for voltage at main burner solonoid.

[Sumar]

Thanks for your reply, Ronald0.
Unfortunately I do not have the skills to check the power as suggested so I chased up the heating engineer. He finally returned and changed the gas valve but the problem still persisted. He then said it might be the PCB and swopped it for a spare he had in his van. The boiler now appears to be working fine. He has taken the PCB he removed back to the supplier as it was less than a year old; it had been changed last year to try to resolve the same problem. So I am not sure whether a faulty gas valve damaged the PCB or whether I was unlucky with the replacement PCB in that it was faulty. I still have the old gas valve if there is any way of checking it once it is out of the boiler?

[htg engineer]

Could just be a problem with the flame sensing electrode/lead.

htg