Worcester Bosch repeatedly cutting out and going to reset

Hi Forum members,

We have a problem with our 7 year old WB Greenstar 24 or 28i Junior (not sure which one it is as there is nothing on the boiler to distinguish) and already had a plumber specialised in WB boilers here several times who cannot find the fault.

The boiler initially cut out after a while of running hot water, running two tabs or the central heating at the same time. The blue light is then normally off and flashes up once followed by two flashes of the red reset light.

The plumber changed the PCB which seemed to make a difference in the first 1/2 hour and at the next visit adjusted the CO2 content to within the limits which hasn't helped at all in so far as the central heating is concerned. Well, we now can have a 4 minute shower (easy with short hair!) - if at all - provided the boiler system is cold. If not, it doesn't last at all and goes into reset pretty much immediately when the tap is opened. The heating doesn't last either, it cuts out pretty quickly too now - I am actually sitting in a freezing cold house as I'm typing.

There is also an anomaly with the green flame light. It sounds as if the boiler is igniting and heating up but the green light for the flame doesn't come on until later ... that's if it get's a chance at all before the boiler goes into reset mode.

The helpline at WB doesn't seem to have a clue what the cause is either. The pressure is ok, the system does not seem to lose or build up pressure anywhere in excess of the normal.

We have already spent some £300, have now a replacement PCB laying about (but the money out of the pocket) and the plumber has other jobs to go to.

Has anyone out there come across this or a similar problem and can help us?

Thanks a lot in advance!