Ariston Combi Boiler goes over pressure

I have an Ariston Micro Genus Combi boiler. When it turns on in the morning generally the pressure guage builds up to about 4 bar then the boiler cuts out. I had an engineer in about 6 months ago but as usual the problem didn't show then. He suggested it may be the pump and mentioned something about checking the water pipes to see if they're hot. I can't remember exactly what he said to check though. Anyone have any suggestions ? It is a Wilo Gold pump in the boiler if that helps any.



Thanks, Stephen.

Get the expansion vessel checked for correct pressure

I can't say I have had the pressure checked this time but the first time the problem happened the engineer checked it and it was fine. At the moment the bolier is going fine. Wouldn't a drop in expansion vessel pressure be a solid fault rather than intermittant as it now seems to be ?

Thanks again, Stephen.