I had an issue on my combi and I called out a gas safe engineer for a repair and to service it, was an intermittent issue which I thought was related to the PCB.
His diagnosis was dunno could be anything chief, you need a new boiler as I’ve never seen one of these🙄, it’s a Halstead clunk whoosh type of boiler not terribly modern but easy to work on.
So I dug out my kit off the van and traced it to a dry solder joint on PCB ordered a replacement of ebay 2nd hand and that was seven years ago.