I doubt this will be the problem so don't bother trying but I'll share my boiler woes from probably ten years ago now.
Combi boiler packed in around November so during the time you want it on. We had that British gas monthly payment scheme so called them out.
Guy turned up next day, spent five minutes and said it was the PCB, he would order one and it'd take 5 days.
Part came to house in 2 days, called but was told it'd still be on the 5th day before someone could come fit it. I wanted to do it myself but was persuaded not to by better half.
Different Guy came out, fitted it, still not working. He said it was the thermistors that needed replacing and said too many plumbers just think they need to change PCB without checking them.
Once again part was ordered and it would be five days.
When the plumber came (different one again) and fitted the new thermistors it still wouldn't fire up properly. He said he wasn't sure of the problem and will get someone to call me later.
Got a call to say they will send out someone to have a proper look at it, in yes you guessed it, 5 days!! My wife took kids and went to stay with her mum as she couldn't cope anymore ( I think our youngest was around 3 so not good for her either)
I got fed up so decided to pull the thing apart myself and look. I knew the issue was that it was only partially igniting and then it would go out. After a lot of checking and experimenting I noticed that the igniting wire (piezoelectric type) was quite low down. I got a pair of long nose pliers and pulled it up a bit and voila it fired up perfectly!!!
In fact it performed faultlessly for another five years until we moved house.
I've always wondered if the guy that was supposed to come would of discovered the problem or ordered more new parts.