Hello again, before I retired I was a licensed boiler contractor in in California and worked with literally thousands of boilers... however, that was still less than 1% of all boilers... over a 50 year period. Even then I relied on manuals for much of what I did... and I still rely on manuals today.
That is true in this case as well, it is something like flat tires, you do not need experience on each tire size and type to diagnose it as a flat tire, then you inspect, and sure enough its a nail in the tire for instance.
Same here, so far I have pulled the manual on your boiler, because you left out the full model number and age if the boiler. So I opened with a trouble shooting tactic that works every time on any system at all with a similar problem.... even in your case if there is no pump or fan wired directly to your printed circuit card.
By unplugging one circuit at a time, you can determine which has the short.
That works every time, because it is a basic ground floor level approach. So far here on just answer my ratings score is 98.8% successful. Let me know if you would like to continue with me on that basis or not, if you do, get me the full model number and age of your baxi boiler.
If not I can opt out, what ever you wish