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To reset the code, turn the power off to the boiler, wait 5 minutes then turn the power back on.
Meantime tell me if there is a sticker on the boiler that tells you what the 25e code indicates.
You could be having a problem with water scale build up in the boiler, and it might need cleaning with white vinegar solution....
If you can use the paper clip icon above to send me a few photo's of the piping connected to the bottom of the boiler, ideally taken from a low angle so that I can see where each pipe connects, that will help.
Take the pictures from 5 or 6 feet away and provide some strong lighting. such as a lamp or drop light, so that I can see if the installer had fit acid flush valves or not.
We can go from there.