Can mice enter sealed room boiler from outside the house?

I have a problem with mice getting inside a sealed room boiler - I have sealed the garage door and some other possible entry points around the garage, but I was wondering if it might also be possible for a mouse to climb into the boiler through the horizontal flue, which is only a couple of meters or so from the ground outside. Or I was also wondering about the copper overflow pipe, which I think is either 15 or 19 mm?? Thank you.

Are you sure they are just not sealed in there now?

Check any cardboard boxes. They like living in those.

mm means you are from across the pond. Welcome!

Thanks. Just wondering if it would be possible for a mouse to get in through the flue and then into the garage, bearing in mind that they can get in through very small spaces. I don't know if a mouse would be able to get all the way.