If your boiler is in a cupboard, is it surrounded with clutter? (coats, shoes, bags etc). Boilers need ventilation, so the space around the appliance should be kept free from clutter.  It should be easy to access, so always make sure your cupboard is reasonably tidy.  Your plumber or engineer should be able to advise on whether your boiler has adequate ventilation.  Why not ask this question to them on their next visit!