The reason for failures isn’t usually anything to do with the heating part of the boiler itself, it’s more likely to be one of the associated parts or a problem in another part of the system. So which of the other parts can fail and leave you looking at a repair? Time switches tend to be fairly simple things, as they are mechanical devices and not much can go wrong with them. They can get a bit gummed up over time, so the clock dial might run slow and make the timing go off. They are easily replaceable if required and could be done as part of your regular service.