Overheat safty thermostate?

I have replaced the overheat saftey thermostate 1 and 2 on the S24 compact boiler. The boiler functions perfectly for the first 24 hours and then the problem re-occurs, i.e. the boiler indictates that the safety thermostate is faulty.

Any ideas what can be wrong? also if I was to purchase a new safety thermostate, before installing it how can I check that its correct and not a faulty part?

Thanks very much in advance. Any help, tips welcome.