Post again with the make and model of the boiler. Are there any error lights or messages showing?
As Anon said: it could be the thermocouple (not thermocoupler) but whether there is one at all will depend on exactly whoch boiler you are talking about.
Post a fresh question with enough information for people to look up the make, model and symptoms - or do a Google search or two for yourself.
Acp, August 2017
