yes you're right it is a internal sensor,the A03 error code is related to the over temperature protection heating sensor ,this is fairly common in your boiler especially if there is a demand for heating and hot water.now you could try resetting the boiler,in some cases it resolves the problem but more often than not a engineer will be needed ,before he extends the time limit on the ophs he will do various other check including filter change cleaning out the venturer and so on ,it is not something you should try yourself .
mech monster, October 2011
