First of all you have to discover whether your problem is related to mechanical failure or related to temperature. When heating controls are calling for heat ie thermostats on boiler programmer and room stat if fitted. Is there a poblem with the diverter valve assembly to the left hand side of boiler the demand for heating will cause the pin on the upperside of the valve to move and make contact with themicroswitch you can see this action visibibly if you look. If your hot water is all ready working then this action will also happen at the front of the diverter assembly (the same action will apply to the top section also) if this is not the case is pin showing signs of being scaled or check diaphram condition, this envolves disassembly of valve. If pin is moving and engaging micro switch then change microswitch. If problem is temperature related then go to temp sensor on flow pipe in left hand side top corner of boiler check for resistance on pins after removing plug (check when cold, should be about 10-11 ohms) when hot boiler will shut down when resistance is around 1-2 ohms. Replace if in any doubt. Also in some cases the problem may relate to the main PCB which means it will have to be changed. i hope this info will help you.
Mark Cadan (heating engineer), February 2007
