You do not mention the boiler Make & Model and neither do you mention the failure mode...
There will sometimes be a microswitch in the Air Pressure Switch - to indicate that the fan is running OK .
There will sometimes be Two Microswitches in the Divertor valve (inside the box cover) - one makes at the half way point to make the motor stall and deliver C/H & H/W at the same time - the other makes at full travel when only C/H is required - it sends a "please fire-up" signal to the boiler when operated.
I am not qualified or experienced to give you this advice - there may well be other miocroswitches that I know nothing about...
Good luck...
Peccavi
July 2010