I have a "Main 12 HE" boiler, which I think is a Baxi/Potterton? (certainly looks alike - but there's some confusion on the web as to whether "Main" is actually a manufacturer in it's own right)
In any case, looks identical to that given here:-
Anyway, as look would have it, to diagnose further requires some form of gymnastics normally associated with 1/2 bottle of vodka and a "Twister" game mat - as the boiler is installed in a cupboard in the corner, overlooking the sink.
Easier, would be to remove the thermistor and test on a bench.
Now, I'm not clued up as to how these boilers work - is the thermistor "wet" (i.e. if I remove it, does the hot water flow from the HW pipe that it's screwed into), or if dry, can I remove without draining the system?
Are these fairly standard items? Are the casting and flow thermistors one and the same type of device (in which case, considering natural ageing, is it worth replacing both?)
I did try getting a spanner on one, to slowly remove, but havn't the right size to hand, and it seemed to require gorilla strength (I was worried about putting too much force on with an ill-fitting spanner) - is this normal? (the installation manual suggested hand-tight then 1/2 turn).
Any info, as always, gratefully received.
Mike