Where can the thermister sensor be found and how can it be replaced?

you probably mean the thermocouple device these can be found sitting on the pilot light flame,

ANON
May 2011

how many boiler manufacturers are there?
how many models does each manufacturer produce?
that narrows the search down to only a few thousand boilers
al with their own individual eccentric component layout
now all i have to do is somehow manage to pick your exact make & model boiler from the thousands available with no further information available
its like taking your car to a garage
telling the mechanic somethings wrong with it
& not letting the mechanic know or see what make/model car you have
essential information:-
boiler make
boiler model
any fault indicator lights flashing
in what sequence
does the problem affect DHW or CH or BOTH

manwithnoname
May 2011

Look in the manual

b
May 2011