Just in case anyone is getting the same error:

Odd one on a friends boiler (Vaillant Ecotec 824+ combi). Turn it on
from cold, and it goes through a moment of self test before reporting
fault code 76 "main heat exchanger over temperature". Which is quite an
achievement for a cold boiler that has not even lit the burner yet ;-)

So I popped the cover off, had a fiddle with the connectors on the
temperature sensors on the pipes connecting to the input and output of
the main HE. Basically just to check they were seated properly (to be
fair they did not feel loose the touch). Then I turned it on again, and
this time it all sprang into life normally. So probably just a lose
connection or even a bit of condensation in a connector (the boiler had
been relatively unused for a couple of months).

So I figured we could count that as a false positive, so went into the
installer mode[1] menu and erased the fault code history.

[1] The default installer code is 17 for many Vaillant boilers.




--
Cheers,

John.

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