Quick fix for a nonsensical Vaillant boiler fault

[John Rumm]

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.

[Brian Gaff]

One other issue is corrosion of the pins or socket and moving in and out can fix the issue. The low current through these sensors does not keep them clean. This is not just on boilers seen it in all sorts of equipment with connectors unfortunately. I doubt if they are gold plated are they! Brian

[Graham.]

Assuming the sensor had a negative temperature coefficient, isn't that what you would expect from an open circuit sensor?

[John Rumm]

Yup pretty much - pretty sure its just an NTC thermister.