It refers to a communication fault between the two circuit boards, I believe the display actually reads CON? It could be a fatal board fault or a poor connection between the two, with the power off, try separating and reconnecting the boards
Carefree
December 2016
You do a Google search for - say "Valliant 'Model' Com Fault" but you replace 'Model' with the exact model number of your boiler.
XXX
December 2016
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