I've got to repair a Protherm 80e Combi boiler which operates properly sometimes and other times just stops delivering both heat and HW. No fault codes shown on the display. Seems to fix itself randomly and fail again later on.
Microswitches seem to be OK.
In the past (for other reasons) I've changed the PCB (it wasn't faulty after all) and also the DHW thermistor and the diverter valve diaphragm.
Parts still to check are the fan flow switch, Overheat stat and CH thermistor.
When it's actually working it seems to work normally for both HW and CH, although turning the room stat down and then up again seems to trigger it into failure mode.
I've got the manual but there are no clues for this fault..... anybody with any ideas?