Have HVAC Questions? Ask an HVAC Expert for Answers
Hello, the fact that the problem is *intermittent* points directly and almost without exception to bad printed circuit card in the zone control panel.
Use a hair drier to slowly warm the printed circuit card up to 120F or a bit warmer, but not over 140F... if there is any change in the error codes or other system behavior... the card has a cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.
No change does not prove the card is good however... but a change proves the card is definitely bad.
Let me know what you can discover.