This part number is cnt02216, I've been told by a heat/air company they will replace with new board and retro kit for $800.00. Well I called another company and he gave me price of $450.00 with new board part #cnt03457 and #kit06839 (conversion kit). Now I was thinking why not just change out bad component on board? Which part would go bad the relay switch or what? How could I test it? Maybe with meter tester? Thanks for any help at all.

One company has the old control board instock. The other Is selling you a board that is a retrofit to work with newer and older models. Either or will work and is just as safe. One is 1st generation one is 3rd generation controls.