Having an odd controller problem with a 17 yr old Carrier 58SXB. It has the original controller (HK42FZ002) and is showing code 43 for a draft proving fault. The pressure switch lines are clear and the switch is working so I replaced the controller with a used one that I believe to be good. Same problem!

I chased the signal from the switch down to the controller, and I can see the 24vac signal come and go on the board connector as the inducer motor starts and stops. The furnace runs just fine on emergency heat on both controllers, it just won't recognize that the LP switch has closed.

I have a hard time believing that two boards have failed in exactly the same manner. Has anyone seen a grounding problem or anything else that would affect both controllers?

My next move is probably to replace the controller with the upgrade, but I don't want to go to the expense and trouble if it still isn't going to run.