One factor in the repair/replace question is how efficient is the furnace? If it is a 60% one, the heat exchanger is just one large round housing, then you can add savings on gas with a newer 80-90% efficient one to savings on maintenance. You already have most of the controls on an expensive to replace and hard to diagnose circuit board.
10 years ago when I replaced my 1970 60% one, the contractor convinced me I wouldn't save enough on on gas with a 90% over an 80% to pay for the increased maintenance. When I look at the problems here, it is mostly things like the control board, inducer blower, flame sensor, and limit switch which my 80% furnace has too. I see very few problems specific to the higher efficiency furnaces. Could be my contractor was looking at past problems that had been solved.
It is possible the blower motor is on its way out, drawing excessive current and burning up the relay on the board. It is also possible replacing the board jiggled something around and got it making better contact for a while.
If the blower won't run on manual, that bypasses the limit switches leaving the relay on the board and the motor itself as the most likely problems. Can you pull the plug to the blower motor off the board and jumper 120V to it? If it runs, the relay on the board could be bad again. Note, be careful not to jumper the 120V to yourself.
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