Quoted from slochar:

If it were the mpu when you disconnected J2 (playfield switch harness) it wouldn't have gone away.

Don't fall into the trap of thinking it's going to be the worst possible outcome. It's very likely that it's something simple like a mashed diode somewhere, or something else shorting out that the game interprets as that switch.

You really want to spend between $100-$200 on a board that might not even fix it? You're out the $$ and you have to fix the original issue anyway.