After the batteries have been replaced, it'll take a little while for switch errors to report again. The game keeps a note of which switches haven't been activated for a while, but that gets reset when the batteries are out. When a switch hasn't been hit in a certain number of games, then you'll get the "check switch" message popping up.

Careful. The diode board under the playfield only handles the switched (A/C relay controlled) coils / flashers. All of the other coils (regular controlled and special) absolutely do need the diode on them. It wasn't until Big Guns that all the diodes were removed from the coils, they were then mounted on a dedicated auxiliary board inside the backbox.

The frayed / chafed wire in the picture above could cause issues though - if that brushed against some other component you'd get voltages doing bad things, enough to fry a transistor or two for sure.