I'd try reseating the ASIC chip. Any acid damage around the game ROM or ASIC? At a last resort maybe pull the CPU out of another game, swap ROMS and see what the other game's CPU does in NF and what the NF CPU does in the other game. At least it could tell you if it's ROM or board issues.

It's likely that a switch hasn't been activated after so many games ... aka bad switch. Take the glass off, start a game, take ball out and press all the switches. Put the ball back in. Shut off. Turn on. Error still exist? Also ensure time and date are set. If both of those ideas fail I suggest you hit 6 bongs.

Is there a credit dot? That's the first place to start.

The bongs from the sound board won't have anything to do with any CPU issues.

The meanings are right in the NO FEAR manual...
2019-07-03 18_14_19-Williams No Fear Manual (resized).png