Have all the targets up, ball out, put the machine in switch test mode (button inside front door).  Should show ''0'' (no switches closed)

Close switches one at a time and make sure the correct number comes up (refer to manual).

One odd one can be a dud diode, if lots are showing the wrong numbers it is most likely the signals from the MPU (bottom left connector) to the SDB (small middle right hand connector)

Report back your findings