The game reports a missing ball.

"Pinball Missing" message

Cause: A ball is stuck on the playfield, or the outhole switch is not registering (broken wire or diode, its LM339, or U20).

Checks

• Look for a stuck ball.
• Test the outhole switch in switch test and check its wires and diode.

Fix

• Free the ball, or repair the switch wiring or diode; replace the comparator or U20 if needed.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• "Pinball Missing" message
• Balls hang in the trough: divots in the trough metal
• Old and new WPC ball trough designs
• Opto ball troughs from Indiana Jones on
• Two balls served at start and the lock spits its ball out: count the balls
• Full ball trough opto test in switch edge test

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part One, Clay Harrell (pinrepair.com)