The display lists a switch to check.

"Check Switch #" message at start-up

Cause: That switch has not closed for many games (roughly 30). The game works around it, but the switch, its wires or diode, its LM339 or U20 may be bad.

Checks

• Find the switch and check for broken wires or a broken diode.
• Test it in switch test.
• If wiring is fine, check its row LM339 and U20.

Fix

• Repair the switch, wiring or diode; replace a failed chip.

Systems

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

Related tips

• "Check Switch ##" message
• "Ground Short Row" message
• "Row Short" message
• Reading WPC switch numbers and the switch test display
• A switch does not register in the switch test
• "Switch is stuck On" message
• Using the switch edge test
• Check playfield switches with the switch edges test

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