The game reports "Check Switch" with a number.

"Check Switch ##" message

Cause: That switch is stuck closed or has not closed in the last 30 games.

Checks

• Find the switch with the switch number and test it in the switch test.

Fix

• Repair or adjust the switch as for any matrix switch fault.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• "Check Switch #" message at start-up
• Clearing a switch flagged in the power-on test report
• Reading WPC switch numbers and the switch test display
• "Row Short" message
• WPC switch numbers give the column and row
• "Switch is stuck On" message
• Ball lock bar holds balls and Check Switch 74 shows: top lock switch not being hit
• Using the switch edge test

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer