Switches 11 and 12 do not register, or flipper lane change does nothing.

Flipper buttons not seen by the game (pre-Fliptronics)

Cause: The 4N25 opto-isolators on the driver board (U7 left, U8 right), or less often U20 or U18 on the CPU board.

Checks

• Watch switches 11 and 12 in switch test while pressing the buttons.

Fix

• Replace U7 or U8 (4N25) on the driver board; if still dead, check CPU U20 and U18. Fliptronics boards no longer carry these chips.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix

Related tips

• Pre-Fliptronics: flipper lane change does not work
• Funhouse: flipper buttons won't step through high score initials
• Row or column "ground short" errors
• Pre-Fliptronics: a flipper does nothing
• Lane change not working on Taxi and later System 11 games
• Pre-Fliptronics: weak flipper
• Phantom switch closure that survives a new CPU board: bad 4N25 on the driver board
• Upper right flipper only buzzes while the magnet holds the ball: failed opto-isolator

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