Closing one switch (here the shooter lane) also shows a second, unused switch in switch test. New diodes, a new opto board, new switch matrix chips and even a new CPU board do not cure it.

Phantom switch closure that survives a new CPU board: bad 4N25 on the driver board

Cause: One of the two 4N25 optocouplers on the -1 version of the power driver board is damaged. They exist to tell older (pre-Fliptronics) games when a flipper fires, and a bad one loads the switch matrix through the ribbon cable.

Checks

• Unplug the playfield switch connectors at the CPU board and simulate the switches with a wire and a diode; if the phantom remains, it is not under the playfield.
• Unplug the ribbon cable from the CPU board to the power driver board: if the phantom disappears, suspect the 4N25 optocouplers on the driver board.

Fix

• On a Fliptronics game the 4N25s are not needed, so the damaged one can simply be clipped off the power driver board. On older games replace it.

Systems

• WPC Fliptronics

Games

• Doctor Who

Related tips

• WPC: phantom switch closures traced to a 4N25 on the -1 driver board
• Row or column "ground short" errors
• Flipper buttons not seen by the game (pre-Fliptronics)
• Pre-Fliptronics: flipper lane change does not work
• Switch matrix chips on the CPU board
• Lane change not working on Taxi and later System 11 games
• Upper right flipper only buzzes while the magnet holds the ball: failed opto-isolator
• Narrowing a switch matrix fault to the CPU board, wiring or opto boards

Source: Pinball Repair help group!! (Facebook group)