The left dome flashers on top of the backbox stay dark (LEDs show only a faint pulse) while the knocker on the same driver transistor works. Incandescent bulbs don't light either.

One topper dome flasher pair dead while its shared knocker works: broken wire

Cause: A wire in that flasher circuit had almost broken off at one end, at a connector.

Checks

• Confirm the circuit with incandescent bulbs first, then go back to LEDs once it works.
• Compare the dead side with the working side, and swap the matching resistor on the interconnect board with a good one to rule it out.
• Trace the whole wire run from the board to the dome sockets and tug-test each terminal.

Fix

• Repair the broken wire and fit a new connector terminal.

Systems

• Williams System 11B

Games

• Police Force

Related tips

• Flashers dead on Banzai Run and later: interconnect board resistors
• Where the flash lamp resistors are by era
• Lane change not working on Taxi and later System 11 games
• LED flashers blink when the flipper buttons are pressed
• Isolating the 50 V flasher circuits for LED flashers
• Flash lamp dead on an early System 11 game
• System 11 backbox boards and supplies
• GI connectors left the power supply after Cyclone

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