Every time ball 1 is locked for multiball the game thinks the scanner midway eject switch is closed, keeps firing that coil and then shows a coil malfunction alert. All switches pass switch test.

Midway eject switch falsely triggers when a ball is locked: shooter switch wiring

Cause: The shooter lane switch had been wired incorrectly, so with the lock and trough switches closed a phantom closure appeared on the midway eject switch. A broken diode on the shooter lane switch gives the same symptom.

Checks

• Find the switches that are closed when the fault happens and work out which matrix 'square' would make the phantom switch.
• Check the wiring and diode on each switch in that square, starting with the shooter lane switch.

Fix

• Rewire the shooter lane switch correctly (or replace its diode if broken).

Systems

• Sega/Stern Whitestar

Games

• Nascar

Related tips

• Trough and other switches on one column read open: broken column wire at the trough
• Destroy the Ring stalls on the second shot: spinner switch misses one direction

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