During the Final Frontier multiball mode the game loses count of the balls in play; when it gets down to one ball the flippers die and the turn ends.

Turn ends while a ball is still in play in Final Frontier: flaky trough opto

Cause: One of the trough exit optos was cutting in and out, so the game thought it had an extra ball. The trough optos and the opto pairs at the two side ball lock (cannon) kickers all count balls on this game, and a false reading at any of them gives the same symptom.

Checks

• Make sure the game has the correct number of balls installed.
• Put the game in switch test with the balls out and tap the trough and the middle of the playfield; watch for any opto switch that flickers on and off.
• Check the trough optos first, then the optos at both side kickers.

Fix

• Replace the flickering opto (transmitter and receiver together if you are not sure which is bad) or repair its board or wiring.

Systems

• WPC DCS

Games

• Star Trek: The Next Generation

Related tips

• Cannon won't turn on the third multiball shot though it tests fine: flaky opto wire
• All playfield optos dead after replacing one opto: failed 16-opto board
• Indiana Jones to Demolition Man games have two LM339 opto boards
• Eddy sensors: electronic ball switches under the playfield
• Adjusting an STNG, Theatre of Magic or Roadshow eddy sensor
• Eddy sensor board R1 values by game
• Random multiball or game never ends on early opto troughs
• Flipper cabinet opto switch boards

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