Check all of your opto switches.

Mine did this exact same thing a few years ago and replacing BR5 and C8 on the main power board fixed all of my issues. Mine was firing balls out but I had also lost my display. Check the voltage on TP1.

This was my thread on it: https://pinside.com/pinball/forum/topic/oh-nosttng-go-boom

You shouldnt have all six in the trough. Some are kept below the playfield. If those optos are dirty(in the subway) the game will think it has too many balls and spit them out.

I had multiple issues on mine. I had to repair a connector, reflow the optos, the 3A fuse was rusty and causing 12vdc issues. The simptom was the same though, spitting out balls.

My guess is that it's trying to pre-load the three VUKs, but due to a ball diverter problem it's unable too complete the action. A very easy check to do is to manually eject all the balls, drop one into each VUK, turn on the game and see if it stops doing what it's doing. Also note if balls were ejected to both cannons; if they weren't, you know where to start looking.

Another easy check is to look at your switch matrix and verify that all three VUK's show loaded balls; it should look like the image below where only through balls 4, 5, and 6 are blocking the optical beam, as three balls should have preloaded to VUK shown with colored dots 1, 2, and 3. The red dots 1-6 on the switch matrix show where balls are blocking the optical beam. This is typically where the balls should be at the start of every game.

Normal Matrix (resized).png

edit: corrected ball trough location

Failure of the top diverter will prevent the loading of under R.Gun (37); failure of the lower diverter will prevent loading of Under L. Gun (36).

call me I've fixed many of these

I agree with Pin_guy - this behavior is due to either balls not being detected in the staging locations (subway opto issues - optos or board issue) or the diverter not being able to divert the balls to the proper locations (diverter mechanical issue, solenoid issue).