Not understanding the problem here. You say it plays game start sound and kicks ball out so what does 'will not start game' mean? Maybe post a video so we can see what's going on.

Take the ball out of the game. Make sure all the targets are reset to the up position.

Go to switch test. The display should show no switches down. Press the start button and see if it shows correct switch number in display (06).

If you see any numbers with nothing being pressed, troubleshoot why it is stuck. Page 17 of the manual shows the switch display numbers. Page 14 shows how to get to switch test.

Ok, good. That means the MPU is not getting the signal from the start button. Just to make sure other stuff works, try pushing some other switches on the game and see if they register. See if you can find another switch in the same row, then col of the credit switch and try them. Also disconnect and reconnect the connector to the coin door.

It could be something as simple as a broken wire on the credit button, so do a good visual and see if you notice any broken wires.

You can check the diode with the game off with a DMM. You may have to disconnect one end of it to get a reliable test.

Awesome!

When you get some spare time, just take that connector apart and clean the pins and receptacles with some alcohol. If any of the metal receptacles are loose on the female side, very gently push them together with needle nose or screw driver, to tighten them up and hug the pin better.

Worst case, you may have to replace the pins and/or the receptacles at some point--but now you know where the problem is, if it comes up again.

check the solder on the board for cracks around the header pins and change the connector and pins and you should be good