I had the exact same problem with my Blackout. Turned out to be a bad connector where the special switch wiring plugs into the CPU board (J4). The "Up/Down" switch signal was not making it to the board.

I would inspect the toggle switch on the coin door to ensure the wiring looks good. With the machine off, reseat the molex connector at the coin door, at the harness where the wires enter the head and at the CPU board (J4 - the rightmost 4 pin connector at the upper left corner of the CPU board).

If that all checks out, try shorting the 2 wires on the toggle switch (I used a wire with alligator clips on both ends) and start the diagnostics procedure. With the toggle switch shorted, it should go into settings and you will know you have a bad switch. If it still cycles through display diagnostics, you have a wiring, connector or board issue.

Do you have a logic probe or a meter? We can confirm if it is wiring or board issue if you do.

Did you try manually adding a credit to see if that will allow a game to start?

With the game on, measure the voltage at pin 13 of IC28 on the CPU board. IC28 is a resistor matrix just below J6 near the top, center of the board. Pin 13 should be second from the bottom on the left side of the chip.

Switch the UP/Down coin door switch to the other position and measure again.

If you get 0V in one switch position and 5V in the other, you have a board problem (the board is not reacting properly to the switch setting).

If you get 5V in both switch positions, you have a switch, wiring or connector problem.

Normally open. There are pull-up resistors on the board that will make the header pins read 5V if the switches are open. When closed, the switches ground the inputs.