I'll be able to check that button later today to be sure, if service credit is the same as test then I believe the repeated presses raise the credit by 1 each press, but I'll make sure....
You're misunderstanding the buttons...it's NOT the same as the Test button.
The Test button is the little white button that's next to the power switch or behind it....on older models, it's located on top of the door connector Molex coming out of the MPU enclosure. It's for switching between pages in the Accounting menus or Diagnostic test pages.
The Service Credit button is the small button located on the bottom of the coin-in optics circuit board uder the CC.
Each press adds a credit to the machine, up to max credits the SS chip allows.
The unplugging and re-plugging you've tried...but you haven't yet tested the harnesses themselves that relate to the Coin-In optics circuit board.
You need to test the harness going to that optics board for continuity.
Now's a good time to take it back out and pull out a multimeter on that harness.
The shortcut of course is to just swap the harness from the known good cabinet into this one to see if it works with this one.
Anyways, there's 10 pins coming off of that Coin-In circuit board....follow each wire from there along the harness, and make sure you have continuity at the other end.
You cannot "see" under the vinyl covering if a wire is broken.
That's why we use multimeters.
Below are the wires coming out of an IGT S+ Coin-In optics circuit board...>>>
Top wire is orange w/blue stripe...that goes to location 1 on the 3-pin Molex (pointy side) for the CC.
2nd wire is red w/black stripe...that goes to location 1 on the 12 pin Molex.
( You may need a magnifying glass to see the pin location numbers molded on the back of the housings.)
3rd wire is solid green...that goes to location 10 on the 12 pin Molex.
4th wire is orange w/green stripe...that goes to the flat side or pin 2 location on the 2-pin Molex for the Gate solenoid.
5th wire is orange w/red stripe...that goes to location 4 on the 12-pin Molex.
6th wire is orange w/yellow stripe...that goes to location 6 on the 12-pin Molex.
7th wire is orange w/brown stripe...that goes to location 3 on the 12-pin Molex.
8th wire is orange w/black stripe...that goes to location 2 on the 12-pin Molex.
You don't need to worry about the other wires coming FROM the 12-pin Molex to the Gate solenoid or the CC.
If any one or both of those two smaller connectors are taken apart, neither one will make the MPU shoot out the [Code 21].
You only get a [Code 21] if there's a problem with the Coin-In optics board, and/or the wiring going to it.
Good luck!
Below is a photo of an IGT S+ Coin harness and a Coin-In circuit board on the coin mech frame.
You can see on this particular board version, the little white Service Credit button on the bottom of the Coin-In board.
Click on it to enlarge if needed...>>>