S Plus 21 code

[t28c34]

I acquired a new project S plus machine with a 61 code. After getting it in the shop, I took care of the 61 code. But it turned into a 21. I did all the quick checks on the optic board, cleaning the lenses, popsicle stick test with no luck. I popped the coin comparitor and optic card from from one of my other S plus machines that work. Ran the same tests with the same failing results. I read a post that talked about possibly the wiring harness could be the culprit. But before I tested that, I pulled the MPU to check the battery, which tested a 3.7 volts. You can see what else I found. Anyone ever see anything like that? The MPU looks good like nothing blew up or burned. But the board that it plugs into (I forget what it’s called) doesn’t look to good when I pulled it. I guess I need one of those. But would that damage have anything to do with the coin system for where it’s at?
I am so great full for the help that found here on NLG.
Thanks

[Stayouttadabunker]

The circuit board installed flat-down onto the floor of an S+ cabinet, is called the "motherboard."
The upright circuit board with the GAME and REEL chips is called the "MPU"...it must never be installed or removed with the power switch [ON].

The motherboard is heavily corroded. 
There are circuit traces are completely exposed and could very well lack continuity from one end to another, causing the problems that you're having.
Unless you know how to do circuit board repairs, I must suggest that you go shopping for replacement board.
There are several member vendors here on NLG.
If you're a Contributing Member, you can go to the Classifieds section, and place a WTB ad for an "IGT S+ Motherboard."
Maybe someone will have an extra motherboard to sell ya. 
While you're shopping, get yourself another Coin-In optics board.
It's the small circuit board that's directly underneath the coin comparitor unit.
Problems with the coin-in optics can cause the [21] error code to pop up.
You need to learn how to use a multimeter to check wiring harnesses for continuity if you plan on fixing slot machines.

[t28c34]

Yes, I'm planning on replacing the motherboard, But I was wondering if that could be the problems with the 21 code.

[Stayouttadabunker]

Could be...considering the corrosion that's prevalent on your motherboard.
Something shorted out and the traces on the motherboard burned right up.
There's no way that I can tell you from here what caused the short.
Have you even checked any fuses?

Another scenario is that, whenever there's something wrong with the coin-in optics board, the MPU shoots out that [21] error code as well.
This is why I suggested that you get a spare set of coin-in optics.

You'd have to utilize schematic drawings along with a multimeter, to find out if the corroded damage on the motherboard has anything to do with the error presented...that motherboard was severely short-circuited...very likely beyond a reasonable price of repair.
I hate to use use the word "kaput". 
I'd toss it before it does any more damage to the rest of the machine.

Obtaining a replacement motherboard and coin-in optics is your best option for getting this machine to work again - as quickly as possible.
Without you telling us if you've tried this questionable MPU in one of your other S+'s, I couldn't tell you whether or not if this MPU is any good either.
I cannot possibly know from here, how extensive the damage is inside your machine.
You're going to have to test the coin-in optics board, and at least the MPU in a known good working cabinet to try to verify if those parts are any good.

or

You could stick your good motherboard from another machine into this problem cabinet to see if this machine will work.
No matter what, you will lose some Benjamin's from your wallet. lol

[t28c34]

For how much I paid for the machine, I have room for a few Benjamin’s. I will probably have a mother board coming by tomorrow. That’s a good idea, swapping one of my other machine motherboards to test this machine with.

[t28c34]

Problem solved. Replaced the mother board and I’m up and running. A big huge thanks to Barry and Alan at K LAR for helping me out! And thanks to NLG for having this website and followers to assist in troubleshooting.

[Stayouttadabunker]

We are happy that you have it up and running! 
Have fun with it!