I think you are missing a P.
The followings is for an XTPS1 not a XTS1 as given.

Here are your parts
Replacement parts for WHIRLPOOL GU2400XTPS1 Undercounter Dishwasher | AppliancePartsPros.com

Here is the tech sheet
http://www.servicematters.com/docs/wiring/Tech%20Sheet%20-%208535451.pdf

Often this is a stuck key (always closed).
Usually trouble shot by checking the keyboard with a meter to confirm this.
If the keyboard checks out OK then it is probably the control board but it is still a bit of a crap shoot.

If you do not own a meter, I would suggest you purchase a one. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.

There is a good STICKY at the start of this forum about it's use.

[COLOR="Blue"]How can I test if the button is stuck?[/COLOR]

Looks like Soak & Scour is P1-11 and P1-3, see the tech sheet.

Remove the ribbon cable from the control board.
Place the positive lead of the meter on P1-11 and it's negative lead on P1-3.
Use a fairly high meter scale, 20K should do it. If not try 200 K.
You should see the ohms drop when the key is pushed.
I am not sure what the ohms reading will be.
So try this on a good key (normal or start) so you can see what to expect.

I do not know how much electronics you know so this may help explain things.
The reason it is important to have the meter leads used noting their polarity is that you are measuring across the switch and a diode in series. A diode only passes current in one direction. So if you use the meter leads opposite to that shown when you push the key nothing will happen as the diode is basically open.

I hope I explained this OK.

If I'm fortunate and it is just a stuck button how can I fix it?

I would not say fortunate as the only way to fix it is to replace the keypad (console assembly)