How do you fix the error f21 on a whirlpool duet washer

SERIAL COMMUNICATION ERROR
The communication between the Central Control Unit (CCU) and the user interface board (UI) cannot be sent correctly.
F21
Possible Causes/Procedure
1. Run Quick Diagnostics Test to check the User Interface boards and motor.
2. Unplug washer or disconnect power.
3. Check wire harness connections to the user interface and Central Control Unit (CCU).
Check connections of the CCU board within the housing.
Make sure all grounding switches are engaged.
4. Check that the serial harness at the UI.
5. Plug in washer or reconnect power.
Manuals: https://dl.dropboxusercontent.com/u/8010163/Whirlpool%2024%20inch%20Front-Loading%20Automatic%20Washer%20Service%20Manual.pd f
https://dl.dropboxusercontent.com/u/8010163/Whirlpool%20Washer%20L-61%205th%20Generation%20Solid%20State%20Electronic %20Control%20System%20Service%20Manual.pdf
Duet washer: https://dl.dropboxusercontent.com/u/8010163/Whirlpool_Duet%20Job%20Aid.pdf
https://dl.dropboxusercontent.com/u/8010163/Whirlpool-Kenmore%20washers.zip
https://dl.dropboxusercontent.com/u/8010163/Understanding%20automatic%20washer.pdf
https://dl.dropboxusercontent.com/u/8010163/How%20to%20use%20Test%20Equipment%20Service%20Manu al.pdf
https://dl.dropboxusercontent.com/u/8010163/How%20to%20Diagnose%20and%20Fix%20Everything%20Ele ctronic.pdf
You need to buy a good digital multimeter from Home Depot, Lowe's, or Radio Shack and get familiar with using it. I would get a clamp meter so you can test amperage if necessary to check a breaker or a motor. A Commercial Electric MS2002 Clamp Meter is inexpensive and every bit as good as a $200 meter. I found it at Home Depot for about $42.38 plus tax (See: Commercial Electric Digital Clamp Meter-MS2002 at The Home Depot (http://www.homedepot.com/p/Commercial-Electric-Digital-Clamp-Meter-MS2002/202353289?keyword=commercial+electric+MS2002+meter ) ). It will do everything you need to do. Make sure to buy some contact cleaner that is for electronics to clean connections in your washer. Look for corrosion where wiring harnesses plug together. I have fixed many with a little spray and a good soft wire brush and an old toothbrush.