On the old control board I got an error code of E41. When I replaced the control board I got E21.
The first problem was that I did not know that the top was supposed to slide backwards to allow access to the screws that hold the control panel.
It turns out that the selector knob was out of sync with the control panel. It took a great deal of maneuvering. I finally figured to rotate the knob through its 16 positions while plugged in until it got to the place where it lit up only the spin cycle LED which is position #1 for the knob. Then I removed the knob stem from the control board and aligned it and reinserted (somewhat against its will) it so that the knob would coordinate with the board.
I must've gone back and forth uninstalling and reinstalling 20 times. And I think I dropped the knobs and springs at least 5 times!
To do it again I think it would take about 15 minutes.
I still don't know why it showed an E21.