[Mr. Buckeye]

I have a Proform 795 treadmill. I get the dreaded e21 error code. I swapped both boards with a friend whose is working and still get the code so I know it's neither board. I keep reading about adjusting the speed sensor. Where is the speed sensor, what does it look like, and how do I adjust it? The incline works, the running belt itself runs for about 5 seconds and then the code appears.