Frigidaire washer model fex831fs4 won't agitate but will drain and spin

You will have to actually take off the back and look at it. You are just guessing otherwise and there is no substitution for knowing what the actual problem is that's causing it not to agitate. My guess would be a belt or the transmission, but here are some manuals to help: https://dl.dropbox.com/u/8010163/Fri...%20Washers.zip
You could even have a problem in the timer and if that's the case you will need to use a meter to find out. Here is a manual so you can learn to use a meter: https://dl.dropbox.com/u/8010163/Whi...e%20Manual.pdf
Don't shy away from learning to use a meter. They are a lot of fun to learn to use. You can test all sorts of things like which batteries are bad or good in a drawer full and which light bulb is bad in a string of Christmas lights. I think everyone needs to learn to use a meter and I have a girl who works for me who is very good at diagnosing AC and refrigeration units and she puts many men to shame with that skill. I have taught girls as young as 7 how to use a meter because I love to see their hunger to learn and watch them practice and have fun doing things adults have not learned. Everyone should learn to use a meter or you are just guessing which doesn't work in the real world.