"How can I fix Error 99 on my Canon EOS Digital Rebel?"に関する回答With a 30 D and a 20 D, it turned out to be the lens in my case. It was not able to communicate with the camera. For this, you will need two lens's and a camera body or two camera bodies and one lens to do a process of elimination. Take the camera and lens that is getting an error 99. Put the other lens, in place of the lens in question, on the camera body. If it works then it should be in the lens that you are having problems with. Try the same thing using an additional camera body if you don't have a second lens. This should help you to narrow the issue to either the lens or to the camera.
"How can I fix Error 99 on my Canon EOS Digital Rebel?"に関する回答My 20D Canon showed the error 99 today and it seems to be pointing toward the EFS 17-85mm lens and the focusing mechanism. I just started messing with it , and now I am able to get it to work without the error 99 when I have the lens zoomed out further than 25 mm, but that is all I know now. See if you have the same problem with a different lens. Hope that narrows what your problem may be.