I have the problem with my quite old 7D. Few months ago it stopped autofocusing when shooting through the viewfinder (and it works fine if I switch to Liveview). And it's not only stops working when I switch to viewfinder, it's showing Error 40 and I can't switch the camera off or on until I remove the battery and install it back.
My lenses are Canon EF 28-135mm f/3.5-5.6 IS USM and it works fine with other cameras, so it have to be the body problem.
An Error 40 is a power source malfunction. It means that an internal error in getting power from the battery to some part of the camera has been detected. It could be caused by a weak battery or a third party battery. We recommend trying a fully charged geunine Canon battery in the camera. If that does not clear the error message, then the camera would need to go in for service. If you are in the United States, you can set up a repair HERE. If you are outside of the United States, you can find Canon in your country HERE.
It's just seems odd to me that there are no problems at all in shooting pictures and video with MF-lenses and the problem only occurs when I attach AF-lenses and only when I shoot through the viewfinder. I were hoping to find someone that used to have exactly the same problem and yet found the right answer to share.
Thanks! I just discovered that there are two AF-systems in my camera, so- you're right, I think. Anyway, the body is in the service now, so soon I'll know for sure.
7D and Err 40 can be a loose screw. I opened up my 7D two days ago to check but all screws were sitting really good. Please ask the service center when you get back the camera.
