My Canon EOS 400D (XTi in the states) along with my EFS 17-85mm f/4-5.6 was giving the famous Err 99 – “Shooting is not possible. Turn the power switch to <OFF> and <ON> again or re-install the battery” when I fully depressed the shutter button. It did not do it at all times, and also the problem did not occur with my EF 50mm f/1.8.
I found through a process of elimination that the problem (Err 99) would only occur on the wide angle (17-35mm) when using an aperture smaller than f/4. So I guessed that either the lens was the problem, or perhaps the communication between the lens and body. I cleaned the contact points with an eraser… but no go… same problem.
Took both the out-of-warranty body and lens into the Canon Service Centre in Singapore. They found the problem to be with the len’s power diaphragm assembly, and replaced it as a cost of S$111.28 (Part S$24, Labor S$80 & GST S$7.28). They also did a great job cleaning the internal lens elements and external housing, so it looks as good as new now.
Unrelated they found my focus screen on the body to be really scratched too, and replaced it for S$26.75 (Part S$5, Labor S$20 & GST S$1.75).
Image credit to Rowen Atkinson