I have a Canon EF-S 17-85mm that developed the dreaded broken ribbon cable problem. depending on which camera body you use the error will be reported as Err 01 or Err 99. Unfortunately, this is a rather common fault with this lens -- Google and confirm for yourself.
I am visiting a mate of mine in the UK -- he has the same lens with exactly the same problem.
In both cases setting your camera to AV Mode, the lens aperture to wide open and zooming all the way out allows the camera to take photos without a problem. Zoom in beyond a certain point +- 30 to 40mm and an error occurs when attempting to take a picture regardless of the aperture setting. My suggestion is set your camera to Av mode and play around a bit with your Aperture setting and Lens Zoom settings -- then see what works.
Also, I suspect that what is being reported as an "aperture mechanism failure" by the Canon repair centers is actually a broken ribbon cable. IMHO. it would be more appropriate and honest for Canon to report it as a "Canon Manufacturing Failure".
Chaps, IMHO this is a Canon Lens with a very serious manufacturing fault. Do not buy this lens! If you already have one then I hope yours does not fail like mine and countless others have. Canon should accept to repair all Canon 17-85 lenses with a broken ribbon cable for free! Canon is seriously tarnishing it's reputation with such an unreliable lens. Mine gave up the ghost while on a visit to Zanzibar... Thanks Canon!
Personally, I have 3 Canon SLRs (10D, 40D and 5DmkII) and numerous Canon Compacts/Bridge cameras (SX1 IS, S3 IS and A480IS) and a couple of compacts from other manufacturers. Considering photography is only a hobby and light usage was made of this lens this is a shockingly poor performance by Canon.
I am considering buying the EF 70-200mm f/2.8L II but at close to £1800 I am a little worried about my continued trust in Canon. I'll wait a while longer