How to fix a Canon EF 70-300mm f/4-5.6IS USM Lens?

SOURCE: Manual Focussing failure with Canon EF 100-400mm SLR zoom lens

I doubt if there is anything you can do yourself. The manual focus may have debris under the ring or is in need of parts. I would leave this one to a tech.

SOURCE: I have a Canon EF 300mm f/2.8L USM Lens

Here is a link to many Canon products in PDF form: Canon Parts Lists


Unfortunately, the only 300mm 2.8 lens listed is the IS version

The procedure is not that complicated, but you need to pay attention to the connections on the circuit board. The mount is removed, and you need to remove the small screws that hold the connector board to the mount ring first.

I cannot do live chat due to I use a Mac, but I will monitor the thread if you have questions,

Kevin

SOURCE: Canon EF-S 17-85MM F/4-5.6 IS USM prevents features from working on 40D

something sounds strange about this. aperture 00 is an invalid aperture value. stands to reason if that is the case the camera would complain about it. via error code. being that its only six mos. old i would give canon a ring.

SOURCE: Canon EF 75-300mm f/4-5.6 III USM Lens

If your AF motor is hunting then the lens probably needs to be looked at. I wouldn't recommend this as something to do at home so you will need to either find a repair shop if it is out of warranty or return it to Canon if it is in warranty.

SOURCE: Canon Zoom Lens EF-S 17-85mm 1:4-5.6 USM: I

I hate to re-invent the wheel (and I don't want to cut 'n paste someone else's hard work to present as my own) so please click here for a likely practical solution which usually works and for Canon's own suggestions on the error 99 fault.

If this has assisted you then please rate my answer; if not then add a comment to provide feedback and I'll attempt to provide an alternative solution.