allanmb@gmail.com wrote:

I wouldn't bother with sending it in for repair - it's just not a cost
effective idea. But you might want to consider taking it apart yourself
if you're handy with tools and are mechanically inclined. It could be a
simple lubrication issue or dirt. Or loose connection. It won't hurt to
take a look if you're careful.

Or you can do what someone else suggested and sell it for parts.

If it's uneconomical to repair it and you end up buying something new,
mail the lens
to Canon along with a note thanking them for such a pathetic design.
Unless the barrel focus autolocks when the AF is engaged, they REALLY
screwed-up here, no doubt to save production costs.  I wonder if any
other companies have this same issue?

Off your meds again there Rich?

Rich, be reasonable. IINM, this lens is comparable to the 75-300mm
that sells for around $160 US. I have the 75-300mm (an unfortunate
gift from a friend who meant well), and it is not a good lens. And
even though I use it from time to time (I don't have another lens with
that reach), I would not be heartbroken if it died.

To the OP....

Check to make sure the contacts on the lens are clean. Dirty contacts
can give an Err 99. If that isn't the problem, save up for something
like a 70-200mm f/4L and maybe a teleconverter. IMO, repairing this
lens is a waste of time and money.

You didn't mention if you get an Err 99 when the lens is set for
manual focus. If everything works fine in manual focus, I'd turn off
the AF and keep using the lens until I had a suitable replacement.

TR