Sticking AF on 100-400L

I am really bummed out. I bought a new 100-400L to use with a 40D two weeks ago and it worked like a charm until today. Suddenly the AF is sticking and at it’s worst with the zoom extended to 400. If for example I focus on something close say fifteen feet away then try to refocus on something distant the focus doesn’t move and vise-versa. The only way I can get to the distant object is by focusing in two or three progressively further (or closer) steps till I get there. The other way is to “help” it with the manual focus first. It was perfectly fine until an hour ago. I note that in the lens manual there is no “trouble shooting” section. Has anyone else experienced this? It’s a huge bummer because a week after receiving the lens I came to Colombia where I will be for the next six months. Trying to return the lens to Adorama from here and have them send me a new copy would be an enormously expensive hassle. With high shipping costs and duty charges (and risk of theft) it’s not an attractive option. Any ideas?

well possible send it to the nearest canon service centre, I never had a problem like that with mine
have you tried it with the different focus min lengths, maybe you are trying to focus below the min distance?, switching between the 2

I've been having this problem recently. Can't tell if it's because I'm shooting in low light, low -contrast conditions, or if there is something wrong.
Am putting off sending it away in hopes that I figure something else out.

Have you set the custom function 3 'lens drive when AF impossible'?

::John:: wrote in post #4241648
Have you set the custom function 3 'lens drive when AF impossible'?

I had it set to "1". Should it be on "0", perhaps?
Sometimes I read the manual and don't really understand what it is saying. I've had it on "1" - just switched it back to "0" and things seem better. This would be wonderful if that was the problem.

It would make some sense if it was the answer. The function appears to be designed to stop the lens hunting to far and wide for a focal lock (keeps the focal interest within a smaller parameter to try and stop rapid OOF hunting when tracking a bird in flight, for example).

I turned it to '0' on mine - 'cos sometimes I DO want the focus to shift from one distance to another quickly.

Well I just tried setting it to "focus search on" and it is acheiving focus between two very different distances but it´s slow (about like on my S3) compared to focus speed with "focus search off". The lens is definately not functioning like it was.

By the way sorry I posted this lens question under the wrong heading.

Do you have the distance switch set to 'near'. There is both a near and far setting. It keeps it from hunting in the other range.

Yes I checked and double checked that.