My canon 17-85mm lens does not focus properly

It sounds like the circuit board flex has gone bad, It folds back and forth when you zoom the lens, And needs to be replaced by a repair shop and readjusted. Tony

Replace the broken ribbon cable in the lens

I also have a 40D with exactly the same lens and fault. Over the course of a few months the lens became "sloppy" so if the lens was pointing towards the ground it began to creep. Towards the end it dropped to its full extent. About 10 days ago it locked up completely and I was unable to zoom at all. Returned to Canon as mine was just in warranty and they have fixed and returned it within 7 days. Very effecient even sending me a text message to say that it had been dispatched. The lens is now just like new and I only hope that this is not an annual trip.
Bottom line; I am an engineer by profession and I wouldn't touch it with a barge pole. To do these properly you need special tools and jigs to make sure things line up properly. It also needs to be done in a "CLEAN" environment.Do yourself a favour and leave it alone.

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.

I don't know your level of expertise...
Here goes. Your AF sensors in the camera need a certain amount of light to confirm focus. F/4-5.6 is a little dark, but under normal daylight its ok. Under darker conditions, it just can't get enough light to confirm focus, and it just keeps "hunting" for the correct focus. Using a teleconverter makes it even worse. The red focus assist beam sent out by that external flash unit only helps focus within about 20 feet.

Solution? Find a place with more available light, or buy one of the lenses with f/2.8 rating.

I had this problem too, then my camera started getting Error 99 (there is some problem) and not working.
If you look at this website and follow the steps under Update 2
http://www.richardsnotes.org/archives/2005/04/29/50mm-lens-contact-points/
I think you will fix your problem. The connections between lens and camera probably have a little dust in them, use a soft rubber eraser to clean them. Make sure no dust or pieces fall into the camera or lens when cleaning!

Hello. First, check the contacts on the lens mount--you're looking for bent pins, oxidized(odd-colored) metal, damaged contact points, etc. The camera must communicate reliably with the internal circuitry of the lens unit. If there is any dirt, moisture, intermittent contact going on, the sensitive focusing electronics will be effectively blinded. If all is fine with the contacts, then the problem lies with the circuits in the lens that provide critical focusing data to the camera. Movement of certain parts causes electrical changes to occur at a calibrated rate--if the camera loses connection, it can't tell if the lens is actually adjusting. You'll have to send the lens in for repair--http://www.usa.canon.com/consumer/controller?act=SupportIndexAct. This address should get you started.--Hope I helped!---Rick

I own Rebel XT and Canon 50mm f1.8 MKII lens with plastic mount and noticed many pictures with missed focus, either front or back focusing. Meaning objects slight forward or below the intended subject is in focus. I've sent my body and the lens to Canon for evaluation and thet adjusted my focus sensors on the body. I still have problems and there are many threads related to focusing on this camera with 50mm f1.8 that the sensors on this camera do not do so well with fast lens and prone to missed focus.
Test your camera's focusing ability with a book or magazine where you focus on just one specific line and see if it is truly in focus when you review the picture.

I don't know if I can help you. But I will try a guess. Some cameras focus on 1 point and it is not necessary the center. Maybe if you give your camera a point to focus on then keeping your fingers half cocked on the shutter shift the focus off-center to however you want to compose it, then click.
But if it was working properly before and then stopped working properly, did you lend your camera to someone, drop it or subject it to impact? I don't think anything is really wrong but I could be wrong. Have you checked the AF/MF button. Could someone may have accidentally pressed it?
If you are using the macro lens, did you check the camera's distance limits? I understand that the 7D actually focuses faster than the Canon EOS and Pentax. Canon and Pentax need to hunt that's why they take longer.

If you are unable to focus in broad daylight and it just hunts through the whole focus range without locking, then it sounds like a problem with sub-mirror pin (also called the second mirror pin). My 300D just had this problem and I had to send to service to get fixed. I sent to http://www.keh.com/ as they were at least $50 cheaper than going straight to Canon. Just got it backed and it works like a charm again.