I was previously failing Aircare (IM 240 test) for consistantly high HC. I've had my car for around 6 years now and over time have replaced nearly every component. After getting a basically new cat from a friend and still failing the test I was really wracking my brain for fixes.
Based on only high HC, New O2's, my diagnostic trace report and a new CAT, I basically had it narrowed down to a gas fumes problem. A friend had mentioned it could be my canister and I would smell a gas smell from the drivers fenderwell. I couldn't smell a thing there though, so I thought I'd take it to a shop to get fixed. Anyhow, the problem was actually a saturated canister and some very bad tubing. I belive it was the tube that goes from the top of the tank to the canister that had become quite porous. While it was on the hoist with my nose near the gas tank i could then smell raw gas fumes, the hose apprently was the main culprit. Taking the old hose off the mechanic actually found saturated charcoal in the hose too!

After replacing the charcoal canister and the tube from the tank I managed to pass with flying colors and basically non existant HC or CO. NoX was about average

It sucked getting raped at a dealer but, but fortunatly the mechanic I was dealing with really knew his stuff (I've dealt with him in the past). Hopefully this helps other people with a High HC Fail reading. I would imagine that if my hose was breaking down it could be happening to nearly everyone else who DD's their Z