OBD all clear won't reset
 

Hi Hyundai People!
Let me start by saying that my '05 Tucson is the most reliable auto I've ever owned. I love it! It's been instrumental in getting my kayaks and fishing gear to the water for almost six years, nearly 130k miles and nary a complaint so far... which is why I didn't mind at all when this year's inspection cost me about a grand. One annoying little problem:
After having my rear O2 sensor replaced during said inspection, the CEL went out, but 600 miles later, the OBD "all clear" still won't re-set. This is preventing me from getting my emissions sticker, as well as my regular inspection cert.
Can anyone weigh-in regarding this issue, and offer some advice about how I can resolve this? My mechanic said that if the issue persists it might be necessary to take it to a dealer to get the computer flashed (re-set).
Help, please?
Have a great day, everyone!

What is the code causing the CEL? Also, what engine? My guess is, the O2 replacement wasn't the problem or wasn't the whole problem.

Thanks very much for your reply!
The CEL has not come back on since the rear O2 sensor was replaced, and what I was told is that one module/sensor/whatever still had not reported back to the computer to enable the "all clear" status. They didn't specify which module/sensor was reluctant, and I'm not sure if they know... I can only assume the original CEL event was due to the O2 sensor, as that was the code they originally read and the light did not come back on after that sensor was replaced. I'll be going back Saturday morning for another read, but this is getting a little scary. :)