99 Diamante Cat Questions Options

[Tailfin]

Ok a summary... My father has a 99 Diamante with California emissions (just the car, we are in New York), with a cat on either side of the engine, and another under the car. He once had one of the side cats replaced, and the part was over a grand. Mitsubishi parts are notorious I know, but that's over the top I feel :-P. Anyhoo, he has an intermittent service engine soon light, and has been told by the mechanic that it's a result of the cat being clogged (the one under the car), which I assume is an O2 sensor code. I have a few questions here...

I looked on Rockauto, and there are cats for around $200, but they all specify not for cars "licensed" in California. Does this mean it actually won't fit on the California emissions car, or that it simply won't pass California standards? Basically, I want to know if one of these can be put on my father's without any problems or codes tripping (for inspection and such). I wasn't sure if the computer or any other issues were different or problematic--like if the service light would trip because of the fact that it's a California car. I have a hunch it would fit, as the bit on Rockauto says not "legal" for sale out of CA, not that it won't work, but I don't want to advise him of this and then have the light trip and have to get the more expensive one anyway.

Also, is there a way to press anything on the dash/stereo area to pull up a list of tripped codes, and also clear them? I'm a Cadillac guy, and there is such a feature, but I'm unfamiliar with this setup. If not, how does one check and clear codes on this car?