@TwinTurbo, what do you think about this? I live in Calif and we have pretty strict emissions testing. Every two years. And for the past 2 times (4 years), my car has passed, but just barely. Because it barely passes, each time I do the test, I have to always go the next time to a place where they put it on a treadmill and test it at 15 mph and also I think at 35 mph. Most people get by w/just an idle test, but the state of Calif considers my car special! It gets the full treadmill treatment.
Anyway, on my car, all the emissions measurement's are fine except for the HC emissions at 15 mph. That measurement just barely passes. When the car was new, it passed easily. So something has caused the HC at 15 mph to go up significantly.
The reason I'm posting is for your opinion on what my best gambit would be if it fails the next time? Like you, I get one free repeat test. The car has always had the routine maintenance up to date, so that's not likely the issue. And I always do what the "How to pass emissions test" websites say to prepare (like put in a new air filter, new oil&filter, drive the car on the freeway 50 miles at 65 mph), and what they recommend just prior to arriving at the test (engine at full operating temp, gas tank full) .
I think the problem w/my car is either the O2 sensor or the cat. Do you agree? And if I fail next time, which should I replace before the 2nd free test? The O2 sensor? The cat? Or both?