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I recently did not pass the emissions portion of my smog check (CA). Specifically the "HC (PPM)"section failed at 15mph. It measured 101 with a max allowed being 61. To remedy this, I had my mechanic replace the oxygen sensor (which was a previous problem months before) as well as a major tune-up and fuel injector cleaning. I went back to retake the smog test and I didn't pass again due to the same area previously mentioned. This time it measured 141, so it's worse than before. My mechanic says it's most likely the catalytic converter at this point. I'm hesitant to replace it as it is expensive. I've heard that running the car for 50 or so miles on the freeway should clear out the hydrocarbons before taking a smog test. Do you think this will work instead of replacing the cat? or should I just replace the cat? Also, do you think that the oxygen sensor not working properly for a while (over a year) contributed to this problem? Your time and thoughts are greatly appreciated!

Answers from the Automotive Experts

Amanda J. Pierce, MBA Cheri, If you have had an O2 sensor that needed to be replaced for over a year, then yes it can cause permanent damage to your CAT. You can run your vehicle down the highway to blow excessive carbon build-up, however, you may have waited too long before replacing your O2 sensor.

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