So i went for my emissions test today and failed. I had a hc of 43 (62 is the limit) CO% of .54 (.35 limit, so i failed bad, ive got a new cat im going to put on tomorow, should i get a new 02 sensor?) and a PMM of 128 (467 limit) So i failed because of my high carbon monoxide content and they told me i need a new gas cap. I have cel 31, and ive read about it alot and how it is a huge pain in the ass and its easier to buy a new car then it is to fix this. Im wondering is anything to do with the EVAP system would cause me to fail the emissions test or if i simply failed because of a shitty cat. Ive heard that a high CO% is caused by bad afr which would be caused by a bad 02 sensor right? So should i just go ahead and change the sensor anyways, and should i change the up or downstream sensor? Thanks for any help, id like to search even more then i did, but i need a strait up anwser.

what sort of mods do you have? I know mine failed pretty badly the first time, did some timing changes and got about all but HC's down close, but they were still like x5 too high. Replaced my oiled cone air filter with a generic paper one, put on a new PCV breather filter, and put my cams back to factory timing (Had -2 intake, +3 exhaust) and it passed with flying colors. I think the cam gears made the largest difference, IMO. Oil change never hurts, either.

i only have a cold air intake, but ill replace the old ass cone filter for something else tonight. I changed the oil about 70 miles before the test and ran 91 octane. Ill re-gap the plugs tonight aswell. Im still not sure if i should change that 02 sensor though.

Try runnin some injector cleaner, or that RXP stuff. How many miles on the O2?

i ran compleat fuel system cleaner and i replaced the pcv last year. The 02 has never been changed, unless the guy before me changed it. There are no codes though. Im getting a bit of hesitation at wot in higher gears at low rpms, so i thought that might have somethign to do with the 02.