Thread: help with failed smog

Bad O2 and cats will not affect your mpg like that. Heat your cats.

It could be a million things but I vote for the Italian tune up before the smog test. It made my S4 go from 174ppm HC at idle down to 9ppm. I spent a ton of time (and money) chasing things around trying to figure out the problem. Heating the car up well and good, plus revving it out is way too easy to do.

Hi again, it sounds logical to replace the o2s. I know the PO had one of the o2 sensors replaced, just unsure which one. I will need to look at the receipts. I have a question: after i replace the sensors, how do i clear the learned adaptations in the DME? And, these would be the pre cat sensors, correct?

At 90k the pre-cat O2 sensors are pretty much at the end of their useful life. Pre-cat sensors are a scheduled maintenance item that should be replaced every 100k. There could be other problems, but aged sensors will tend to make the engine run rich. Replace the pre-cat sensors, clear the learned adaptations in the DME and re-test.