I just did the denatured alcohol thing to pass smog test. I was passing everything with the exception of the Nox. I used a 1-to 4 ratio (1 part alcohol). 1994 Suburban 454- All NEW ôChevy crateö motor, cat, O2 sensor, EGR valve, plug and wires and oil change. And it still would not pass, until I tried this trick! I found this same advice on two other sites but was still a little reluctant to try. So IÆm posting my story to help build the confidence of others to try.