ive been a smog tech for the last 3 years.. additives dont work all the time, they can help if its right on the border, never heard of using denatured alcahol, if its an obd2 car (96 and newer) you cant smog another car in its place, carhospt u have no idea what your talking about, shutup. theres lots of shady places that will do it, u have to ask and your gonna have to pay, 91 octane does indeed help, alex 71 is exempt from smog and if timing isnt within spec it will fail, ric c02 is good and cant fail for it, hcs could also be a ignition or 02s problem, sean you have to drive anywhere from 30 - 50 miles for your pcm to reboot if the light didnt come back on.. hope this helped fellas.