I'd have to agree and then disagree with most people on this. A/F is only useless if you use a narrow-band O2 sensor which comes on most cars. Narrow-band O2 sensors are cheap because they aren't very accurate and they only operate well around stoichimetric A/F ratios. Modern cars are run near stoichiometric because of the limited operating range of the catalytic converter. For catalysts to be effective (assume stoichiometric A/F is 14.6:1 for gasonline) they need to run with A/F between 14 - 15.jbhebert said:On a stock engine, EGT and A/F are useless. Arguably, A/F is useless all the time, since most people have found that it bounces around too much to be useful. EGT will not get dangerous until you are running more boost or more compression.