Vehicle is a '94 Ranger XLT, w/ 3.0 liter eng, auto trans w/153K miles. Failed the smog NOX test (barely) at the 15 mph test level. Reqmts ( ) and readings are: HC CO NOX CO2 O2 RPM 15 MPH (80) 56 (0.66) 0.32 (517) 520 15.2 0.3 1893 25 MPH (54) 38 (0.80) 0.22 (770) 378 15.2 0.2 1836 I don't know how they load the dyno, put in a certain % grade or what? Judging from the rpm, it appears that at 25 mph, the truck shifted into a higher gear, probably 2nd. So it appears the problem is in 1st gear. My question is, what is the most likely cause for this NOX level? I know that the higher NOX levels are attributed to high combustion temps. EGR issues? Oxygen sensor? Catalyctic converter?(hope not). By the way, no check engine light problems, so there are no codes. The problem is now I have to take it in to an official "Gold Shield" smog repair shop for repair. Additional info: in '06 the 15 mph NOX reading was 502, It just passed. The other readings were pretty much the same. So what happenned in 2years and 7K miles. Am I a victim in the normal variation in test equipment? The change is less than 4% and I realize it was marginal pass in the first place. In '04, 124 K miles, the reading was 279. So there was significant jump in that time frame. Any ideas on what is happenning?