My old '92 Lumina (3.1L V6, Auto, 201k miles) failed its biennial emissions test. The results are below.
HC and CO numbers are "normal"; NOx is sky high. However, it failed by under 20 points. My experience has been most all cars if they're going to fail, fail at 15MPH.
Suspecting a lean mixture, I hooked up my scan tool and monitored the BLM's at both test speeds (road and engine) and the mixture is way rich:
BLM at 15mph = 108
BLM at 25mph = 112
BLM at 65mph = 115
What doesn't add up in my mind is High NOx and a rich mixture.
Next steps would be to check EGR function and passages followed by the cooling system. Hopefully, I'll find the problem someplace in these two places and not the converter; the replacement converter is only 4 years old (21k miles).
The O2 sensor is original to the car.
Another observation: when I first hooked up the scan tool, I noticed right away that the TPS is reading 0.62V at idle. I believe that this should be closer to 0.50V. The TPS on this car doesn't have any adjustment.
Any comments, etc. are welcomed.