How does the PCM determine air-fuel ratio when its running "rich" using a narrow-band O2 sensor and how is it incorporated into the closed-loop engine management system? I have a 2003 STS engine and its running fine. I'm just trying to get this question answered for a mechanical engineering class. From what I've readabout NB O2 sensors, they are very sensitive near stochiometry (sp?) but either pegged at 0.9V when rich and 0.15V when lean. Then how is our PCM able to determine that AF ratio i ~13.0 orless under load and still operating in closed-loop control mode? It does go open-loop when it get "richer" or more load but its still report an AR ratio that still seems correct. We also have a wide-band O2 sensor and Horiba exhaust gas anaysis system that we are comparing AF ratio with and all three numbers are pretty consistent.

Thanks,
Ben