I finally got my car to passe all the OBD2 smog checks after replacing the downstream O2 sensor and using the ol' "screw a 'spark plug non-fouler' from Autozone onto the end of the sensor" hack for good measure. The O2 sensor test refused to complete for weeks, and I was ready to resign myself to the fact that I was going to have to sacrifice some of my silver stack for a new car. Then after one last desperate interrogation of Grok and ChatGPT about smog check drive cycles for GM ECUs, I drove the car around the subdivision at low speed with periods of gradual acceleration and deceleration for 10 minutes. I didn't think this would actually work, but miraculously, the O2 sensor test finally cleared when I pulled into the driveway and ran the smog check one final time. Saved myself the grief of being forced to part with 50-100 ozt of my stack just to obtain a legal wagemobile.