Yesterday my e30 started generating a persistant CEL (Check Engine Light) error 1221 which indicates the o2 sensor has failed or is sensing values so far out of spec it can’t be interpretted. I suspect that driving around with a nagging misfire for almost a month fouled the sensor. The sensor is only a couple months old, which is a shame. Thankfully I kept my old sensor which I’ll use until it fails or I finally swap in a s52 engine.