This popped up a few days ago in my 2018 20d, stands for SCR nox Catalyst Efficiency Below Threshold, a potentially serious problem. Also showed code p20fb, but that is proprietary and I haven't found its meaning. My dealer is 45 min away, and has no current service openings (!), tho am waiting for my service rep to actually speak to me. I am hoping this is just a bad sensor, which has happened before (o2). But want to know if anyone else encountered this?