I got the error right after ignition and there was no error on the way home the previous night.
I suspect it ran an automated test when the engine was still cold and the screwy reading it got triggered the engine code. According to the info given testing this sensor should only be done on a warm engine when the car is parked. Air temp at the time was around 20F if I remember correctly.
To confirm it was P0420 I miss-typed the error code in my first post. Error has not come back with even more commuting. (115 miles round trip every word day.)
If I start getting multiple errors I'll just go for a full delete.
Also the guy that worked on it recommended a different shop about 20 miles away. The other shop does ~80% diesel work including tractors, semis, and dozers, etc. So if I need it, I got real diesel techs nearby, not just some tech who took a course and really only knows gas engines.