Was it P0240 or P0420? It only caught my attention because I was just dealing with a P0420 on my wife's van and knew that was a catalyst efficiency code. I think P0240 is a actually a boost sensor code.
If it is P0420, and it comes back, it's probably a reference to the Diesel Oxidation Catalyst, which is the top section of the DPF unit. There is only one O2 sensor, which screws directly into the DOC portion of the DPF unit. If you're DIYing a repair for this, cleaning the O2 sensor would probably be your first line of defense, since it's cheap and easy to do. You could try to discern what's happening by watching the output of the O2 sensor and related values, if you have access to a code reader or scan tool that can do it, but not sure what you'll find. Replacing the O2 sensor would be a lot cheaper than replacing the DOC/DPF unit if you wanted to take the chance, but it might not be the problem either. A problem with the O2 sensor itself should also throw a P0133 code, as well. If the code persists, it could be a DOC/DPF unit problem, but seems unlikely if there's not other codes along with it. Could also be a fuel/air or EGR problem, or a combustion issue causing problems with the catalyst efficiency or overwhelming the ability of the catalyst to do its job, all of which would require further diagnosis, of course.
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.