It will come back, the egr ports might be clogged with carbon, or the
egr valve itself. That or the DPFE sensor is bad. Make sure you are
getting vacuum to the egr valve as well. My 2000 Taurus wagon w/ 45k
miles has this at the moment. I scanned it with an OBD-II device I
have. I turned off the MIL but it came back later the same day.
Apparently a very common problem on my model Taurus. Try google I
found some good links on this DTC P0401 code explaining how to fix
this. I have not myself but when I get around to it I do plan on
cleaning any carbon out first before spending $75 on a new DPFE sensor.