P0401 is an Insufficient EGR Flow code. The most common cause of this is the blockage of the EGR passages behind the throttle body with gook. It's a pretty easy job to remove the TB and clean out the passages - just make sure you purchase a new TB gasket first, and make sure you plug the intake so junk doesn't get in there.
If you do a search on P0401 over all forums and all dates, you'll get some more detailed procedures.
- Mike
