Most of the time this is a false indication, due to a bad DPFE sensor. No
way to tell for sure unless the system is tested. There are other
possibilities, from disconnected tubes to a clogged EGR valve, to a bad
solenoid. All these parts are inexpensive. Probably an hour of labor to
diagnose and replace a failed part, at most. I wouldn't expect to pay more
than $150, hopefully less.
If you are a gambler and not afraid of bruising a knuckle, replace the DPFE
yourself and see what happens. Nothing more than a small wrench (can't
recall the size) needed.