This is a very common Ford fault code as I learned from my net surfing for PO174 - Insufficient EGR Flow. I had this nagging problem on my 2002 Moutaineer - I changed the EGR - cleared the code - code came back. Change the DPFE - cleared the codes - problem solved!
As always with my luck - Ford just put out a TSB for the 'Mounty' related to problems with the DPFE switch. I would say that is your problem.
99 Countour SVT
