My shop pulled an EGR code, found nothing wrong with the EGR and went to the next logical step. Apparently the DFE WAS bad since the car is running/idling better than before.
I don't think that replacing the DPFE would make the car idle any better, since the EGR system does not kick in at idle.
There are no "easy" remedies to an EGR problem, since there are a lot of possible fixes. The following text comes from the Ford Technical Service Publication (TSP) on a P0401:
Diagnostic Trouble Code (DTC) P0401 indicates that Continuous Memory Self-Test has detected insufficient EGR flow.
Possible causes:
-- EGR valve stuck closed or iced.
-- EGR valve diaphragm leaks.
-- EGR valve or flow path restricted.
-- EGR vacuum hose off, plugged or leaks.
-- VPWR circuit open to EGR Vacuum Regulator (EGR V.R.) solenoid.
-- EVR circuit to PCM open.
-- EVR circuit to PCM shorted to PWR.
-- Differential Pressure Feedback EGR (D.P.F. EGR) sensor pressure hoses both off.
-- D.P.F. EGR sensor pressure hoses reversed.
-- D.P.F. EGR sensor VREF circuit open.
-- Downstream pressure hose off.
-- Downstream pressure hose plugged.
-- Damaged orifice tube assembly.
-- Damaged EGR V.R. solenoid.
-- Damaged D.P.F. EGR sensor.
-- Damaged PCM.
The easiest thing that you can do is visually inspect the vacuum hoses to make sure that there are no cracks or leaks.
After that, you will need tools to test each of the components. I have found Alex Peper's Vehicle Explorer software to be the best tool for this situation, since it will give readings on the DPFE and EVR (aka EGR VR). Another good tool is a hand-held vacuum gauge.
If you just want to replace parts one at a time, then the next one to replace would probably be the EVR (since you already replaced the DPFE and you say that the EGR valve is good). This part costs around $15-25 and is easy to replace (just remove two screws, the electrical connecter, and the two vacuum hoses). You could probably do it yourself.
One last thing, how did your shop check the EGR valve? Are you positive that it is OK?
