I really think the 401 is an exhaust leak issue. Pressure loss. The lower egr hose port of the dpfe sensor did not see a difference from itself and the upper hose port of the sensor (ref), when the PCM commanded it and expected it.
You can apply vacuum to the egr valve while the engine is idling and listen for a stumble. Stumble =EGR valve good
The DPFE is iffy. even a new one. Try to find a metal one somewhere.
The only other thing is the vacuum, and the vacuum regulator that switches vacuum to the egr valve when the PCM tells it to. it is an air valve solenoid. I guess you could apply some voltage to it, which should make it actuate and result in the stumble idle as above. That would verify the system is working, when told to.
Now, if the DPFE doesn't see a difference in pressure between the 2 hoses when the PCM says it should, p0401 will be set.
I think if the valve stays open it will trigger a p0402.
If you look at all of the EGR code possible though I think the troubleshooting part has been done for us.
http://www.explorerforum.com/forums/index.php?threads/obd-ii-diagnostic-trouble-codes-dtcs.228242/
P0400 Exhaust Gas Recirculation Flow Malfunction
P0401
Exhaust Gas Recirculation Flow Insufficient Detected
P0402 Exhaust Gas Recirculation Flow Excessive Detected
P0403 Exhaust Gas Recirculation Circuit Malfunction
P0404 Exhaust Gas Recirculation Circuit Range/Performance
P0405 Exhaust Gas Recirculation Sensor A Circuit Low
P0406 Exhaust Gas Recirculation Sensor A Circuit High
P0407 Exhaust Gas Recirculation Sensor B Circuit Low
P0408
Exhaust Gas Recirculation Sensor B Circuit High
So, IMO the 401 low flow is from either a leak somewhere ( exhaust manifold gasket, egr tube, egr tube hoses to DPFE split or blown out, DPFE blown out ) or possibly a clog in the egr metal tube between the orifices .
If the hoses and DPFE continue to blow out, or if you experience the hoses being blown off the EGR tube connections,
clogged cats.
Hope this helps.