I have a truck (98 F150 4.6) that keeps setting a PO401. I checked the EGR passages, all clean. I replaced several hoses and also tried a used DPFE. Still getting a PO401 everytime I clear the codes and drive 30-40 miles (without turning the key off). I tried the tip about using vaccum on the EGR valve, and the engine does almost die. I've heard of reprogramming the PCM, but not sure if this is the way I should go.

 

Here is an overview on the fault.
401 code is low flow detected when the PCM does a system test for an 'expected' flow rate as measured by the DPFE.
The test limits are very tight so failure of even the DPFE can be a cause of the test failure.
The test involves measuring the pressure difference across a calibrated orface in the metal tube to which the DPFE is connected accross.
Problem could be from a rusted through tube, blocked orface, DPFE out of calibration, leaky attaching hoses.
The vacuum source to the EGR regulator is not solid, the reg is not in good condition etc.
Anything in the system involved can cause the issue.
Inspect all the areas the n if you can't come up with a reason, try a new Ford DPFE.
Clear the code and run 2 drive cycles to see if it was the trouble.
The system takes 2 drive cycles to see if the test fails 2 times in a row before logicing it a failure. This test sequence is intentional by design in software because of moisture build up in below freezing temps that might block a passage until it melts from engine bay heat.
Good luck.