well i have a ford f150 5.4L and for the longest i have a EGR valve fault code of 401 my scan tool tells me it is insufficient flow. i replaced the EGR and still got the code i went to the manual and did the test procedures on all the sensors. the only sensor i got a bad test on was the DPFS (differential pressure feedback sensor) when the engine is hot the signal voltage to the sensor should be 5-6VDC so the manual said, but i only get 1.7VDC after the temp is hot, i also don't get vacuum on the hose that connects to the top of the EGR valve. from looking at the system pic in the manual it is the DPFS that controls the opening of the EGR. i don't know what to do or what else to tell you any help would be great to keep it away from the mechanic who over charges!

 

Replace the DPFE sensor. It is a square box about 4 inches to the left of the EGR Valve (if you are standing on the drivers side of the truck). This sensor is prone to failure on the F150. you can drive it with the SES light on from that code. You might lose 1 to 2 MPG, but it will not affect the engine. I had this happen on my 2000 lightning and ended up turning the EGR system off in the ECU. It was a pain in the azz. I had to replace the Sensors every 10,000 miles. I can almost garuntee that the DPFE sensor is the one that has failed. (BTW, it is called the EGR pressure sensor at Autozone). This should fix your problem. Did you clean the carbon build up when you replaced the EGR?

Edit: This is one expensive sensor. $60 at Autozone.

 

You stated, "The manual said the signal voltage to the sensor should be 5-6VDC". The key words here are "signal voltage to the sensor". I believe what you are referring to is the "reference voltage" supplied to the DPFE by the EEC computer. If you are only getting 1.7VDC on the terminal that is supposed to read 5-6VDC then that indicates to me that the problem is not with the DPFE but, rather with the voltage being supplied to the sensor. Be sure you are measuring the voltage on the proper terminal.

 

Usually, the DPFE sensor is the first part to go bad in the EGR system and is the leading cause of code P0401 in my experience. From what you posted, it looks to me as the sensor is bad.

Also check to see if the EGR ports (from the EGR valve into the intake) are clogged. The EGR valve itself seldom goes bad.