yo,
I had DTC PO401 too; went through the diagnostics and checked possible causes shown below; when I pulled REF hose off, fine particles fell out of DPFE
DPFE Location pic in a 96 5.8
Source: by bergs
P0401 - EGR Flow Insufficient Detected The EGR system is monitored during steady state driving conditions while the EGR is commanded on. The test fails when the signal from the DPFE sensor indicates that EGR flow is less than the desired minimum. Vacuum supply
EGR valve stuck closed
EGR valve leaks vacuum
EGR flow path restricted
EGRVR circuit shorted to PWR
VREF open to DPFE sensor
DPFE sensor downstream hose off or plugged
EGRVR circuit open to PCM
VPWR open to EGRVR solenoid
DPFE sensor hoses both off
DPFE sensor hoses reversed
Damaged EGR orifice tube
Damaged EGRVR solenoid
Damaged PCM
Perform KOER self-test and look for DTC P1408 as an indication of a hard fault. If P1408 is not present, look for contamination, restrictions, leaks, and intermittents.
DTC PO401; Most of the time, because of hot exh gases flowing thru the DPFE, it is a failed DPFE sensor; Before you buy a new one; ck the two high temperature silicone hoses that go fromthe EGT Tube's 2 small diameter tubes to the DPFE sensor. They are available at your local Ford Dealer. The dealer sells this special silicone hose by the foot ( I paid $10.00 for each hose & you trim it to exact length. PNs 1L3Z-9P761-AA - 1/4" for REF - Downstream hose (INTAKE) & 1L3Z-9P761-BA - 5/16" for Upstream hose (EXHAUST);
For DPFE Sensor, The new DPFE sensor will be plastic as your original is still probably die cast metal; dealers in area want $150.00 for the DPFE Sensor and will not discount; so I bought the MOTORCRAFT DPFE15 thru Amazon w/free S&H for $60.00 and no tax;
Also read the DPFE Testing & Vacuum/Voltage Parameters & pics on both older Aluminum & Newer Plastic Version
@
http://www.tomco-inc.com/Tech_Tips/ttt36.pdf
backprobe connector using long straight pin (I use my better-half's sewing kit pins with large plastic balls on end and clip one meter's lead to it)
Select Volts DC on your Multimeter
Connect the BLACK lead of the Multimeter to a good ground point on the engine or to the Battery Negative Terminal.
for the tests req. a hand held vacuum pump
go to local parts store and use their Loan-A-Tool Program