P0401 is an EGR system code not specific to DPFE sensor.
EGR system has the EGR valve/tube, then the EGR regulator/vacuum valve.
The DPFE sensor is used by the computer to monitor the pressure difference as the EGR valve opens.
With any EGR system code you should first check the EGR valve.
With engine warmed up and idling, remove the vacuum hose on the EGR Valve.
There should be no vacuum on that hose and engine idle should not change.
If there is vacuum then problem is with the EGR regulator, in any case follow that hose back to find the regulator.
It will have a second hose to the intake manifold, make sure that hose is "air tight" this hose should have a strong vacuum with engine idling.
After checking the regulator place a vacuum hose on the EGR valve and apply a vacuum(suck on the hose), engine should start to idle rough and then die.
If not then EGR valve/tube is clogged up or bad, remove and clean or replace.
The DPFE sensor has two hose that connect to exhaust system, one is for 'normal' exhaust pressure, the other is closer to the EGR valve and will have less pressure if EGR valve opens, so the DPFE measures the pressure difference between the two hoses so the computer can monitor EGR valve flow.
The hoses and exhaust ports can get clogged with carbon so good idea to check that when checking EGR valve/tube
P0401 = Insufficient EGR Flow
So it could mean DPFE sensor is not working correctly, but best to check if EGR valve can open and exhaust gases can flow into engine first, because that is what the DPFE sensor is monitoring.