Visual inspect first. The DPFE sensor is by the EGR valve, it will have one connector and two silicone hoses attached to it, the other ends of the hoses attaches to the EGR tube (from manifold to EGR valve). Verify both hoses are attached and not broken or split. Verify the connector is OK/properly connected, no pins are pushed out/no corrosion/water intrusion etc.
The vac to the EGR valve is controlled by the EVR (EGR Vacuum Regulator). Follow the vac line from the EGR valve. Engine at idle, there should be no apppreciable vac at the EGR valve vac port, you might feel some sort of airflow/vibration at the vac connector but unless there is an actual vacuum, don't worry about it, it's just normal EVR flutter. If there is appreciable vac (>2hg)verify the little filter on the EVR (under a round snap on cap) is clean. A clogged EVR filter will not let the EVR release the vac to the EGR valve when the PCM commands it. Longshot, but can occur in dusty climates.
Apply vac to the EGR and release it a few times to verify operation, you can do it at idle to check for sticking (note it will roughen idle and possibly set a code if you do this).
I don't know the correct DPFE voltage at idle. There were different null voltages on different sensors, some were .4V, some were 1.0V. Voltage will increase as the EGR flow increases. If you are confident, you can carefully backprobe the connector and read the voltages with a DVOM, but that won't tell you that the processor is seeing the same thing. Scan tools are very handy. I do know one tech (I won't elaborate) got a similar code when the parts dept gave hime the wrong sensor.
11 years at Ford, 10 years working with modular series engines, I replaced 1 EGR valve........because I broke it while doing other work on the vehicle.
