This 1999 Mercury Sable came in with the check engine light on and a code P0401 for insufficient EGR flow. Luckily the EGR valve and the DPFE sensor are easily accessible on this car.
I set may scan tool up to view DPFE voltage and connected my hand vacuum pump to the EGR valve. This can also be done with a voltmeter connected to the DPFE signal wire.
The voltage with the key on and the engine off was .44 volts. The voltage with the key on and the engine running was the same voltage. The voltage with the key on, the engine running and the EGR valve opened with the vacuum pump was also .44 volts. The engine also started running poorly and almost stalled, this indicated that the EGR valve and passages are open, good. Looks like probably the DPFE sensor is faulty.
I turned the engine off and switched the key back on, jumped the 5 volt reference wire to the DPFE signal wire and monitored voltage. The voltage went close to 5 volts. Diagnosis and replacement of a faulty DPFE sensor.
The original DPFE sensor is on the right and was of a metallic design the replacement has been improved and now is made of plastic. The metal one tended to corrode badly.