Could be a bad DPFE sensor. you should also check the hoses running to and from the DPFE and the EGR Tube and make sure they aren't clogged, pinched or burned. If you end up replacing remove the negative battery cable
To test the sensor
Disconnect the DPFE sensor harness connector. With the ignition on and engine off, measure the voltage between VREF and SIGRTN terminals of the DPFE harness connector. If the voltage is 4 -5.5V, the power circuits to the sensor are okay.