From what I've found:
If the Electronic Vacuum Regulator (EVR) duty cycle is very high (greater than 80% duty cycle), the differential pressure indicated by the DPFE sensor is evaluated to determine the amount of EGR system restriction. If the differential pressure is below a calibrated threshold, a low flow malfunction is indicated (P0401).
The DPFE sensor has two lines connected to exhaust ports, one to the exhaust stream and the other to the EGR exhaust feed. The DPFE sensor generates a 0 to 5 volt signal based on the differences between the two pressures.When the EGR is fl owing, that pressure will drop and the signal reading will be high. It is a typical three wire sensor with a signal, a 5 volt reference and a ground supply. Check for cracked or brittle lines before condemning the sensor. If the vehicle has a tube-mounted DPFE
sensor and one of these codes are present, the bulletin instructs the technician to install a new sensor. Ford has recognized a problem with these DPFE sensors. There is a redesigned sensor available.