Took my '99 5.4l in for the SES light on and thought I would write it up,
since there isn't a lot of info about it and hasn't been posted much.
Truck started sounding a little funny on cold start-up, fast idle wasn't very fast anymore, and seemed to kick down a lot quicker than usual.
There was also a stumble, or misfire between 30-40 mph. The code was po402-egr excessive flow detected. I thought maybe a COP was going out. I hadn't even heard of this one before.

There is a good write up on how to test it here:
https://www.ford-trucks.com/forums/3...fe-sensor.html

The mechanic told me that if it is faulty long enough, it can destroy your cat converter.

Here's another good write up for insufficient flow--
https://www.ford-trucks.com/forums/4...e-testing.html

Here's another good explanation from the Ford Focus forum--
Definitions:

EGR means "Exhaust Gas Recirculation", the process of sucking exhaust gases back into the engine to re-burn them. It is a technique that reduces the pollutants emitted by the engine.
DPFE means "Delta Pressure Feedback of EGR", Delta means "difference" or "change", so the DPFE Sensor reads changes in the pressure of the EGR system.
Description of the EGR and DPFE system:

There's a vacuum line coming off the intake manifold to the EGR actuator. The actuator is on the firewall and it's an electric device controlled by the PCM that modulates how much vacuum pressure gets past it. From there, another hose goes to the EGR Valve. Vacuum pressure opens the EGR Valve, allowing exhaust gases to be pulled from the exhaust manifold, past the DPFE tubes, then back into the intake manifold.
When gas flows through the EGR tube, the two tubes leading to the DPFE Sensor get pressurized. There's a differential in the pressure, one tube has more pressure than the other one. The DPFE registers the amount of pressure difference and can tell how much exhaust gas is flowing through the EGR system. It tells the PCM how much is flowing, and The PCM uses this reading to control the actuator.
DPFE Sensor Failure

When the DPFE Sensor starts failing, it gives the PCM bad information. The DPFE Sensor is less sensitive, and the PCM thinks less recirculated gas is being burned than is actually so. Trying to compensate, the PCM opens up the actuator too much. This excessive amount of EGR combined with the air coming in via the throttle body leans out the fuel mixture, causing a lean misfire, which is the "stumble" or "hesitation" that is common as the DPFE Sensor fails. The DPFE sensor in the Focus is not very robust, requiring frequent (every 30k miles or sometimes less) replacement or a work around. Note that complete DPFE sensor failure is a very slow process, and the check engine light won't come on until the DPFE sensor is pretty much done for.