How it works:
The following conditions must be met for the EGR to open:

• Engine coolant > 113°
• TPS (Throttle Position Sensor) is at part throttle
• MAF (Mass Air Flow) sensor is in mid range

Look for:

• Leaky or clogged hoses
• EGR valve stuck closed
• EGR valve leaks vacuum
• EGR flow path restricted
• EGRVR circuit shorted to PWR
• VREF open to D.P.F. EGR (DPFE) sensor
• DPFE sensor downstream hose off or plugged
• Defective DPF EGR sensor
• EGRVR circuit open to PCM
• VPWR open to EGRVR solenoid
• DPFE sensor hoses both off
• DPFE sensor hoses reversed
• Damaged EGR orifice tube
• Damaged EGRVR solenoid
• Damaged PCM (Computer)
• Perform KOER (Key On, Engine Running) self-test and look for DTC P1408 as an indication of a hard fault. If P1408 is not present, look for contamination, restrictions, leaks, and intermittents.
• Bad Vacume line

Trouble shooting:

1. When applying vacume (from a vacuum pump or other source) to the EGR valve with the engine off you should feel movement of the EGR diaphram.
   Apply vacuum to EGR valve with engine idling. It should idle rough or stall if exhaust gas enters the intake manifold. It should also hold the vacume.
   You can remove and clean the EGR valve. Do not use gasoline or solvents which will destroy the diaphram.
2. Apple 12V to the EVR. It should cause vacume in the hose to the EGR.
3. Turn the key on (engine not running) and check for voltage to the EVR solenoid connector.
4. Check resistance of the EVR solenoid. It should be 30 - 70 ohms.
5. Check voltage to the DPFE.
6. Disconnect hoses to EVR and blow air to verify that air does flow thru.

Ford had a new version of the DPFE that I had to get from them. After replacing the one I bought from Autozone (and correcting a leaking EGR gasket), my problem was solved. Source: AutomotiveForums.com/vbulletin/t193848.html

After removing and disassebling the Throttle Body (TB) I've found that there was a metal tube that originated at the very essence of an exhaust section, in the visinity of the headers, and leads all the way to TB, which then recycles the CO gas. That's where I've found tonns of carbon buildup that was restricting EGR system to vent into TB eventually causing this DTC. My problem was solved right there. Source: OBDII.com/ubb/Forum1/HTML/002127.html

Try replacing all the vacume lines. my Grand Marquis had that problem... Source: ford-trucks.com/forums/showthread.php?t=344591

I've replaced the EGR valve, which is a diaphram (vacuum controlled) valve that lets the exhaust gas back into the intake. Next, I replaced the EGR solenoid, which is electronically controlled to open the vacuum to the EGR valve. Neither fixed my problem. A vacuum leak is a possible cause between the EGR solenoid and EGR valve, but I can't find one.

The fault could lie in a blocked EGR passage, a marginal EGR valve, or possibly a component outside the EGR system that monitors EGR rate.
The easiest way to diagnose this code is with a computer scan tool which will give you more detail than just the trouble code. With a computer scan tool the voltage signal from the DPF sensor can be monitored and compared to values in a diagnostic flow chart. DPF sensors do fail, but you want to be sure that it is bad before you spend about $100 to replace it. Source: AllData.com/techtips/2004/20040405b.html

locate the EGR DPFE sensor where it lives...remove it and tap it in your hand with the holes down. If moisture comes out, your EGR system may be plugged. Spray the inside of the sensor with computer grade solvent to get the water out. Source: CarTrackers.com/Forums/