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P0401 (EGR Flow Insufficient)
The EGR (Exhaust Gas Recirculation) system causes some of the exhaust gas to be redirected back into the intake manifold. The idea is to reduce the combustion temperature by adding inert gas to the air fule mixture to reduceNOx (nitrous oxides) cause by high combustion temperatures.
How it works:
This starts flow from the exhaust manifold to the intake manifold which is sensed by the EGR flow sensor (called a Differential Pressure Feedback EGR (DPFE) sensor, which sends a signal back to the computer which turns the EVR off and on to get the desired flow.
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.
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/