This code has been coming up for a few days now. I have also noticed that the transmission seems to be shifting pretty hard since it came on. Could these 2 problems be related?

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1996 E300 D
1992 300D
1990 Ford E-350 7.3 Diesel AKA "the Deep Fryer"

96E300d here, "P0400 Exhaust Gas Recirculation Flow Malfunction" another poster brought up the fact that the EGR valve is also in a circuit with two other vacuum pressure sensors located on the side of the air filter box. Both sensors are the exact same part number and can be interchanged, one sensor controls the egr valve, the other the resonance flaps. Also in this vacuum circuit, is that small hose that comes off the intake manifold and feeds into a small black sensor. It seems that all three work in conjunction with one another along with the TPS, and send information to the computer system. In my case, I cleaned off all the electrical connectors at the vacuum pressure sensors, cleared the code, and it has not reappeared. The other thing you should check are all the small vacuum lines that activate the sensors. A leak in one of them could cause a problem. In my case, the car did not have any driving issues at all, just kept throwing a Cel code. Good luck...
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96 E300d

Your EGR valve could also need to be cleaned, it might be getting stuck open or closed.. Its fairly easy to remove and clean with brake cleaner

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD