before diving into spending alot of money it could also be a faulty vac line to the solenoid or to the egr

to see if the egr is whats gone get a hand vac start the car and pop the vac line off the top of the egr and connect the vac while running apply vacuum to the egr if your car starts to idle bad or even stall out the egr is functioning.
while you have it connected and the stalled out from applying vacuum, keep a vacuum applied (about 5 lbs) and see if it stays and holds,if it cannot hold 5lbs for more then 2 minutes it would mean the EGR's diaphragm is split/torn.

if the EGR checks out then its most likely the EGR's Vacuum solenoid. but again check the vacuum lines to each because they do go bad

Below is a borrowed picture you can see the EGR on the bottom of the pic the vac line that is on top is the line from the vacuum solenoid which is located on the firewall of the enginebay

Also Borrowed this is the EGR Solenoid which is attached to the back of the engine bay on the firewall

 

OP confused the issue by asking 'WHICH PART' on a car that has NEITHER.

Single electric EGR valve/sensor located at the end of the cylinder head, and it's water cooled with a return hose off of it so watch out for that if you remove/clean/check it out.

Don't forget to check WIRING/connector first before assuming a bad valve, and looking in the head to see if it might be gummed up with crud would help as well. You can blow both ways to make sure that's clear.

 

good point and now i feel sheepish (baaaaaa) havent owned a duratec long enough to notice that.. dont expect to own a duratec much longer either

 

2006 Focus has an EEGR....and it's more than likely a faulty EEGR that's setting the P0400. Of course, it's also possible that the DPFEGR sensor is faulty.

 

Posting here to echo what sailor said at post#3.

Only component here is the EGR (aka EGR Valve, aka EGR Stepper Motor). Other system items are the PCM and wiring.

My 2004 EGR has 6 wires. 2 power wires and 4 control wires going to the PCM.

 

Hehe - but TOTALLY right when you consider the OP was talking about a vacuum controlled EGR valve.

I'm not admitting wrong this time - just for chuckles.

 

hey i made the same error.. i havent had my duratec for long so hell if i knew.. as everyone knows the SAP is my very first Duratec all 20+ of my other cars are all zetecs so i used the off the top of my head zetec part numbers to fish the pictures

 

Thank guys, the 06's do not have a DPFE or EGR Vacuum solenoid so ill just change the EEGR valve and/or check for blockages in the line