Since your post suggests an EGR problem... P0 400 on that car..

Here are the posibilities:

The EGR valve doesn't open...doesn't hold vacuum...doesn't get vacuum.

The vacuum switch-over valve doesn't have a vacuum source....doesn't get signal to open.. doesn't open.

The EGR tube is PLUGGED or partially plugged.......THAT is the most common EGR problem IF the above things pass first!

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