e320 are known for this problem. Fortunately, it's an easy DIY.
The EGR tube snakes from the EGR at exhaust manifold around back of engine and enters head just below 2nd and 3rd intake tubes. Tube has a 90 deg turn here. There's a carbon build up in the elbow, just inside the head or both.
Put car up on jack stands. Remove two 10 mm nuts [or bolts - I forget] securing tube to manifold. Use 1/2" nylon diameter brush [available from your kitchen's cleaning equipment drawer] to clean out both head & tube. I used lacquer thinner as cleaning agent. Reassemble. Clear code. If still persists, there's a build up in the tube probably where it goes around back of engine. You'll need spiral cable as used on the throttle cable of a lawn mower. Disconnect tube at head and at EGR. Attach spiral cable to electric drill and snake thru tube.
Hardest part is R&Ring the nuts or bolts on the tube where it attaches to the head. Best to do one from top other from bottom with a box wrench and a 1/4" drive socket. Tedious, have a bear or two while wrenching, The <$2 gasket will often fall off while assembling.
TIP
If you use the spiral cable to do tube. I threaded it thru a plastic tube to prevent it from wrapping up on itself while feeding into EGR tube. If you ever snakes a drain, it's the same procedure. Lawn mower cable and plastic tub avail at Home depot, Lowes etc.
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