A shop just did this for me. I failed NOX. EGR valve was failing and tube was plugged. They replaced valve and solenoid and cleaned the tube. I asked them how they clean the tube and he said he did it "thru the throttle body". Apparently the tube typically clogs at the inaccessible intake end and not at the accessible egr valve end. As I mentioned in another thread, since I failed at a Test Only facility, the state of CA paid $500 of the $860 bill to get the car to pass.

glenmore
1991 300CE
2000 C280
1990 LS400