Hi Signals -- I'm just seeing a message from Feb 2013, and now it is Sept 2013 -- My answer is that if you reset all your codes, and then P0400 comes on again, and you have not cleaned the tube: then it is likely the EGR tube. In any case, that tube should be cleaned to allow the exhaust system to function according to design. If you clean it and the code still comes up (again, after a code reset) then you could chase other potential issues -- though most people are seeing the code disappear after this tube cleaning.

97 maxima - KS replacement.
Points to avoid facing same access issues again.
Major access problem is EGRC valve solenoid (green plug) nutted below a horizontal plate.
Idea is:
1) move valve away by replacing 3 short tubings by longer ones. *
2) cut or bend down mtg plate & keep it ever away. Reuse cut part to relocate if needed. Or tie the valve safely to nearby tubing?
Notes:
1) Hard to remove hoses! Locations & clamps very awkward. Change squeeze clamps to belt & screw type.
2) Pulling out hoses is very PIA at many locations. Right size open ended wrench helps to push out hose ends after W40 pre-soaking. HF tool ($9) for hoses is useful too-insert tool below hose & twist to loosen while tugging it with other hand. Lubricate inside of hose ends before re- insertion.
3) Don't over tight bolt on KS. It may crack & wont be noticed! Hand tight fully & then a click or two with a socket driver. Found *std. 3/8" socket driver right length to lever it forward & loosen KS bolt.