Could be any thing in the egr system... A vaccume leak, a malfuntiioning or stuck egr valve, or solonoid, the list goes on.
I would head up to your local nissan parts department, ask to view a fsm for your year (all the ones ive been to have been happy to let me look at one, although I have my own I couldnt afford to buy a fsm for every idea I get

)... When you get it thumb through the "ec" section till you get to the egr system testing pages. Then say Id like a copy of ec-88 to ec -96 (so to say) if you wouldnt mind please. As long as its not a ton of pages and they arent extreamely busy they should hand them out....
of and the mil / cel story was just something I get a kick out of.... I really dont know why I posted it, oh wait its because that was the night it went out again hehe.