The next thing you'd do is check for intake leaks - starting at the MAF and then onto everything on the manifold including the intake manifold itself. I usually start with a careful visual inspection and then move on to an unlit propane torch feeding around all possible leak points. One thing to inspect very carefully is the intake snorkel itself. Cracks & splits can be very hard to see - twist, pull, bend, etc.
There is a TSB out for this code (NTB06-039a) but it specifies it is for Altimas. It may be worth a call to a dealer service dept though just to find out for sure.