Initial rough idle could be a bad injector, but that may not throw a code
for weeks.  I had 2 injectors go bad on my 95 (one under warranty), and
the check engine light did not come on until about a month after I noticed
the rough idle during start.  But it could be some other sensor or water
in the gas.

I never got a shop manual, so I do not know Nissan diagnostics, but my
T-bird had a key-on engine running test that would kill one injector at a
time to see if one would NOT drop the idle (indicating it was bad).