I have a 96 Max with over 285k miles.  I have never really had many
issues with it beyond the usual maintenance and wear and tear.
However, for the last 3 years, almost all of my emission sensors have
been replaced.  I thought I was through with all that for another
200k
when my "Check Engine" light just recently came back on.  It was
0503,
the RH Heated O2 Sensor.

I purchased the normally $80 part online for about $50.  It was an
SNG
sensor.  I replaced it, reset the computer and drove.  I also
replaced
my fuel filter and added a short-ram intake kit all on the same day.
Almost a week later, the light came back on.  It was the same code.
I
cleared it, and it came back on the within miles.

I have reset it a few times.  Since all of this, my car has been
getting less MPG, from about 27MPG to 18.  Moreover, it has now
started to hesitate when I am accelerating from a stop or from less
than 20mph.  It is almost acting as if it is not getting gas or
something.

It drives fine on at highway speeds.  It accelerates a little faster
to 60, 70, 80 and so on.  In addition, I have been able to drive at
80, 90 and even 100 without the car going over 3,000RPM.  I attribute
that to the air filter mod, but I could be wrong.

Therefore, Nissan community, what do you think the problem is?  Is my
new O2 sensor bad?  If I had bought it locally, would you have
suggested I returned it already?  On the other hand, is it the mod or
something else?

* I will let you all know that I am no expert but I have done most of
the work on my car (CV axle replacement, radiator and such), so I am
no slouch.  However, I am lost on what I should do next.