To get the codes from you altima you have to get access to the "brain" of
the car. This is located under the center console under the radio. Once
you get to it (it'll have a lot of wires going to it), you should see some
LED lights on it. Also there should be some sort of place to insert a
screwdriver, Phillips (tm) or flat head. This screw is the mode selector,
and can be used to set the ECCS ("Brain") into Diagnostics mode. I can't
recall exactly how to get it into this mode, I think you have to turn the
screw clockwise, then hold it there for 2 seconds, and turn it back. After
that it will blink the LED lights in a manner to indicate the codes. If
there are 2 LEDs then the first will blink to indicate the ten's place
number (long blinks), and the second will pulse faster to indicate the
ones place. This code can then be used to determine what sensor tripped
the problem.