From another forum and a self proclaimed Nissan master tech:
Engine Control Unit (a.k.a "the engine computer"). It sits behind your central console.
1.To access it, remove the plastic panel next to where your front passenger would put its feet. It's a metal box bolted down right behind your heater and AC controls.
2. Remove the 2 bolts and yank the ECU loose (gently!) and don't disconnect anything. At the back of it there is a little screw, it may be covered by a piece of plastic tape.
3. Turn your ignition key to the "On" position but don't start the car.
4. On the side of the ECU that was originally facing your passenger's feet, a little red little should've come on.
5. Using a screwdriver, turn that little screw all the way (about half a turn) as far as it goes without forcing.
6. Count slowly to 2.
7. Turn the screw all the way back as far as it will go without forcing.
8. Look at the little red light, it should be blinking. Count the long blinks and then the fast blinks. The long blinks indicate the tens, the short blinks indicate the units.
9. Look-up that number in a trouble code chart. Code 55 is "No problem found".
10. The ECU will cycle through all the codes it needs to report. When you're done, turn the screw all the way over again, count to 2, then turn it all the way back and turn your ignition key to off.