Code 31 is a load signal not sensed during diagnotics. "Load", as in aircon or headlights and stuff. That is how the ECU knows to bump up the idle speed when you run the aircon.
If you don't have aircon then you can switch something else on during that step in the diagnostics.
The ECU has part numbers on it, both Nissan and JECS part numbers. What part numbers you should have depends on the year, federal/Cali, auto/manual.
The JWT website has a list of part numbers and applications, and lists are probably available elsewhere.
Even obscure things like the Nissan parts microfiche will list part numbers.
Auto ECU should work with a manual box, but auto ECUs want the motor to idle higher in neutral, generally speaking.