NICO Club's site will give you access to a free, online, factory service manual for your vehicle. You'll want to click on the "ec.pdf" which is the Engine Control section of the FSM. On page EC-128, there is a "Symptom Matrix" you can use. It will help you get to the right section you need, which will give you step-by-step diagnostics to help you pinpoint your problem.
Nissan Factory Service Manuals