On Board Diagnosis Logic
WARNING: This page is about a different car, the 2004 Nissan Sentra. However, it is still accessible from the selected car via links, so may be relevant.
To judge the malfunction, the A/F signal computed by ECM from the air fuel ratio (A/F) sensor 1 signal is monitored not to be shifted "lean" side or "rich" side. When the A/F signal is shifting to the lean side, the malfunction will be detected.