On-Board Diagnostics
WARNING: This page is about a different car, the 2003 Nissan Altima. However, it is still accessible from the selected car via links, so may be relevant.
The ECM has an on board diagnostic system, which detects malfunctions related to engine sensors or actuators. The ECM also records various emission-related diagnostic information. See Fig 1.
The information in Fig 1 can be checked using procedures listed in Fig 2.
The Malfunction Indicator Lamp (MIL) on the instrument panel lights up when the same malfunction is detected in 2 consecutive trips (two trip detection logic), or when the ECM enters fail-safe mode.