Fuel System Monitor

WARNING: This page is about a different car, the 2003 Dodge Dakota. However, it is still accessible from the selected car via links, so may be relevant.

To comply with clean air regulations, vehicles are equipped with a catalytic converters. These converters reduce the emission of hydrocarbons, oxides of nitrogen and carbon monoxide. The catalyst works best when the air/fuel ratio is at or near the optimum of 14.7:1. The Powertrain Control Module (PCM) is programmed to maintain the optimum air/fuel ratio of 14.7:1. This is done by making short term corrections in the fuel injector pulse width based on the Oxygen (O2) sensor output. The programmed memory acts as a self calibration tool that the engine controller uses to compensate for variations in engine specifications, sensor tolerances and engine fatigue over the life span of the engine. By monitoring the actual air/fuel ratio with the O2 sensor (short term) and multiplying that with the program long-term (adaptive) memory and comparing that to the limit, it can be determined whether it will pass an emissions test. If a malfunction occurs such that the PCM cannot maintain the optimum air/fuel ratio, then the MIL will be illuminated.