Air Measurement System

With this condition, the engine will run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the PCM is not able to compensate enough to correct for the condition. One possibility is that the mass of air entering the engine is actually less than what the MAF sensor is indicating to the PCM. For example, a contaminated MAF sensor, the engine will run rich at idle because the PCM will deliver fuel for more air than is actually entering the engine.

Examples: MAF sensor measurement inaccurate due to a corroded connector, contamination/dirt. A contaminated MAF sensor typically results in a rich system at low airflows (PCM will reduce fuel) and a lean system at high airflows (PCM will increase fuel).