Air Measurement System:

WARNING: This page does not describe the selected car, but rather 30 other vehicles, including the 2004 Mercury Sable, 2004 Mercury Mountaineer, 2004 Mercury Monterey, 2004 Mercury Marauder, and 2004 Mercury Grand Marquis. However, it is still accessible from the selected car via links, so may be relevant.

With this condition, the engine may actually run rich or lean of stoichiometry (14.7:1 air/fuel ratio) if the Powertrain Control Module (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, with a contaminated MAF sensor, the engine would run rich at idle because the PCM would deliver fuel for more air than is actually entering the engine.

Examples: MAF sensor measurement inaccurate (corroded connector, contamination/dirty (a contaminated MAF sensor will typically result in a rich system at low airflows (PCM will reduce fuel) and a lean system at high airflows (PCM will increase fuel), etc.).