Three Way Catalytic Converter (TWC) Conversion Efficiency
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2011 Mercury Milan, 2011 Mercury Mariner, 2011 Mercury Grand Marquis, 2011 Lincoln Town Car, and 2011 Lincoln Navigator L. However, it is still accessible from the selected car via links, so may be relevant.
A TWC requires a stoichiometric air fuel ratio of 14.7 pounds of air to 1 pound of gasoline, or 14.7:1, for high conversion efficiency. In order to achieve these high efficiencies, the air/fuel ratio must be tightly controlled with a narrow window of stoichiometry. Deviations outside of this window greatly decrease the conversion efficiency. For example a rich mixture decreases the HC and CO conversion efficiency while a lean mixture decreases the NOx conversion efficiency.
For vehicles using E85 the required air/fuel ratio is 9.8:1. Other gasoline/ethanol mixtures require a variable air/fuel ratio between 14.7:1 to 9.8:1 dependent on the percentage of ethanol content.