Exhaust System Leaks

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In this type of condition, the engine runs rich of stoichiometry 14.7 to 1 (9 to 1 E100) air to fuel ratio, because the fuel control system is adding fuel to compensate for a perceived (not actual) lean condition. This condition is caused by the heated oxygen sensor (HO2S) sensing the oxygen (air) entering the exhaust system from an external source. The PCM reacts to this exhaust leak by increasing fuel delivery. This condition causes the exhaust gas mixture from the cylinder to be rich.

Examples of this include:

exhaust system leaks upstream or near the HO2S
cracked or leaking HO2S boss