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Exhaust System Leaks

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

For example, exhaust system leaks upstream or near the HO2S, and poorly welded/leaking HO2S boss.

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