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Message: <blockquote data-quote="haywire" data-source="post: 537542" data-attributes="member: 16474">Pre-cats is actually used to control the combustion in the engine, the ECU reads the o2 level in the exhaust manifold and alters the combustion according to the reading it gets from the pre-cat o2 sensor. If lots of o2 is detected in the exhause manifold the engine is not burning in optimum performance, isnt it?Post cat is more for monitoring purposes only, and detects whether the cat is properly working.Many people would advise changing the pre-cat only and leave the post-cat alone as it doesnt do much to the engine performance.</blockquote>