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Message: <blockquote data-quote="duff daddy" data-source="post: 679346" data-attributes="member: 10730"><p>Up stream oxygen sensors (in conjunction with other sensors) will read how the vehicle is running then suggest to add fuel or pull fuel depending on how things look based on exhaust gases. The down streams job is to say OK after going though the converter everything is where it should be so no other adjustments are needed at this time. When you took out the cats there is on change so its going to turn on the MIL for (normally) catalytic converter efficiency. Sometimes if your old sensors are weak enough, and respond slow enough, then you wont turn on a light because of the sensors being "lazy" or slow.</p></blockquote><p></p>