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I would then replace the upstream oxygen sensor (near the exhaust manifold). This sensor determines if your precat is bad or not. Since you replaced the precat, the sensor is sending an inaccurate reading which is why your engine light is on. I heard most oxygen sensors last around 100,000 miles or so.
I am not 100% sure if it is your upstream oxygen sensor, but that would be the first thing I would replace. This is all assuming the code is P0420.
Last edited by bob1234; 11-08-2010 at 08:34 PM.