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How many miles on the "primary" or otherwise known as "precat" O2 sensors? It could be as simple as one of them not working as well as the other. Best to replace in pairs and to not exceed 50k on them. I replaced my originals at 60k and there was a noticeable improvement in driveability which means they actually needed to be replaced sooner than 60k. My car is completely stock, no tune.
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