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Not tryin to jack your thread but, I don't want to start another topic. I am having a similar problem. Getting PO420 code,Crappy MPG, Also exhaust smells very rich. I have a high flow cat would that set a code? I have had it for a year though and the code just came up a month ago? I don't want to replace the O2 sensors if I don't HAVE to. My car has 127,000 miles if that's any help. I know GM recommends to change O2 sensors at 100,000 miles but, is it necessary
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