Hello I performed a back pressure exhaust test on the upstream 02 port and got a zero reading even after increasing rpm. So according to what I have read and viewed the converters are good.. I know the rear one is because it is new along with the muffler. I am thinking if I install the upstream converter the PO420 code will still cause the check engine light to reappear. I have changed both O2 sensors with Bosch, OEM.. the upstream about 2 years ago the and rear with the new exhaust replacement parts. The gas mileage is as it should be 22 city and more on highway.
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