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Message: <blockquote data-quote="2007LucerneCXL" data-source="post: 374873" data-attributes="member: 38546">On the code if a shop or if you are doing the repairs. There's two O2 sensors and it's possible, depending on sequence of repairs that the harness / O2 sensor may have been damaged or it's a poor quality aftermarket O2 sensors. A O2 sensor extender may resolve the problem, the type of catalytic converter may also be the cause. How many miles and what type of driving, highway or city, since installing?Another site that may help.[URL unfurl="true"]https://troubleshootmyvehicle.com/gm/3.8L/how-to-test-trouble-code-P0420-page-1[/URL]</blockquote>