It's not so much what the average voltage is for the oxygen sensor, but what the rich-lean switching frequency ratio is between the upstream and downstream sensors on each back that is used to determine efficiency. The rear sensor should have approximately half of the rich-lean cycling frequency compared to that of the front sensor's while cruising down the highway at normal operating temperature. One would have to graph the voltage signals of the front and rear sensors in order to compare them, which is easier said then done if you don't have someone else to drive the vehicle while you watch the scantool, not to mention how much traffic you may encounter. If there are no exhaust leaks and the oxygen sensors appear to be working properly, and there is no noticeable excessive rich or excessive lean running condition, it is usually an inefficient, upstream catalytic converter that is the cause of a P0420 or P0430 trouble code. If you have under 8 years-80000 miles on your vehicle, check to see if it's covered under your long term emissions warranty.
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