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Hey guys need your help! I keep getting that P0420 code and cannot seem to get rid of it. I have replaced all 4 oxygen sensors and the catalytic converters. It comes on at different times; like idling, standing in the gas, or just driving down the road. I also noticed the tailpipes were black and sooty. Any thoughts?
 

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All the down stream sensor does is look for the change in resistance caused by the drop in temp between it and the up stream. If it does not indicate a change of 100* +/- it sets that code (left bank) and or 430 (right), the code reads '...below threshold' or words to that effect. It is seen as a cat fault. If you get your hands on a temp gun shoot the up and down sides of the cat, if that temp drop is not 100 or better either the cat has failed or (most AM cats) it is not as efficient as the programing wants it to be. Either put OEM replacement cats on or take it to a tuner. O2 sensors do not fail very often and should be one of the last things to look at.
 
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