1. Feb 11, 2010. #1. I am the proud owner of a 2004 Avy Z71. I have the 4.3L Vortec V-8 and 4WD. There is no cladding on my rig. I have to replace two O2 Sensors and I am confused as to which ones need replacing. I know that the truck has four. The guy at Auto Zone said that I have to replace the Zone 1/Sensor 2 and Zone 2/Sensor 2. More @Wikipedia
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