Q: Is there a difference between front and rear oxygen sensors. Should I replace 4? on 2004 GMC Sierra 1500

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2004 4.8 ltr v8. I had to replace catalyic converters at 84,000 mi. I replaced 2 sensors and got a code for a weak rear sensor. So I replaced the other 2 but it is still coding a weak sensor.
(1) Answer
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Yes replace your O2 sensors as a package not as an individual item. Reason is because the computer will read the sensors and compare the values to set the fuel management system air fuel ratio. If one is not reading correctly it will cause your computer to have a invalid comparison. Which will cost you in fuel consumption. If the problem still exists as you have stated, check your connections for water,corrosion or a broken or grounded wire.