when people say to replace the o2 sensor do they meent o replace both of them, i was reading in the repair manual and the picture shows the truck has to of themm but it is a pic of the 4.7L i have the 3.9 v6 just wondering why noone says to replace both or just one?

 

my personal experence is to change both of them at once...however you do not have to if say that just one of them is throwing a code...

 

also most of the time it is the downstream (second)that throws a code...

 

the first O2 sensor infront of the cat is for your air/fuel mixture. and the second one after the cat just senses if your cat is doing it's job.

 

wait am i rite on this...? now i cant rember i think its post that fails...lol idk now

 

ya, it's usually the 1st one before the cat that throws a code and gives you bad gas milage. but they always dont throw a cade. mine didnt throw a code till the truck would hardly even idle because it got soo bad, usually you can tell when your gas milage drops all of a sudden. I went from 20-21mpg down to 15mpg

 

yea thats what mine did roughly from 20m to 12-15m to gal now and i have the small 6cyl getting 8cyl gas milage