98 ram 1500 5.2l 2wd. check engine lights come on every week or some after city driving and its always the o2 sensor so my question to you is does the downstream o2 sensor effect gas milage because ive heard its only for emmissions is this true?

 

That is correct. You can try this to eliminate the CEL. https://dodgeforum.com/forum/2nd-gen...converter.html

 

Unlike upstream O2s, downstream O2s have no bearing on gas mileage, A/F ratio, etc. It is simply a CEL that comes on to tell you that the cat is not doing the job it is supposed to.

 

but like you said it is telling the computer that the cat is not doing its job. well dont you get bad gas milage if your cat is bad?

 

Well all kinds of things happen if the cat is clogged. But that has nothing to do with your question about the post cat o2 sensors. Which has already been answered. And from what I've read here, you have no cat(s) anyway. And since gutting the cat will usually result in a loss of low end, you may be on the gas harder to get the truck rolling. Which in turn, will result in worse mpg.

 

no i still have a cat on my truck

 

A CEL about the downstream O2 usually means one of two things.

1. The catalytic converter is bad, which doesn't necessarily mean its clogged, but it may be.
OR
2. That O2 sensor is bad.

Like others have said, a bad downstream O2 will not have any effects on performance/MPGs, but you are right that a bad catalytic converter may.

 

Sure a clogged cat will effect gas mileage, but that wasn't the question...

 

how do i tell if my cat is bad?

 

replace the O2, clear the CEL. If the CEL comes back then in all probability you have a bad cat...