Ok i have a engine light (bad 02 sensor) am i supposed to just geuss which one it is or is there some way to troubleshoot these? 2001 dodge ram 5.9. 105000 miles It started hapening after the the cat was replaced.

 

It's more than likely the upstream one. The one behind the cat doesn't really do anything.

 

How did you check the code? The code # tells you which sensor is faulty.

 

If you had the cat replaced, you should have had the O2 sensor changed as well. You mentioned the CEL on, is it giving a code? More than likely, its the pre cat one that is bad. Be careful when purchasing one, the connectors are different, depending on when the truck was built. NTK(NGK) and the OEM sensors seem to work best with our trucks.

 

It does serve a purpose and can cause the CEL to come on regardless. Hence the question about the code #. Why Guess.

 

Ok the code is P 0420.

 

Here's what a simple search through the FAQ section will get you, https://dodgeforum.com/forum/2nd-gen...code-list.html

 

P0420 1/1 Catalytic Conv Eff were is this exactly pointing me?

 

Post cat sensor.

 

http://www.obd-codes.com/p0420

http://wiki.answers.com/Q/What_does_code_p0420_mean

hold on.

420 means the post cat and pre-cat readings are too similar, which means the cat is not working. it doesn't necessarily mean that one or the other is bad. this is a common code for people who remove or gut their cat.