I have a 2001 XJ which has 4 O2 Senors, 2 before cats, and 2 after the 2 small cat converters right off the exaust manifold.

Seeing as O2 sensors are so expensive I really don't want to shell out the 200 plus bucks for four new ones.

My Question is typically it is the post cat converter O2 sensors that need to be replaced correct?

I've seen my fuel milage take a dive lately and this is the most likely cause as everything else seems to be in good running order and the engine is approaching 100k miles. Eventually I will replace all of them, but I would like to just do the two to start off with.

 

Do u have a CEL? I believe there's a specific code for each O2.

 

if you are not throwing a CEL coz of your o2 censor don't worry about it. at close to 100k on the engine i would suggest doing a complete tune-up which will cost less. something inexpensive to start with is buy urself 2 cans of Seafoam. put one can in ur gas tank and another can for ur engine. there is a write-up on it here somewhere on the proper way of using it. it will cost u about $10 for 2 cans. I'v seafoamed my high mileage vehicles and it makes a difference. after that, do the complete tune-up. new plugs, clean or replace ur injectors with the neon 703's u can buy here from forum sellers for less than $100 for all 6, change all fluids, grease ur axle bearings, etc.

 

Thanks for the tips. Everything else is in pretty good running order. I've already swapped out injectors, which was a nice upgrade, and changed all fluids. I inspected the axle bearings when I converted the rear brakes to discs last November and they looked pretty good as well. I put new NGK plugs back in November of 2009 so I might need to put a new set in. As for Seafoam I've never tried it. I put a bottle of lucas oil through it at least once a month, but maybe I'll give seafoam a try.

I have heard nothing but good things from swapping out O2 Sensors so I'm definitely planning on putting some new ones in. Plus I did have the emissions light come on last summer under very hot and humid conditions. The question is do I start with the sensors pre or post cat?

 

I had a CEL come on last summer under hot humid conditions. Nothing since then. However, I've ran into situations in the past where O2 sensors have gone bad but not thrown codes. They are rated for about 100k and I've heard nothing but good things from swapping them out.

 

I took mine up to the parts store and there were able to tell specificaly which one.I wanna say that a scanner told me mine was bank 1 sensor 2.

 

OBD-II has gotten pretty sophisticated....I'd bet 99% of the time a bad O2 will set a code/CEL. Do u remember the code from last summer?