My 2000 XJ is the newest vehicle I have worked on. This is the first hobby car I have owned that has O2 sensors. I want to replace them. I got under my Jeep and it looks like I have 2 upstream sensors and 1 downstream sensor. Is this correct? What are the 2 expanded chambers before the upstream sensors? My vehicle is a 2000 XJ with 4.0 and automatic transmission. Thanks for any help.

 

If it's California emissions you have two cats after the upstream 02 sensors. You should have 2 sensors downstream of those cats.

 

I would certainly not replace the downstream O2 sensors, as their only purpose is to set a code if the Cat is failed

I personally would not replace the front O2 sensors unless you have a good reason, as the replacement items are not as good as OEM

If you do replace the front ones, you should use Mopar, or the most expensive ones you can get

The cheap ones are hopeless

 

If your pipes look like this you should see 2 before the bulges and 2 after..you can see the bung on the left after the cats...the other bung for the right pipe is in a similar place but facing away from us in this picture angle

 

Actually, the SM says downstream sensors DO participate in AFR:

Just tossing that out there...

 

In these days of $5 gas it may sound like a good idea to replace O2 sensors just for the heck of it but as pointed out above most of the available sensors are probably worse than what's in there. If you don't have codes showing and the sensors seem to be alive on your live data scan leave them alone.

Also,if this is the newest car you have worked on it's possible you might not have a scan tool. Get one today. Not just a code reader but something that shows at least some live data. You'll need it

 

Good to know,

the earlier FSM says they dont, but I havent read the later ones that dont apply to my '96

Having said that, they would possibly only start doing their thing if the Cat becomes ineffective ?

In the earlier models, a P0138 code is generated if the difference in O2 output voltage varies by more than whatever the set parameter is

 

I don't know why they decided to do that, but if I were a gambling man I'd bet the house that it has nothing to do with performance and everything to do with emissions.

Just like them denting up my engine pipe for no sane reason.

 

An O2 sensor downstream of the cat can't determine if the fuel mixture is correct. It can only determine if the cat is bad, or possibly the upstream sensor is bad.

 

Well, I can only resort to the "Does too!" method of debate, as the SM says it does:


Browsing around other makes of vehicles shows this to be true in many other cases as well.

 

in my opinion, there is 2 possible answers to this issue

1) The later computer is re-programmed to alter fuel function based on both o2 sensors, ( but its not clear to me from an engineering perspective how this would actually work, give a cat failure)

2) The info in the FSM is not accurate

In any case, I wouldnt change my rear O2 sensors as preventative maintenance, unless I had $ falling out of my pockets, plenty of other items to spend money on

I did change my rear O2 sensor when I scanned my vehicle and got a P0138 code (no CEL), the replacement parts was cheap, the thread was not quite right, and the code came back in 3 months.
I replaced that with a used OEM, no problems since

 

I'm looking at it from a technical perspective too. If the cat is working, then it's reducing the oxygen content of the exhaust. Thereby preventing a downstream sensor from accurately determining if the engine mixture is correct. Downstream is typically used to determine if something looks wrong though. For example showing the same or higher oxygen as the pre-cat O2, or never changing when it should.

Re-reading that description, it sounds like maybe it's using the post-cat O2 to assess the exhaust. Perhaps it can influence the fuel mixture within a narrow range to adjust the final exhaust O2 content, so long as the upstream O2 still shows a reasonable value.

Here's another odd question. Since there are two sets, is the engine computer adjusting those banks separately? I guess you'd have to look at live data and see if there are two sets of fuel trim numbers.