ron3865

I was on the Technical Forum looking at O2 posts and my eyes glazed over. My questions I think are pretty basic as I'm not a Cherokee aficionado but merely a guy that drives a '99 4.0 and have done so for over 200,000 miles. I get consistent
MPG of 16.5/22+, burn no oil and have changed oil every 3-4000 miles since new. Good, even compression and no oil leaks.

Last smog check about 15 months ago (Riverside County, Calif.) it had a hard time passing. The NO reading at 15mph was 486ppm and the limit is 508ppm. Over the years these numbers have gotten closer together and it might not pass smog this year because of that. All the rest of the numbers are OK. Original CATs and O2 sensors.

I've been on Rock Auto, O'rielly's and YouTube re replacing O2 sensors and figured it would be worthwhile as I'm not selling anytime soon and Rock Auto recommends new sensors at 60-100,000 miles so:

1) There are 4 sensors, 2 up and 2 down. Some say the 2 up are the same and 2 down are the same. Other places say there are right and left ups and the same for down. So, what's the difference between left, right, up, and down? One site, using the same photo, had different prices and different part numbers.
2) On Rock Auto's site all most of the brands including Bosch, will cost around $200 for all but Mopar is way more expensive. Is there any reason to buy Mopar other that the brand name? Will I get the same performance with Bosch? or Denso or AC Delco?
3) Will new sensors have any effect on the NO reading?
4) Lastly...My battery has not been disconnected in a long time and the OBD II shows no codes, pending or otherwise. Every once once in a while I plug in the OBD to see what's going on. Now it's showing the CAT monitor is inconclusive or not on line. I forgot what the term is. I can pass smog with one monitor out but it would be nice to see it working. It didn't happen for any of the usual reasons ( battery disconnect, delete codes, etc.) so it's a bit of a mystery.
Any help here would be appreciated.

5-Speed

1 - dont worry about the down stream sensor for now. It doesnt affect how it runs. Just replace the top one to start. Good luck getting the original one out too.

2 -NTK is the same as OEM.

3 - if they have never been replaced after 200k, I am going to say yes.

4 - this could be multiple things. Need to see your last smog readings to be sure.

Cummins93

https://www.cherokeeforum.com/f2/pas...2/#post3556920

And I thought I had it rough in CT!

00t444e

The 99 models only have 2 o2 sensors, upstream and downstream, the upstream one is the one that affects performance. They didn't switch to the 4 o2 sensor setup until sometime in 2000.

BlueRidgeMark

Heat is your friend. Get your tools ready, take a half hour drive, and then pop it out, OR, use a torch on the bung (not on the sensor). Makes a HUGE difference.


Yes, and that's the only thing you should use. Do NOT get an aftermarket. If it's not Mopar/NTK/NGK, you are rolling the dice.


I'm thinking that might not be true of a California car. I think they went to 4 sensors sooner than the rest of the country.

97grand4.0

Your monitor is showing incomplete. I think you may be allowed to have 1 incomplete monitor. You are wise to investigate the cause. What this means is the rear 02 sensor is saying the cat is not operating efficiently. I had this recently. If you look to your STFT and LTFT, those numbers should both ideally be around zero. I suspect your LTFT is somewhat negative, maybe -7 or more. Not enough to set a code. The absolute value of the STFT and LTFT should be not above 10 in my opinion. If so your PCM is struggling to maintain the fuel trim to an acceptable level. You want to be able to rely on your O2 data, so if in doubt the first but not the last step is to replace the front sensor.
Then if you still get about the same numbers, start looking for something causing you to run rich, if the LTFT is negative, because this means the PCM is having to subtract fuel. If the LTFT is a higher than zero..a positive number, look for some thing causing you to run lean like a vacuum leak or maybe fuel restriction or plugged injectors or low pressure.
In my case I had replaced my injectors with 4 hole Chinese, and tho I think they were made ok, I do think they sent me the wrong injectors meaning ones with to high of a flow rate.
There are actually a ton of injectors so they made a mistake despite e bay claiming that they fit the vehicle. Drove like that for 2 years before I really figured it out. My LTFT was -8 and STFT was around 3.
So I put the original, bosch 200k mile injectors back in, the car ran leaner, quieter, mileage went up, and..wait for it...the CAT(alytic converter) monitor 'ran' and reset on it's own to Complete within 24 hours (no problem) the first time I drove on the highway. Immediately on start up the st and ltft numbers were much closer to zero, too. Hope this heps.

*forgot to add, if the numbers are ok, I would start to think that maybe the cat itself or the rear sensor has seen better days.**

ron3865

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A bit late on the response but, yes, Calif GCs have 4 sensors. I guess we're lucky that way Anyway, the 4.0 jeep passed So Cal emissions. The only one I ever had any problem with was the 15mph NOx reading. It barly passed but that was enough. The first time the smog guy ran it the 15mph NOx was too high. He started the test over and it passed. That's the way this car has been for a lot of years. Replacing the upstream O2 sensors didn't do anything on the smog but it did run just a tad smoother. And even though they were original 1999 with 200,000+ miles they came out without a fight.
A couple of smog guys said one reason for the high NOx reading could be the thermostat. Mine's the stock 195 degree that works correctly but they suggest a 160 degree might reduce the NOx reading as they say NOx is a function of, among other things, engine heat.

cruiser54

I would NOT put a different thermostat in it.

Buy a can of BG 44K. Add it to a full tank of fuel. Run the tank down to 1/2 and refill with gas. Run that tank down to about 1/4. See what your readings are then.

318SixPack

They are doing emissions on a 20 year old vehicle there? Escape to a normal jurisdiction.

cruiser54

Old vehicles, but not on people crapping on the sidewalks........Priorities of Californistan.

ron3865

Yeah,..they smog back to 1973 or 74. The next vehicle i need to do is a 1988 Vanagon. It wouldn't be so bad if the "fail" numbers went up with age but they stay the same. I've got smog reports on my Jeep that go back to the early 2000's and the limits never change.

5-Speed

1975-99 gets a probe up the rear and stuck on a dyno (awd excluded). 00 and up gets probed in the obd2 port. Both are a painful process to go through. On 00+, if we replace our ECU with a junkyard unit, it would fail due to vins not matching. If you swap from a manual trans ecu to an auto ECU, you will not pass cause the operating system # isnt the same. Both of those situations have nothing to do with smog/emissions, yet you will still fail. Pretty sad. #komifornia

Ralph77

So this brings up something I wondered about.
You live in a Federal Emissions state and own a Federal Emissions vehicle. Perfectly legal, no changes, and exactly like it came from the factory.
You move to CA and register your car in CA.
What happens when it comes time for Emissions?

97grand4.0

I have never seen a Jeep or any other car that was not CA emissions. Automakers don't want to have to be excluded from NY and CA markets or make 2 different cars, fed and CA. Correct me if I’m wrong because that’s an interesting question. ,, The logic is kind of like the electrical code where nothing is explicitly said but all is ruled out, if you follow me.

Cummins93

Yes, there are CA and Federal emissions equipped XJs. I thought it was just for the JTEC ecu years but after sourcing a new ecu for my 95 that was also a different unit.