Does anyone have a definitive answer on the Downstream 02 sensor debate?

[Muddy98]

I have a 2000 cherokee with the ca emissions exhaust system and it has the popular 0331 cracked head problem. The previous owner has toasted the rest of the motor so I want to swap it with a 99 motor i have and use the exhaust manifold from the donor vehicle without having to use an adapter that hinders the flow of the head in anyway. i have searched through the web for the last 36 hours trying my best to find a definitive answer on if the downstream 02 sensors after the mini cats have any purpose other than seeing if the cats are in fact still there.

All I'm curious about is the down stream sensors

All I have read is debate after debate half say its essential to the air and fuel ratio and it makes sure the cats come up to the correct temp and that it'll continue to run rich forever trying to heat the cats up.

The other half say they are just there to see if the cat is still in place and only throw a cef light and checks to see if the cat is there.

does anyone have a real answer that has done it themselves? Or have some kind of reference to an ex jeep employee or documentation,
or just plain experience?

[Muddy98]

Was trying to add this pic in my original post

[RCP Phx]

The upstream(s) are for the ECM and Air/Fuel ratio's, the downstream just checks to see if the cat is performing properly.

[Muddy98]

Originally Posted by RCP Phx

The upstream(s) are for the ECM and Air/Fuel ratio's, the downstream just checks to see if the cat is performing properly.

From reading until my eyes hurt and learning way too much about jeep O2 sensors I could hardly comprehend i believe that the downward o2 sensors are there to check the voltage between .3 - .6 v and as long as those sensors stay between those numbers it doesn't throw a code or affect performance or mpg.


I think I may be ready to move forward and put 2 bungs on the 99 exhaust manifold and find a way to give the downward o2 sensors a constant 0.45 voltage so they can keep the check engine light at bay.

Now seeing as nobody has done a writeup on this and nobody informs me otherwise before I start this project I'm going to document the process.

I think there is some unclaimed horsepower to be had with the combination of the 00' exhaust manifold, the better performing coil packs and 0630 head in place of the smaller ported 0331. How much I'm not sure?

[Da XJ nut]

You can fool the ECM by using 2 old school non filers that were designed to keep oil from getting on plugs. Get 2 per down stream o2 and still the bottom out of one, screw in o2 sensor and screw the 2 together and reinstall to pipe. It will sample the air thru the restricted hole and help monitor the A/F ratio. I've done it a few times with vehicles I removed cats and drove anyway. No loss if fuel milage I could tell

[maxbraketorque]

This is apparently a CA emissions 2000 XJ. If the vehicle were running, you'd be able to tell from the upstream O2 sensor voltage whether the down stream O2 sensors were affecting AFR. If down stream O2 sensors were for instance causing the ECU to run rich to attempt to heat up the cats, you'd see a consistent rich indication in the upstream O2 voltage during this condition. You could probably track this with an OBD-II logger set to only log upstream O2 voltage. (OBD-II logging is so slow that meaningful time resolution the data is only possible when one or two items are being monitored.) Or alternatively, you could slap a wideband in the exhaust, and grab data from that.

If you don't want to figure it out on your own by logging data, you probably want to contact someone who is likely to have had direct exposure to the engine control algorithms such as Chris at christuned or Dave at BG Tuning.

Also, you may find that O2 simulators do not get the job done. I've written my own code to fake a rear O2, and for the ECU I was reprogramming (not a Jeep), it wasn't as simple as faking a constant voltage. The ECU needed to see a varying down stream voltage in order to believe that it was seeing a real signal.

[Tim_MN]

My 00 has 48 state emissions, no pre-cats. Shouldn't be that difficult to locate the necessary wire harness and parts.

[75SV1]

Originally Posted by Tim_MN

My 00 has 48 state emissions, no pre-cats. Shouldn't be that difficult to locate the necessary wire harness and parts.

That might depend on what emission checks his State does, or rules on the age of vehicle. I hear it gets expensive if caught. Still, I'm in IN. I see the Rolling Coal.

[maxbraketorque]

Originally Posted by 75SV1

That might depend on what emission checks his State does, or rules on the age of vehicle. I hear it gets expensive if caught. Still, I'm in IN. I see the Rolling Coal.

Not only that but the ECU will be expecting to see two upstream and two down stream O2 sensors. It won't be happy seeing just one of each. There may also be other emissions-related differences in the vehicle that translate to differences in wiring harness and sensors that would need to be changed as well as changing the ECU to a 48-state version.

[75SV1]

Originally Posted by maxbraketorque

Not only that but the ECU will be expecting to see two upstream and two down stream O2 sensors. It won't be happy seeing just one of each. There may also be other emissions-related differences in the vehicle that translate to differences in wiring harness and sensors that would need to be changed as well as changing the ECU to a 48-state version.

I will note I am planning this on an '01. For some reason, I like the '00 Fed setup. The other aspect is if it is for On Road or Off Road.

[Muddy98]

Originally Posted by Tim_MN

My 00 has 48 state emissions, no pre-cats. Shouldn't be that difficult to locate the necessary wire harness and parts.

Can you emphasize? I was trying to see if I'd be able to just swap the pcm and harness to a 2000' Federal emissions one and avoid this issue altogether but was getting the same bag of mixed results.

Thank you to everyone that has chimed in, I'm in oregon and after 20 years there's is no emissions testing where I am at. I am perfectly fine with running the main cat I just hate the idea of having 4 o2 sensors and 2 precats that heat the engine bay and restrict exhaust right there on the manifold.

I called and went to several junkyards today to try and find an 02/03-04 grand cherokee with the tupy head because it's a direct bolt in but I couldn't find one not even just a tupy head, so may have to buy the Clearwater one but I'm trying to avoid spending that much on getting this thing going again.

The body is super clean with a good color and the tranny and transfer case work great so I'm happy I got it but can't wait to get over this issue and start working on it.

[75SV1]

Did you try :
https://www.car-part.com/

Showed a 2000 in Kalamuth for $200. One Stop Auto. Still, that might not be a Tupy. Maybe try the Northwest filter. I tired the Oregon.

[Muddy98]

Originally Posted by 75SV1

Did you try :
https://www.car-part.com/

Showed a 2000 in Kalamuth for $200. One Stop Auto. Still, that might not be a Tupy. Maybe try the Northwest filter. I tired the Oregon.

Thank you for that link I started to look but couldn't get my mind off the pcm swap possibility cause if it's possible to just swap in another 2000 pcm that had the federal emissions ill be good to go so I'm now scouring the web trying to find that info.

[RCP Phx]

You can easily swap the PCM's(you'll need the vin coded in), the harness will be the hard part.https://naxja.org/forum/showthread.php?t=1145993

[Muddy98]

Originally Posted by maxbraketorque

This is apparently a CA emissions 2000 XJ. If the vehicle were running, you'd be able to tell from the upstream O2 sensor voltage whether the down stream O2 sensors were affecting AFR. If down stream O2 sensors were for instance causing the ECU to run rich to attempt to heat up the cats, you'd see a consistent rich indication in the upstream O2 voltage during this condition. You could probably track this with an OBD-II logger set to only log upstream O2 voltage. (OBD-II logging is so slow that meaningful time resolution the data is only possible when one or two items are being monitored.) Or alternatively, you could slap a wideband in the exhaust, and grab data from that.

If you don't want to figure it out on your own by logging data, you probably want to contact someone who is likely to have had direct exposure to the engine control algorithms such as Chris at christuned or Dave at BG Tuning.

Also, you may find that O2 simulators do not get the job done. I've written my own code to fake a rear O2, and for the ECU I was reprogramming (not a Jeep), it wasn't as simple as faking a constant voltage. The ECU needed to see a varying down stream voltage in order to believe that it was seeing a real signal.

I spoke to Dave on the phone about flashing my pcm to a federal emissions one and remove the 02 sensors from the computer so that is one option I have

but then he also confirmed what da XJ Nut was saying and that the 2 downstream o2 sensors indeed do not affect the fuel trim and that some spark plug extenders for the downstream sensors should do the job...