Its a 2000. I thought the rear O2s jsut make sure the cat is working? I know when you delete the cats you just need a different reading from the front O2s for the computer to think the cat is still there.

 

[QUOTE=No4x4Yet;2240214]Cat is original to my knowledge. I havent had any issues with it.

Thanks for the input guys. Now to clarify I need the two upstream O2 correct? To my knowledge those are the ones that affect MPG, the downstream, after cat O2s just make sure the cat is working correct?[/QUOTE]

Sort of.

Here's a little reading for you about the downstream O2 Sensors and how they work with the upstream O2 Sensors:

Upstream Oxygen Sensor Goal Voltage

Since the introduction of OBDII with both upstream and downstream O2 sensors, the downstream O2 sensor has been used to evaluate catalytic converter operation and to control converter efficiency.

When looking at the upstream goal voltage, it should fluctuate above and below the normal center (.4 to .6 volts) based upon what the downstream O2 sensor is reading. The vehicle should be driven through all speed ranges when checking goal voltage. If the goal stays high or low all the time, a thorough check of the downstream oxygen sensor should be made. Be sure you check the sensor, wiring harness, connectors, catalytic converter, and inspect for exhaust leaks.

Although a MIL lamp may not be illuminated, there are instances when the downstream O2 sensor goal voltage is out of range, causing driveability problems or low fuel economy.

If the downstream O2 sensor tends to stay below center all the time, the PCM will adjust the goal voltage higher in an effort to correct the low O2 readings of the downstream sensor. This can result in low fuel mileage.

If the downstream O2 sensor has a tendency to stay above center, the PCM will set a low goal voltage which results in a lean fuel mix which can cause the engine to run hotter and aggravate a spark knock condition.

Sometimes a service technician will reset the PCM and take it for a test drive. The PCM does a recalibration after the reset, and the problem may go away until the PCM has completed a relearn of the system and then begins to attempt to correct for the downstream sensor error. If the symptoms reappear after several days of driving after a service is performed, check the downstream O2 sensor, the exhaust system, the wiring, and the catalytic converter. Note: a worn out catalytic converter can produce symptoms similar to a bad downstream O2 sensor.

 

The front and rear O2 sensors communicate with each other along with the cat. I do not know about rigging a vehicle to run without emmisions as that is not legal making the vehicle not able to pass an inspection.

 

The rear sensor is just there to make the EPA happy. Just swap the front ones.

 

[quote=CCKen;2240330]Where did you get this from? Im just wondering if it it jeep specific or not?

Its not with out emission, completely. I know on my mustang the computer wants to see a difference between the front and rear readings to know the cat is working. If you were to run no cats, pulling the sensor from the exhaust stream with a spacer creates the same affect. I have the rear O2s turned on on my mustang and i pass emissions.

May ill grab the fronts and see what happens if i see nothing Ill do the rears.

I was looking for the O2s and they have two separate part numbers? A front and rear upstream sensor?

 

Wonderful information. I've been wondering about this for some time as there's inconsistency here between reports from folks who do end up replacing their rear sensors.

I suspect this is a generational change in engine management that probably goes like this:

Renix: Upstream O2 for fuel/air
OBD-I: Upstream O2 for fuel/air, downstream to illuminate CEL
OBD-II: Upsteam & downstream participating in fuel/air probably as mandated by EPA

I'm going to go out on a limb here and speculate that the EPA said that just setting off a CEL wasn't good enough as people would ignore the code, remove the light, or put some tape over it. This fits in with the trend in emissions controls - modern vehicles will go into limp mode if things aren't doing well (first example that pops into my mind is if a truck runs out of DEF). Also keep in mind that the 4.0L was discontinued due to Chrysler's inability to make it meet then-current fuel economy and emissions targets, so such behaviour for a 4.0L vehicle with 3 catalytic converters really isn't surprising.

Yes the upstream and downstream sensors are different part numbers. Upstream is NTK 23151 IIRC... RockAuto.com has em listed


EDIT: Regarding Jeep-specific-ness, some things are common across all of the OBD-II line up, for example the CPS behaviour.

 

Im comin up with two different numbers for the upstream O2s, a front and rear specific?

 

Hmm didn't notice that before... not sure what that is. I honestly thought they were all 23151 upstream but there's this 23506... 2001 has 4 total NTK P/Ns listed too.

Do you have the 3-cat system or just the single big one? Perhaps someone can comment on the {Upstream|Downstream} {front|rear} thing - I am currently away from my parts fiche

 

I have two cats right off the exhaust manifold, not sure if there is another uder the vehicle. Prolly but I never really looked.

 

Ok, it's really those two mini ones that matter. The main one is located near the transmission crossmember.

 

The upstream and downstream O2 sensors have different part numbers, basically for two reasons.

1. The elecrical pig tails are different lengths, and

2. The bulb on the O2 sensor is different. The NTK (that's all I use and refuse to talk about any other O2 sensor) upstream sensor will have small round holes around the bulb and the downstream will have a series of slotted holes in the bulb.

Yes, On Board Diagnostics is a Federally mandated progam to address emissions. The OBD I and OBD II systems in our Jeeps satisfy the regs for the years they were produced. The O2 sensors play a key role in reducing emissions so the PCM pays close attention to what they are saying. The communication between the upstream O2 sensor and the downstream O2 sensor is designed to protect the CAT (the main dude in the emissions game). The CEL program is there to alert the operator that there is a emissions related problem...that's it.

CEL flashing is the end of the road as far as emissions faults go. Stop and fix the problem(s). You may end up replacing a CAT.

 

Ken I got all that. There are 3 parts numbers for the O2s from what I see. Two for the upstreams and one for the down stream. Nothin is wrong with my exhaust system, no CEL. I just wanna try and increase the mpgs some and I know the O2s tell the computer how much fuel the engine needs.

 

Okey dokey. So I found my parts fiche, and there are actually 4 separate part numbers listed for all the O2 sensors! No idea why the two upstreams and downstreams are different.

Anyway, the wording in the book is indeed "Before Catalyst Front" and "Before Catalyst Rear" referring to the sensors upstream of the forward and rear converters. So, now we know. lol

For completeness sake these are the OEM P/Ns:

- U/S Front: 56041212AE
- U/S Rear: 56041213AF
- D/S Front: 56044215AA
- D/S Rear: 56041213AG

Who knows... the upstream and downstream sensors are probably the same units but with different length wires lol. Anyway one of each of the 4 NTKs should do the trick.

 

I ordered two upstream sensors the other day. Ill see how they work and go from there. I looked that the part and the front and rear sensors have different wire lengths. Not really sure how big of a deal it is cuz they both have plenty of extra wire?

 

I'd guess that it'd be to run back to some bracket