Will faulty O2 sensor strand me on the roadside?

Runnin'OnEmpty

New member
Hi guys. My 96 4.0L is running great now after
installing a new distributor (due to it's eating the
CMP sensor).

Now I'm gunshy about driving it, after being stranded
on the roadside.

I've read here that faulty O2 sensors will stall the
engine...? If that's true, should I install a new one
just in case? My other vehicles simply trigger a code
with a bad O2, and keep running.

Thanks for any comments.
ROE
 

birchlakeXJ

New member
A faulty oxygen sensor will generally not leave you stranded.

That being said, they have a finite lifespan of between 100k-150k depending on the efficiency of the engine. Use NTK brand oxygen sensors for the best results with the 4.0 H.O engine.
 

techno1154

NAXJA Member
NAXJA Member
They do shut the engine down. The first one I replaced ran rich and caused a very poor fuel mileage. The second one shut the engine down. I think the second one caused the engine to go lean. Neither of the two set a code. I did however get codes after the engine is shutdown indicating that several of the engine sensors were bad or not reading.
 

8Mud

New member
You have to define faulty, they can fail and make the motor have poor mileage or maybe a rough idle or maybe bucking at certain RPM's. But if they short, either the sensor itself or the wiring, no telling what is going to happen. Especially if the sensor ground for the PCM has any significant resistance (a poor ground). If the O2 heater circuit (12 volts) gets crossed up with the O2 sensor signal to the PCM, bad stuff is likely to happen.

I know for a sure one sensor supply voltage supply loop can affect another sensor voltage supply loop. I don't know exactly how it happens (internally in the PCM). But I suspect various sensor supply voltage drivers may be wired in parallel. When I tore the wiring out of my rear O2 sensor and the wire cooked on the exhaust pipe, I popped some seemingly unrelated codes. A real head scratcher that one.

A bad O2 sensor IMO is unlikely to stop you, a shorted O2 sensor (or wiring) and all bets are off. Especially with a poor PCM ground. Voltage is going to take the path of least resistance inside the PCM or outside the PCM.

The problem I have with my 96 is there isn't an FSM for it I've ever been able to find. I use a combination of the 95 FSM and the 97 FSM. A lot of my troubleshooting is guesswork, cause and effect (figured out by experience, empirical rather than analytical).

In Dodge truck school (way back when) I was taught, a known old part is often better than an unknown new part. New isn't always better, a tried and proven part is more often better. They preached to replace only what was broken, like replacing one piston instead of replacing them all. The exception was bearing sets, you never replace one bearing, of a set.
 
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tjmotter

NAXJA Member
NAXJA Member
As I understand it, the oxygen sensor is only used to help tune the engine once it warms up. When the engine is cold, it runs without the O2 sensor in a mode called "open loop" which essentially means it is running rich as noted below. Once the exhaust heats up, the sensor turns on and starts to detect how much gas is coming out of the exhaust so that it can then tell the computer how much to throttle the injectors back (closed loop). This is one of the biggest reasons why emissions shops tell you to warm your car up good before bringing it in for testing cuz if it is still cold, it will run rich and likely fail.

Others have reported an O2 sensor causing their car to stop running which I guess could be a computer related safety feature (say if a wire shorted making it think it detects a lean condition) but if you simply disconnect the wiring, it should default to a rich condition and run (but with poor gas mileage).
 

8Mud

New member
As I understand it, the oxygen sensor is only used to help tune the engine once it warms up. When the engine is cold, it runs without the O2 sensor in a mode called "open loop" which essentially means it is running rich as noted below. Once the exhaust heats up, the sensor turns on and starts to detect how much gas is coming out of the exhaust so that it can then tell the computer how much to throttle the injectors back (closed loop). This is one of the biggest reasons why emissions shops tell you to warm your car up good before bringing it in for testing cuz if it is still cold, it will run rich and likely fail.

Others have reported an O2 sensor causing their car to stop running which I guess could be a computer related safety feature (say if a wire shorted making it think it detects a lean condition) but if you simply disconnect the wiring, it should default to a rich condition and run (but with poor gas mileage).
Open loop and closed loop is misleading, like it is one or the other. Which sensor inputs have priority is constantly changing and the only time they are all relevant is in a pretty narrow RPM, temperature, vacuum and throttle position window.

The engineers did anticipate various failure scenarios and programed in defaults. But how well the defaults integrate into the whole system is unlikely to be perfect or in some instances even marginally functional.

An example might be the CPS, if it misses a few beats it may be hardly noticeable, if it misses a few to many, it can cause fuel overload, misfires and even affect timing. The way I figure it is, the PCM can run out of programmed responses (out of the envelope). It is doubtful the programming has an answer for every every possible scenario.
 

techno1154

NAXJA Member
NAXJA Member
"When the engine is cold, it runs without the O2 sensor ". This quote by tjmotter is exactly what my XJ did and thus the reason I ignored all other codes and replaced the O2 sensor. When cold the XJ did not have a problem. When hot it will run poor until it finally stalls. Leave it for an hour or so and it will go again for a short distance.

8Mud also have a very good understanding of how the O2 works. Now, if I could only get the correct code for a bad O2, my trouble shooting would have taken several days less. Oh well! You learn as you go.

I do have a CD based manual for my 1996 XJ that I ordered from some online seller back in 2006. I find it to be 90%+ accurate for that year model. I am not sure I still have the ordering information.
 

Matthew Currie

NAXJA Member #760
Add to the above that it may depend on which sensor. On later models using two sensors, it seems that the downstream sensor does not affect running at all. I had intermittent codes from the downstream sensor on my 99, and my son had intermittent codes on his 96 too, I think, and nothing at all changed, including gas mileage. The same has occurred a few times on my current Liberty. Mine would clear again after a while, and would often not recur for many miles. I think very slight variations in fuel had an effect, and on the 99 it would almost always occur after warm restarts. If I cleared the code before it cleared itself, it would stay gone.

I know the OBDII on the 99 will set a code for this, and I was under the impression that the 96, which has both I and II, will as well. My son's 96 did.
 

5-90

New member
Hi guys. My 96 4.0L is running great now after
installing a new distributor (due to it's eating the
CMP sensor).

Now I'm gunshy about driving it, after being stranded
on the roadside.

I've read here that faulty O2 sensors will stall the
engine...? If that's true, should I install a new one
just in case? My other vehicles simply trigger a code
with a bad O2, and keep running.

Thanks for any comments.
ROE
They should not.

The "upstream" HEGO provides a feedback loop for fuel trim adjustment. The "downstream" HEGO provides monitoring of the catalyst bed efficiency.

If the dHEGO fails, it will throw a code, and that's about it.

If the uHEGO fails, the ECU will fault back to preprogrammed fuel maps, which are tuned to "slightly rich" to help prevent engine damage (if a lean burn isn't carefully monitored, it can cause serious trouble. If you go too rich, you have cylinder washdown and accelerate wear. "Ideal" burn AFR is about 14.7:1, "failed HEGO" AFR is usually about 13.5-13.8:1)

It should not leave you stranded, but a failed uHEGO will have a fairly significant impact on your fuel economy (this is typically an early Sx of uHEGO failure - and, in OBD-II, verification of uHEGO switching and fuel trim is easily enough done and you can catch the sensor failing before the ECU does. An ELM327 adapter and the Torque app is the first thing I've encountered that make OBD-II actually useful to me...)
 

techno1154

NAXJA Member
NAXJA Member
Well, it seem that my 1996 XJ is the Red Headed Step Child of the family. It have a combination of parts on it from 1994 or earlier up to those found on the 1997 and onwards until the end of the XJ run. Most over the counter parts shop have difficulty finding the correct parts for my XJ except the dealer who always ask for the VIN before looking for the requested parts. They always get them correct. The fact that an up-stream O2 sensor have shut down my XJ on the road is/maybe proof of this Red Headed Step Child. I just went back to my old yellow note pad to look up the error cods I had recorded when the XJ almost drove me insane :tears: ,....Err, to tears.

Here are some of the errors I have recorded on June 04, 2009.

B0000
B0082
P0000
P0012
P0204
P0208
P1391

Bear in mind that on the XJ, there should be not code set # B0000 since there are no wires or sensor in that location.

An O2 sensor fixed all that. Since then my XJ have done over 30,000 miles with nothing more than an engine oil change and two oil filter changes until a few weeks ago, I again replaced the up-stream sensor to fix a high fuel consumption. This time I may have done something to cause the last sensor to go bad. I went back home to the islands for 2 and a half years. On my return, I added 20 ounces of fuel treatment to less than 1/4 tank of fuel drove the XJ to a gas station fill it up then drove it Jacksonville FL from Miami, a trip of more than 350 miles. The down stream sensor will set a code only if it is disconnected. I ran my XJ for a few months with a straight pipe in place of the Cat with a bung welded in to facilitate the O2 sensor in that location. It do have a Cat since 2004. Funny thing is, it did get very hot recently which was the first indication that the engine was running rich.

Now that you have a copy of the codes, tell me of any there that you think will cause the PCM to kill the engine.

:music:



The others I did not record because I knew what they were.
 

Matthew Currie

NAXJA Member #760
I don't know about the codes, but my son definitely had the red headed stepchild syndrome with his. Some parts were unique and not listed as such, some were not unique but were listed as such, some changed midyear. He could never seem to get the right size u-joints.
 

Runnin'OnEmpty

New member
Thanks to all who replied to my post. Lots of
info here on O2 sensors, and it gives me more
confidence in the XJ. I like the fact that the (upstream)
sensor at least is located in an easy to reach location
should it be necessary to replace.
 
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