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O2S problem

Ben H

Ben H

NAXJA Forum User
Location
Fresno
I have replaced both oxygen sensors in my 2000 4.0. I have stock manifolds with stock cat and a flowmaster 50 series. I replaced the cat back O2S about 3 months ago after it threw a code for that sensor. Last week I got another code for the manifold O2S. At the same time I replaced all fluids and it got all new cooling components. I went on a trip the next day and got about 150 mile per tank which killed my wallet. On the return trip home my check engine light popped up. When I got home I scanned it and got the following codes:

PO132- (1of2) high voltage

PO135- heater circuit malfunction

PO138- (bank 1 sensor 2) high voltage

I have the FSM and am not sure which sensor to replace or inspect. Any help would be appriciated greatly.

Another interesting thing that happened was that it attempted to stall out at a couple of different stop lights right after the manifold O2S install last week. I attributed it to either CPS or IAC, but I pulled no codes sending me in that direction. Can this also happen due to a shotty O2S?

Thanks

BEN H
 
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Could be bad sensor, did you disconnect your battery after you did this ? The OBD II systems retain settings. I'd try that first, disconnect for about a 1/2 to 1 hour then reconnect. I will probably run rough initially till the system relearns it's parameters. I often wonder why they have so many 'step one, disconnect the battery' whenever you work on the XJ in the FSM, some I can see for safety but thinking about it some might be to force a reset..
 
yeah I disconnected the battery overnight a few days ago. I also reset the computer through the scanner yesterday. the check engine light off for now. I have not driven it enough to get it up to operating temperature. I am more confused on which sensor it is (front or rear or both). Are the codes listed above serviceable, or do they call for me to replace the sensor?
 
I drove it a little more today. The check engine light remains off, and I burned through a 1/4 tank of gas in 40 miles. It still feels sluggish, but it isnt stalling yet. Is it possible that after I reset the computer with the scanner, it corrected itself to run with a shotty O2 sensor. The gas mileage is whats really got me here.
 
Ben H said:
I drove it a little more today. The check engine light remains off, and I burned through a 1/4 tank of gas in 40 miles. It still feels sluggish, but it isnt stalling yet. Is it possible that after I reset the computer with the scanner, it corrected itself to run with a shotty O2 sensor. The gas mileage is whats really got me here.
Click to expand...

Two things come to mind. First of all, anytime you kill power to the computer, it has to relearn the engine. Secondly, it might take a little longer for the fault to return. I would keep driving it until the check engine light comes on again.

PO138 is the rear post cat sensor.
 
Yeah it came on just after I made that last post. I will change that sensor again tommorrow. Thanks for letting me know which sensor.
 
My 99 has been throwing the same P0138 code on and off for the past 2-3 months now. I'm not replacing the stupid sensor untill I can catch the damn thing acting up. Everytime the light comes on and I hook my dads DRO tool up to it and to a digital read out on it, all sensors read and act completely normal, I turn the light off, and it stays off for a while.
 
Have you noticed a significant drop in MPG? I am down from avg. 16 mpg to 11 mpg in the last week. With 3 different fill ups that is a 30% reduction in fuel economy. Thats a lot of wasted money. I have a hard time believing that this extra fuel consumption is soley because of this one O2 sensor. If it is just because of a sensor then, that freakin sucks.
 
no not really. Actually I noticed a spike in my mpg not too long ago. I've always got about 14-15mpg (all short distance city driving) and about 19-20 (on the two highway trips i've gone on, one of which was about 85-90mph for a good ways, and I don't have cruise control). However on the tank before the last one, in all city driving, I got 19mpg. I never run my a/c unless i'm on a trip on the highway, I usually run with all windows down.

edit: I just noticed you said you reset the computer with the scanner and you are burning through alot of gas. Thats normal. The computer has to re-learn your driving habits. Kinda like with a new car, how you need to break everything in for milage to get better, only it doesnt take near as long.
 
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But how long does it take for the computer to reset itself? This is after 3 fill ups. I was under the assumption it only took about 15 minutes or so. I usually drive it for about that long after reset, just for this particular reason.

What is the likely hood that the IAC or CPS is fouled, and not showing a fault on the scanner?
 
For now disregard the voltage high readings, they are caused by the PO135- heater circuit malfunction fault.
The PO135- heater circuit malfunction fault can be a fuse, a bad relay, a broken wire or the heater is burned out. You need to be sure by testing for voltage to the sensor heater.
 
To function properly, an O2 sensor needs to be at or above a specific temperature--can't remember exactly, but something like 900*??--any way, XJ's have there sensor mounted too far from the exhaust manifold to insure that hot exhaust gas will maintain proper sensor operating temp, so a heater circuit is installed in the sensor. A bad heater could easily result in a sensor that does not properly respond to the fuel mixture. That results in an incorrect fuel mixture. So get the heater fault cleared first--which probably means replacing the sensor anyway.
 
Thank you all for helping me out.
I just looked in the FSM for an explanation of the heater circuit. I didnt find an way to check the voltage on the heater circut. Any pointers, as I am electrically challenged. I see that at ambient temp of 70, I should be reading 6 ohms. I have had it at operating temps (930*-1100*F). Now here comes a question; which wire lead is the heater circuit? I have a 2000, and finding the correct fuse is challenging for this year, so where is the fuse for the heater circuit?
 
The

PDC2000.jpg

The zig zag symbol is the heater.

o2sensors.jpg
 
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Not sure about the 2000 XJ, but my '88 had an O2 relay that the ECM turned on/off depending on various operating parameters. My Chevy FSM for a system with a heated sensor says the heater is always on--that is 12V applied direct from the Ign sw. If you have three wires to your sensor, two are for the heater--one is engine ground and the other is a switched 12v (relay or ign sw). The third wire would be the signal wire to the ECM. Do not put an ohm meter on the sensor's output wire! You can destroy the sensor! You can connect a digital volt meter to the output wire which should rapidly vary between .4 and .9vdc when the engine is at operating temp (this also indicates "closed loop" operation). On my '88, pin A & B were the heater connections--pin b was ground. Pin c went to the ECM. The resistance across pins a & b should be between 5 and 7 ohms (with the connector disconnected).
 
Thanks I have never noticed that section in the FSM. That helps out tremendously.
 
Ben H said:
Thanks I have never noticed that section in the FSM. That helps out tremendously.
Click to expand...

Are you talking about the wiring diagrams? They take up half the f'n FSM, how could you have not noticed that section?

Just giving you a hard time, I have no constructive advice :eeks1:
 
jmsull said:
Are you talking about the wiring diagrams? They take up half the f'n FSM, how could you have not noticed that section?

Just giving you a hard time, I have no constructive advice :eeks1:
Click to expand...


I always reference the index and go directly to the page. I was talking about the 2nd and 3rd page in the Power Distribution section. I may have seen it before, but just never paid any attention to it. That is some information worth holding on to.

Now on to the fix. The 15amp fuse in the 21 spot was missing! I had written down my fuse pattern long before I had the information Langer gave me. I know the fuse was in there about a year ago. But today it was gone. So...
Replaced the fuse, reset computer, drove 42 miles. No check engine light, no more stumbling idle or stallng at lights. It appears that I returned to my previos MPG amount as my gas needle barely moved.

Thank you all, I am assuming now that the problem is fixed.
 
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