I had that code also. I replaced the rear o2 sensor and it went away.

 

you got the wrong sensor. sensor 2 is the one after the cat so you might want to do more research. mike

 

The rear O2 sensor *is* the one after the cat as pointed out by X99J above, so replacing the front won't resolve this code.

The P0138 high voltage code indicates an open in the rear sensor heater circuit. It may be a bad internal heater within the rear O2 sensor, you could check the heater wires (dark green/white and black) for continuity. If the sensor heater checks out OK, you're stuck with checking the rest of the circuit from the PDC to the ground stud next to the dipstick to find the open.

-Ed Rico

 

By checking the circuit and wires do you mean checking for melted insulation or using a tester? If the wiring checks out all good ill replace the rear 02 sensor

thanks

 

What I meant was using a meter to check the circuit for continuity. But to your point, checking for melted insulation would be helpful and could yield some useful clues to finding the problem. Just be careful when checking the sensor itself to probe the 2 heater wires (colors listed above) and NOT the 2 sensor output wires. You don't want to risk damaging the O2 sensor by applying a voltage across the sensor output, and besides it won't help you solve the problem.

-Ed Rico

 

After CAT sensor

What does the 2nd sensor do? Adjust fuel mixture, or is it for emmissions only? I'm getting the P0138 now, it seems to be a fairly common code.
Thanks in advance,
Bnugget

 

Rear O2 sensor on OBD-II monitors oxygen content of exhaust stream downstream of the cat. This information is used to compare to the upstream (pre-cat) O2 value in order to determine if the cat is functioning normally. The rear sensor has no direct effect on air/fuel mixture, but can have an important indirect effect since the internal resistance heaters for both sensors are wired in parallel. If a problem with the rear sensor heater knocks out the fuse in the PDC, the front sensor heater is disabled as well which can negatively affect driveability.

-Ed Rico

 

I've got this code as well. Weird.

 

ya I still haven't figured it out, im gonna try replacing the rear 02 sensor i guess

 

are the front and rear sensors the same?

 

Nope, front adjusts air/fuel ratio, rear just senses oxygen in the exhaust. Read Ed Ricos post above.

 

Front and rear O2 sensors have physically different connectors, otherwise they are the same. That's why the replacement sensors that require splicing your old connector are the same for front or rear applications.

-Ed Rico

 

Fuse location?

My downstream O2 sensor wires (behind the cat) fell on the driveshaft wearing through them. I am having the same hot engine misfires/sputtering problem as described in these posts and the same code. After the sputtering began I spliced the wires together but the problem continues. After reading the Upstream and Downstream O2s run on a parallel circuit I thought I would check the fuse. Do you know where it is located? I thought that possibly the Downstream O2 short may have also shorted the other Upstream o2 sensor? What do you think?

Mike

 

O2 sensor heater circuit fuse is located in the Power Distributon Center under the hood. If you damaged the wiring to the rear sensor, it likely shorted the heater circuit, effectively knocking out the heater to both sensors.

-Ed Rico

 

O2 sensor heater circuit fuse is located in the Power Distributon Center under the hood. If you damaged the wiring to the rear sensor, it likely shorted the heater circuit, effectively knocking out the heater to both sensors.

-Ed Rico

Sorry to bump this old thread but I hate making a new one.

How would you tell if the whole circuit was shorted, a blown fuse? And if so, what should I do to repair it (other than get a new fuse..lol)

I think this is what happened to me. I have checked the fuses (all good) but I've replaced the back O2 sensor. I keep getting the code too. I also have a relay for a "rear O2 sensor" and i swapped it with a different relay and still no luck. Im starting to get ticked. Trying a bottle of fuel cleaner today just because I'm at wit's end. I hope I wont need a new catalytic converter.

 

Man, I just got this exact same code yesterday. I just threw a new downstream O2 sensor on it today so we'll see if it comes back.

It's silly how easily that wire can rub on the driveshaft. I zip-tied mine up and out of the way as much as possible but it still worries me.

 

Crap. Got under the ride a bit more (I know it's not the right forum, but its the same problem) which is a 4.7 v8 grand Cherokee Limited.

It appears there are 2 downstream sensors (1 on each pipe, which turns into one pipe after the cats. I've replaced one but I guess I will try to replace the other one. The wires/connection to the O2 sensor on it are very loose. I'll take a guess that the wires became dislodged in the O2sensor and it's bad. Guess I have the dreaded "California emissions package''

 

99 cherokee 4.0 my p0138 just came on this evening . thats as far as i got . i will just for damage underneath tomorrow and go from there. so if the rear sensor is bad should the front be replaced also? or is the cat bad ? i also have the exhaust manifold cracked with the exhaust leak there. not sure what the long term effects or of not fixing that manifold leak if any.

 

my 99 xj also has this same code. reads ''PO138 O2 circuit high voltage (bank1, sensor2). and im pretty sure my gas mileage is fuuuucked

 

The most common cause for the P0138 code is a faulty oxygen sensor located after the catalytic converter (called the downstream sensor). I just had it on my 99 at 140k and a new oxygen sensor resolved it.

http://www.obd-codes.com/p0138

If you're replacing this and plan on keeping the vehicle, replace the upstream oxygen sensor too. Sensors don't last forever. The average lifespan of an oxygen sensor for OBDII is between 100k and 150k, depending on the efficiency of the engine.

Use NTK sensors; widely available and are the OEM sensors for your vehicle. This is the same sensor you would get if you went to the Jeep dealership.

 

'98 XJ
P0138
Live data showed 1.00V for sensor all the time.
New behind the cat (Bank 1, Sensor 2) O2 sensor did not fix it.
Found broken black wire in harness to this sensor, just upstream of the connector at the sensor.
Looked like something got kicked up and sheared one wire and damaged another.
Hidden by the outer loom cover.
Soldered up, all good.

 

Bear or shorting wire harness

 

For the Jeep 4.0L you should never use Bosch or generic auto parts store O2 sensors, they often cause codes and drive-ability issues that are easily cured by using genuine Jeep or NTK O2 sensors.

 

My 3.6l jeep throws both sensor codes for high temp on both post cat sensors. Replaced both twice now with no change. Checked wires and they look to be fine also. I'm tried of trying to work this one out, any help or ideas would be great only have 260000km on it

 

Welcome to the forum. What year and model? That would indicate an open circuit aka a broken wire(s), or bad/poor connection(s). As you've already replaced both sensors, those connections are likely OK. These get combined with the TPS/MAP/MAF in the ECM to regulate fuel flow. If you do any further work, (such as checking the ECM connectons) it is strongly suggested to disconnect the Negative battery cable before proceeding further. Hopefully some one else with familiarity with your Jeep will chime in after you provide the above.

 

My jeep is a 2012 laredo 4x4
I hope someone can point me in another direction

 

The last XJ Cherokee was built and sold in 2001, you should be looking for a different sub-forum for a 2012 Jeep.

Try here https://www.jeepforum.com/forums/#models.2


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