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YeOlCherokee 08-28-2013 12:59 PM

Misfire in cylinder 4 & 5, and o2 sensor code (138), what could this be?
Hi guys,

98 cherokee 4.0, 130000 miles

So my truck runs good overall, good throttle response, but i keep getting this code for 304,305 and 138 which is misfire in cyl 4 and 5 and one of the o2 sensors.

At idle, it will sometimes run kind of rough. not too bad, but enough to notice that its not idling smooth. When im accelerating though, its A ok. The check engine light will go on once in a while and give these codes, but even when it goes off, my scanner still reads them.

Im assuming the o2 sensor code is because the misfires allowed unburned gas to get into the o2 sensor, messing it up, but im not sure. I just cant figure out why this misfire is happening. Its not noticeable when the check engine light goes on, because its the same idle when its off.

Is there any specific possibility because its cylinder 4&5 ?

The truck had a tune up about 8,000 miles ago, and all of the plugs and wires are good quality and still in good condition?

What could this be ? any ideas?

CJ7-Tim 08-28-2013 04:16 PM

Fix the known problem and see if the other issues change or go away. O2 sensor service life is about 125-175,000 miles, so you should be testing the O2 sensors, and maybe consider buying some new ones.

P0138 Jeep - O2 sensor Circuit high

Possible causes

- Faulty Rear Heated Oxygen Sensor
- Rear Heated Oxygen Sensor harness is open or shorted
- Rear Heated Oxygen Sensor circuit poor electrical connection
- Intake air leaks
- Exhaust gas leaks
- Incorrect fuel pressure
- Faulty fuel injectors

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