Oxygen Sensor

I have a 2009 Jeep Grand Cherokee. Engine is running rough. Check engine light comes on. Code is P0152. The code is defined as high voltage on the front o2 sensor (bank 2). I hooked up the scanner again and checked the voltages. Both O2 sensors on bank 2 are never making it above 0.1V. That seems to contradict what the DTC is telling me. Is that somehow related to the fact that it is not going into closed loop? I’m a little confused.

Here’s the things that can cause a DTC P0152 on your Jeep.


LOL. Yeah. I went there already and it had too many options. Even though it says replacing the sensor usually fixes the problem, I was trying to NOT be the guy who just swaps out parts. That’s why I went the extra step of checking the O2 voltages. But now I’m more confused.

The error codes are stored, which means a high voltage was detected at some point.

I would ignore the “voltage high” sensor code and focus first on why the engine is running poorly. The low sensor voltages suggest the bank is running lean. I would look for intake air leaks, etc, causing lean condition.

Also, you say it’s not going into closed loop. I assume you got that from the scanner. Not going into closed loop means the engine should actually be running slightly rich, not lean as the low sensor voltages suggest, so that’s another clue that you have an intake air leak. Extra air from a leak may be overwhelming the extra fuel from open-loop running and causing the engine to run rough and show low voltage on the sensors.

Check the power brake booster vacuum line and any other possible sources of vacuum/intake air leaks. Also check the PCV valve which could be stuck open.

Reading the actual sensor voltage with a DVM or scope would be more conclusive.

Thanks. That is helpful. The vacuum hoses look to be in good shape. I read you can find a vacuum leak with propane. I didn’t have any luck with that. I even took off one of the vacuum lines and shot propane right in. I didn’t notice any change at all.

Is there any trick to reach that oxygen sensor? I can’t fit my hand up there to reach where the cable plugs in. The only thing I can think of is cutting out the exhaust, but that seems like overkill.

Re: Closed loop

All the O2 sensors (before the cat) have to work for the ECM to be able to compute how much fuel to inject in closed loop operation. If the ECM thinks one or more of the O2 sensor functions aren’t working correctly, it will usually then use it’s open loop operation parameters and ignore the O2 sensors.

Open lool operation isn’t that uncommon, even if everything is working correctly. For example when you first drive the car in the morning the ECM will usually be in open loop for a while, b/c the O2 sensors take some time to warm up before their readings are correct.

Does it seem to run ok while the engine is cold? And then starts to run badly when the engine is warm?