Oxygen Sensor issues

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by d_w, Mar 7, 2025 at 11:20 AM.

  1. d_w

    d_w New Member

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    Hey all,

    My 2007 is throwing a p2195 code and occasionally a p0138. MPGs have dropped to low 30s. There is something up with the O2 sensors, so I am trying to narrow down the issue.

    I recently did a piston soak to help with an oil burning issue. Replaced the PCV, spark plugs, and valve cover gasket. Cleaned the MAG sensor. It all went well, I don’t *think* these codes are related.

    I’ve attached a log of a short city drive between 30-40mph. The A/F sensor data seems erratic to me, as well as the short term fuel trim.

    I have a replacement A/F sensor on order, but was looking for more opinions before changing out the part. Is there anything else it could be? Am I missing something? Any help is greatly appreciated.
     

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    #1 d_w, Mar 7, 2025 at 11:20 AM
  2. d_w

    d_w New Member

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    Here is another short trip log, mostly at highway speeds
     

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    #2 d_w, Mar 7, 2025 at 2:18 PM
  3. dolj

    dolj Senior Member

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    Did you mean MAF sensor? Did you clean both the temperature sensor (at the side of the 'finger') and the airflow sensor (embedded inside the 'finger')?

    Did you disconnect the 12 V battery for a minute or two after you did all the work? You need to do this so it resets the EMC to defaults. Especially import after cleaning the MAF/throttle body.
     
    #3 dolj, Mar 7, 2025 at 10:47 PM
  4. d_w

    d_w New Member

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    Yes, the MAF. Typo. Everything on it got a good clean. I did disconnect the battery to reset the ECM but didn’t see any differences before and after.
     
    #4 d_w, Mar 8, 2025 at 10:08 AM
  5. dolj

    dolj Senior Member

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    After a reset, it takes several drive cycles for the ECM to relearn the optimum values. Anything from 5 minutes to 5 days.
     
    #5 dolj, Mar 8, 2025 at 8:52 PM
  6. mr_guy_mann

    mr_guy_mann Senior Member

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    Do you have freezeframe data for either code? It's a bit odd because P2195 is for the B1S1 Air-Fuel ratio sensor stuck lean. That should show in data as at or near 5.0V. Short Term Fuel Trim should go positive to compensate.

    Your graphs show (unexplained) dips in A-F voltage down to 1.4V, that's indicating (or at least reporting) a rich condition, and STFT is going highly negative in response.

    Here's the bits in the MAF that need careful attention. clean_maf.JPG

    Posted via the PriusChat mobile app.
     
    #6 mr_guy_mann, Mar 9, 2025 at 4:01 PM
  7. d_w

    d_w New Member

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    I’ll post the FF data when I drive a bit more and the code pops again (I recently cleared the codes). I am also confused by the data coming off the sensors as it isn’t really logical.
     
    #7 d_w, Mar 9, 2025 at 8:50 PM
  8. MAX2

    MAX2 Active Member

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    Short term fuel trim means that the gasoline is flowing like a river and screaming to the engine computer "Stop it".
    And the gasoline supply is reduced. The engine growls, but moves. You press the gas pedal again and everything repeats.
    Everything needs to be checked again, spark plugs, coils, fuel supply, injectors, air supply, etc.
     
    #8 MAX2, Mar 10, 2025 at 7:15 AM
  9. mr_guy_mann

    mr_guy_mann Senior Member

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    Close. But you have to be careful about assumptions or they will kick your butt.

    Short Term Fuel Trim is a response to the upstream S1 Air-Fuel (or oxygen) sensor.

    If the S1 reports a rich mixture, STFT makes a negative correction to reduce fuel. If S1 reports lean, then STFT makes a positive correction to increase fuel.

    There are three general areas that can cause fuel trim problems.

    First is an input sensor misreporting that results in the wrong base injector on time. (Think dirty MAF or a vacuum leak).

    Second is a fuel delivery fault. Fuel pump, clogged injectors, even the wrong fuel in the tank.

    Third is that the A-F or O2 sensor is lying. This is also an ECM input, but I keep them separate because it can really trip you up.

    Posted via the PriusChat mobile app.
     
    #9 mr_guy_mann, Mar 10, 2025 at 10:06 AM
    MAX2 likes this.
  10. d_w

    d_w New Member

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    The codes popped again, luckily while I was recording a log. P2195 and P0138. In the logs, it is easy to see when they triggered about halfway through where the A/F sensor spikes up and the O2 sensor drops to 0.

    It doesn’t have freeze frame data for the p2195 code, just p0138 for some reason, but they seemed to happen at the same time so hopefully that is relevant.
     

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    #10 d_w, Mar 11, 2025 at 1:42 PM
  11. mr_guy_mann

    mr_guy_mann Senior Member

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    No, it's the other way around. If you try to line up the RPM graph (by frame numbers along the bottom, around 225-250), the A-F goes high (lean) and O2 goes 0 (lean) when the RPM hits 0 (engine off).

    It would be a little easier to correlate things if you ditched vehicle speed. Right now some of the graphs have different scaling.

    P0138 is the S2 O2 voltage going too high. That can be 1V or more, OR it can be around 100mV higher than normal.

    The freezeframe data doesn't really show anything abnormal. Fuel trim is slightly high but S2 O2 is at 0.7V. Whatever happened, FF didn't catch it.

    However, look earlier at the data graph around frame 130(?), you have a hard accel from around 25mph. Again the S1 A-F sensor goes crazy low voltage (rich), and this time the S2 O2 hits 1V, also full rich. That is likely where it set P0138. Both sensors reporting rich suggests that it's true. Wonder what the MAF reads?

    Some cars go full rich during WOT acceleration, but I don't think the Prius is one. Will have to check on my commute home.

    Posted via the PriusChat mobile app.
     
    #11 mr_guy_mann, Mar 11, 2025 at 3:28 PM
    Last edited: Mar 11, 2025 at 3:36 PM
  12. MAX2

    MAX2 Active Member

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    You will have to start working by the process of elimination.
    Another CEL Thread!!! P0138 and P2195 | PriusChat
     
    #12 MAX2, Mar 12, 2025 at 5:03 AM
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