Changed O2 sensor, code keeps coming back

Discussion in 'Prius v Main Forum' started by ErikK, Oct 14, 2021.

  1. ErikK

    ErikK New Member

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    V
    Short version:
    I changed the upstream O2 sensor, cleared the codes, the check engine light went off for a few seconds and came back on. Tried to clear several times with two different scan tools, it keeps coming back. Any suggestions?

    Long version:
    2016 Prius V. 76k miles. Check engine light came on, codes about the sensor 1 bank 1 which I believe is the upstream wideband sensor. We've had squirrel problems and one of the things the little **##*s did was chew the wires on the upstream sensor. Of course the dealer would not cover it under warranty. So I replaced the sensor with a new Bosch part, it was not a difficult job.

    I cleared the codes but they keep coming back.
    P0031 HO2S heater control circuit low
    P2238 O2 sensor positive positive current control circuit low

    My scan tool shows three categories of codes - stored, pending, and permanent. P0031 occurs in all three, P2238 is only in the "permanent" list. There are no other codes. I've tried to clear them all repeatedly, they keep coming back.

    I did 'read live data' and the voltage output from this sensor is stuck at 3.28V. The lambda estimate is stuck at 0.999. They do not change at all with throttle or RPM or when the engine starts and stops.

    Any advice is welcome. At this point I have two guesses:
    1.. It needs a better scan tool than mine to clear the "permanent" codes. Is there a DIY solution or do I have to go to the dealer?
    2.. When the rodents chewed up the wires they got shorted together and this damaged something in the ECU circuitry that drives the heated wideband sensor. I hope not....!

    Any suggestions? Thanks!
     
  2. ErikK

    ErikK New Member

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  3. mr_guy_mann

    mr_guy_mann Senior Member

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    You need to check for additional damaged wires somewhere. The single permanent code will clear itself once the other fault is fixed and you drive the car long enough (there is a special drive cycle you can do in the service manual to clear things faster).

    Now the 00031 code indicates that the heater circuit is likely open- ie has a broken wire or possibly a defective new sensor. Since the code is stored and pending that means that the fault is happening right now.

    There is a slight chance that the chewed wires did short together and put high current through the heater control circuit and "blew" the driver transistor in the ECM. You may need to test the circuit at the ECM - if there is "12V" present on the control circuit while the ICE is running and the code sets, then the ECM is likely damaged.

    The heater gets 12V and the ECM controls the negative side by switching it to ground on and off very quickly- called pulsewidth modulation. When the heater isn't turned on the ECM expects to see 12V on the negative circuit. If there is no/low voltage then you get the P0031.

    If the heater doesn't work then the air fuel sensor itself won't function and the "3.3V" on the scantool won't change at all.

    Posted via the PriusChat mobile app.
     
    #3 mr_guy_mann, Nov 4, 2021
    Last edited: Nov 4, 2021
    ErikK likes this.
  4. ErikK

    ErikK New Member

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    V
    Thanks for the detailed info! That all makes sense. I think my next steps are:
    - visually inspect for more damaged wires
    - check for 12v at O2 sensor connector
    - measure O2 sensor heater resistance
    - check for PWM at O2 sensor connector (i have an oscilloscope)
    - I could even inject some voltage under 3.3v at the connector and see if the ECU sees it real time.

    Then I guess if I have not found the problem in the wiring harness visually I could repeat all the above at the ECU connector, to determine whether problem is in harness or in ECU.

    Will let you know what I find...
     
  5. mr_guy_mann

    mr_guy_mann Senior Member

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    Do NOT inject voltage into the air fuel sensor circuit. That would fry the Engine Control Module. The ECM keeps a 300mV difference across the two sensor wires (3.3V to 3.3V). The actual air fuel mixture in the exhaust will affect how much current (and what polarity) is required to maintain that difference. Near stoichiometry (Lambda of 1.0 or 14.7:1 ratio) it only takes less than 100 micro amps. At fuel cut -max lean- it will go to a couple milli amps.

    So the ECM measures the tiny amount of current going through the sensor and Toyota engineers decided to convert that into a voltage reading for the scantool display.

    Posted via the PriusChat mobile app.
     
  6. ErikK

    ErikK New Member

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    Got it, I won't inject voltage. Thanks for the warning.
     
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