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P0138 new B1S2 always 1.27v

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by anonymous, Jun 10, 2024.

  1. anonymous

    anonymous Member

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    since cat was stolen the sensor was replaced with a $15 ebay sensor. but since installing the cat and sensor it is always immediately throwing P0138 and the freeze frame data always shows 1.27v (i dont have a way to view live data).

    I have two of the new sensors but both of them behave the same so maybe it is not that. Considering the wires were chopped off I wonder if a short circuit could have damaged something else. any ideas?
     

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  2. anonymous

    anonymous Member

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    I plugged in the other o2 sensor and went for test drive with it dangling in cabin and got P0137 instead...

    Then I plugged back in the installed sensor and so far it is working now...? Also now live data works on this thing...

    CAT monitor is passed. O2S is heater and needs cold start?
     

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  3. anonymous

    anonymous Member

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    No O2S need 10 min idle and 2 min >25mph according to some random pdf I found... Well I got all monitors ready except evap so... it fixed itself??? Well that's annoying, I did not even do anything, so I don't know what was wrong and if it won't come back
     
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  4. anonymous

    anonymous Member

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    Well this is interesting. I put the extra new sensor in an other prius (a gen 3), and initially I was seeing high voltage around 1.2v, and it soon turned on MIL for p0138. But after about a minute of driving the voltage dropped to normal fluctuating levels. I cleared the MIL again and now all monitors are OK. ??? Do new o2 sensors have a break-in period or something?? What is this???
     

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  5. mr_guy_mann

    mr_guy_mann Senior Member

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    See if that "new" sensor does the "high voltage when cold" thing consistently.

    It sounds like there's an internal defect in the sensor that bleeds some voltage from the heater circuit into the signal wire. It "goes away" when the sensor heats up?

    I am pretty sure the voltage is higher than 1.27V, but that's the maximum the engineers allowed for the ECM to display on a scantool.

    Personally, I stay away from cheap AM parts - have had too many problems.

    Posted via the PriusChat mobile app.
     
  6. anonymous

    anonymous Member

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    Yea I guess it is a faulty sensor. The voltage should not be high when the engine is not even running, right? 0.66v before cold starting engine (went up to 0.72 once hybrid system ready), went as high as 1.25v while engine running... after driving 0.015v with engine off which is what it should be? P0138 pending again, this is surely gonna keep causing trouble.

    I wonder if it's just because they're cheap or if it has something to do with having soaked in rain water for a while. Well both have quite a few miles on them and they're not getting any better. I have an old OEM sensor from a gen 3 I'll see if that will cut the cake