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Shim on downstream o2 sensor

Discussion in 'Gen 2 Prius Technical Discussion' started by BigRic, Dec 17, 2018.

  1. BigRic

    BigRic New Member

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    I've had an 05 Prius for about 4yrs, purchased with 242k miles on it. I've had it to a friends place a couple of times (he used to work for Toyota as a mechanic) due to an O2 sensor code. He'd noted several times that the o2 sensor was shim'd (from what I understand, to pull it further from the center of the exhaust flow where it changes its sensitivity to temp?). There's no room to shim it any further and he thought I'd end up replacing the catalytic converter as the next step. Come to find out today, that Toyota was supposed to have replaced the catalytic converter about 5k miles before I bought it (at a price of $1800 per the prior owner). I'm wondering if there is any validity to the o2 sensor shim theory (or if my buddy needs to slow down on his beer consumption)? If in fact the CC was changed out and the o2 sensor replaced, is there anything else other than bad wiring that could cause an issue?

    Thanks in advance!
    BigRic
     
  2. BigRic

    BigRic New Member

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    One added note - codes were P0137 and P0420.
     
  3. valde3

    valde3 Senior Member

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    Yes rear o2 sensor shims put the rear o2 sensor further from the exhaust stream. That will then make the sensor respond slower. ECU will then think that catalytic converter is working better as good catalytic converter will also make sensor respond slower.

    But other than that this whole think doesn’t make any sense to me. If catalytic converter was replaced 5k miles ago then it should not be bad. So was it replaced then? And the other code was for the rear o2 sensor that was also supposed to be replaced? Is the rear sensor functioning now? If those codes are cleared witch will come back?

    You can get angled o2 sensor extenders but if the catalytic converter is bad enough then it should not be replaced because it can/will also be blocked. But if the catalytic converter is only 5k miles old it should work like new.

    Catalytic converters normally go bad because of engine oil consumption. How much oil does yours use?
     
  4. Mendel Leisk

    Mendel Leisk Senior Member

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    Would shimming the sensors do anything to the air/fuel ratio?
     
  5. valde3

    valde3 Senior Member

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    No. Sensor will still read the same exhaust gas. It will just react slower.
    And shimming is done for rear o2 sensor. Air fuel ratio is (mostly or only (depending on car)) measured with front sensor (in a Prius that’s air fuel ratio sensor).
     
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  6. BigRic

    BigRic New Member

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    Sorry, I forgot to update this after getting more detail. One person's memory of 5k miles (especially when said person is 84yrs old) might be slightly different than someone else's! :) Ends up it was more like 5 yrs ago and 90k miles... Seems that around that time (180k on the motor) they replaced everything. CC, Ratio Sensor, O2 Sensor, exhaust, etc. Still makes it pretty old but I've also been told that it's still very young in terms of those specific components. I have it at another friend's shop right now to see if he can identify the root cause. I know the O2 sensor was replaced not long ago (again, after the big job) but I'm not sure about other things that may be going on. Oil consumption has been fine. Only issue I have is gas engine runs pretty rough and I had to replace a coil on one cylinder. At 270k miles, I'm happy it runs at all.
     
  7. SFO

    SFO Senior Member

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    My guess is you're having fuel trim issues caused by the injectors at 270k (use techstream or similar to check the long and short fuel trims), find some reconditioned injectors ($25) online or send off your existing ones for reconditioning (around $60 to $100).

    When were the spark plugs last checked or replaced?
     
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  8. Skibob

    Skibob Senior Member

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    You can make your own fuel injector tester/cleaner for a couple of dollars. Anyone know what the voltage to the injectors is?