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p0420 question

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by twj815, Jul 22, 2024.

  1. twj815

    twj815 New Member

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    Mahomet, Il
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    2008 Prius
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    My 2008 Prius now has 268,000 miles on it. Last fall, at around 245,000 miles, someone stole the catalytic converter while it was parked in a retail parking lot. They also cut off the downstream O2 sensor. Insurance replaced most of the exhaust system including both sensors. Now the check engine light comes on showing a p0420 code shortly after starting a cold engine. If I let the engine to warm up before resting the code, the check engine light will stay off for 100 miles or more until after I park the car and allow it to cool down. After the cool down period of half an hour or longer, the check engine light will come back on with the same code.

    Live data with a Blue Driver code reader looks like the downstream sensor is fairly stable in the .6 to .8 volt range on the highway. I occasionally see a momentary drop to near 0 which seems to usually happen while driving down hill. Maybe the engine temporarily shutting down? In town, stop and go driving is hard to tell what the live data is telling me because of the engine starting and stopping all the time.

    Could the new cat have gone bad that quickly? Is it more likely a problem with the downstream sensor? On other cars I didn't care about a p0420 code but since this car doesn't have a temperature gauge, I won't know if there is a problem with the cooling system or other issues if I can't get the check engine light to stay off.

    Thanks for your advice.
     
    #1 twj815, Jul 22, 2024
  2. Tombukt2

    Tombukt2 Senior Member

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    If it was replaced with a factory Toyota cat because you have emissions where you're located then more than likely you have the proper cat and everything should be good with that unless you're burning lots of oil putting that into the cat and then that destroys the catalytic material I don't know generally generation twos don't burn tons of oil not the kind that floods a converter and where is it out in short order generally speaking so it'll be interesting to see what's going on or is this one of those $100 converters which even those they're made by a big company that makes converters for V8 cars and all that kind of stuff so they get decent reviews for all kinds of models not sure about the Prius but they sell for lots of vehicles the kind that kids put flowmasters on and all that stuff so I put a cheap set of cats on a Mercedes and they lasted a long time so there's always that I do realize the Prius cats have a lot more material in them than other cats do or might have oh well
     
    #2 Tombukt2, Jul 22, 2024
  3. twj815

    twj815 New Member

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    2008 Prius
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    N/A
    I can't say for sure that it is a Toyota cat but it is bolted in, not welded. Removing it from the car would not be difficult. I don't live in an emissions testing are.

    It is used mostly as a paper route car (lots of stop ang driving for several hours a night). It still averages around 50 mpg in the summer, even with the p0420 code.
     
    #3 twj815, Jul 22, 2024
  4. rjparker

    rjparker Tu Humilde Sirviente

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    As Tom said it is possible especially if the replacement was a low cost aftermarket cat. Since you are not in a state that requires the best quality CARB Compliant cats, insurance would have paid for a lower cost version.
     
    #4 rjparker, Jul 22, 2024
  5. twj815

    twj815 New Member

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    2008 Prius
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    N/A
    Here is an interesting thought. I had borrowed a Blue Driver to check the status of the o2 sensors. It said that the upstream sensor (sensor 1) was not supported. Ok, well maybe it's an older unit and not fully compatible with a Prius. So, I bought an OBDLink MX+ unit. The software that they supply on their website shows zero volts from the upstream sensor.

    Can it be that these Bluetooth obd2 devices won't show what sensor 1 is doing in a 2008 Prius? Is it possible that the sensor has failed and that is the cause of the p0420 code?
     
    #5 twj815, Aug 5, 2024
  6. mr_guy_mann

    mr_guy_mann Senior Member

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    So Toyota uses an Air-Fuel Ratio upstream sensor (S1 position). A "normal" oxygen (O2) sensor produces a 0V to 1V signal. Any generic OBD2 scan device can (should be able to) read that data.

    An AFR sensor moderates a really small electrical current signal from the ECM - based on the Air-Fuel mixture in the exhaust. Toyota decided that the ECM will turn that milli to micro-amp current reading into a voltage amount (about 1.5V to 5.0V) on the scan data. That's what you see for any scantool that has "enhanced" (OE style) data. That's what I expect from OBDLink app when you go into the ECM.

    Good generic OBD2 scantools will usually show the S1 milliamp reading for an AFR. Bad ones show 0V or some value between 0-1V that doesn't change.

    If your AFR sensor has completely failed, you would have codes for that.

    Posted via the PriusChat mobile app.
     
    #6 mr_guy_mann, Aug 6, 2024
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