P0420 error - what should I do?

Discussion in 'Gen 2 Prius Technical Discussion' started by enginestarter, Nov 12, 2019.

  1. enginestarter

    enginestarter Junior Member

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    P0420: Catalyst System Efficiency Below Threshold (Bank 1).
    This is what Android Torque gives.

    I had the mechanic look under the car - nothing to report, all looks fine, no part loose and cables connected OK.
    + I drove it around with the mechanic, nothing to report.

    I got the part from Eurocarparts a few months ago - it was a Klarius converter + a Bosch O2 sensor.
    I have the receipt. I can get replaced.

    Problems:

    - How do I know what is faulty? Is it the cat converter? Is it the O2 sensor?
    I'm guessing in most cases only one part is faulty?

    - I have to pay for removing the cat converter and then reinstalling again.
    The mechanic said to go and buy a new one and then he'll take off the old one, which I can then return.

    - If I don't make the right decision about the correct part - then that doubles my costs.

    What shall I do?

    EDIT: I can get rid of the error using Android Torque. It comes back after a few days. Also, sometimes (not frequently) the engine management light will just go out of it's own accord - without me doing anything. I thought I would mention.

    Thanks.
     
  2. bisco

    bisco cookie crumbler

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    would tech stream be more specific?
     
  3. enginestarter

    enginestarter Junior Member

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    what's tech stream? is that an app? i looked up on google play but couldn't see anything. let me know. thanks.
     
  4. valde3

    valde3 Senior Member

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    You can watch the values of (front) air fuel ratio sensor and (rear) oxygen sensor with running engine to determine which is faulty air fuel ratio sensor, oxygen sensor, or catalytic converter. Catalytic converter is the most likely but also the most expensive. Unfortunately it's kind of hard to explain how to read the data basically you need to see how fast signals change depending on running conditions.

    How much oil does your Prius consume? Engine oil consumption will slowly kill the catalytic converter.

    Techstream is Toyotas own diagnosis tool. There is also a cheap Chinese version of that. But many general scan tools are basically as good when diagnosing engine (related) problems like this.

    If it is the catalytic converter only legal options is to fit a new one.
     
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  5. bisco

    bisco cookie crumbler

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    yes, see valde3's response above. if you search here, you will find how and where to buy, along with advice on use
     
  6. Skibob

    Skibob Senior Member

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    You bought a Bosch front O2 sensor instead of a Denso?
     
  7. valde3

    valde3 Senior Member

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    You really shouldn't use anything other than OEM sensor as the front sensor. Front sensor is air-fuel ratio type and differences between sensors can cause problems.
     
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