Catalytic Converter Temperatures... Confusion

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by HerbTheNewPriusOwner, Mar 28, 2020.

  1. HerbTheNewPriusOwner

    HerbTheNewPriusOwner New Member

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    I'm confused about how I'm supposed to read Prius (Gen 3) catalytic converter temps on a scan tool. I believe B1S1 is the first O2 (AF) sensor right in front of the cat. That means B1S2 is the O2 sensor between the cats. Yet, the temps coming from B1S1 are HIGHER than B1S2 on my scanner. I've also see this on pics and vids from other folks on this forum.

    Isn't the exhaust gas temp supposed to be lower in front of the cat and higher after it?

    Can anybody tell me why this is or how (what) I'm reading wrong? I've attached a pic of what my scan tool is saying so you can see what I'm talking about.

    BTW, I'm fairly certain the converters are good as I just installed a new Denso four wire AF sensor and I have no check engine lights and all the exhaust checks are completed.

    Thanks!
     

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    #1 HerbTheNewPriusOwner, Mar 28, 2020
  2. sam spade 2

    sam spade 2 Senior Member

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    I did not believe that ANY of the sensors are designed or intended to "read" the temperature of any exhaust components directly.
    But that display certainly looks like it does.
     
    #2 sam spade 2, Mar 28, 2020
  3. pjksr02

    pjksr02 Active Member

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    I'd be more concerned about the EGR system in a newly acquired Prius. Have you discovered that issue here, yet?

    Welcome to the board.
     
    #3 pjksr02, Mar 28, 2020
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  4. Elektroingenieur

    Elektroingenieur Senior Member

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    Correct. Using Toyota’s terms from the Repair Manual (more info) and the SAE paper on the 2ZR-FXE engine: the air fuel ratio sensor (sensor 1) is located in the exhaust manifold, and the heated oxygen sensor (sensor 2) is located between the starter catalyst and the under floor catalyst.
    It’s true that the main outputs from the sensors are currents or voltages indicating the oxygen content of the exhaust gases, and there are no dedicated EGT sensors, as you’d find on turbine engines, for example.

    As Toyota explains in the Technician Handbook for Course 973A: Advanced Engine Control (available by subscription to techinfo.toyota.com), however, the engine control module (ECM) does detect the approximate temperature of the A/F and O₂ sensor elements by sending pulses and measuring the impedance. (See also the temperature-impedance chart in the Repair Manual procedure for DTC P0136.) This is important because the sensors don’t work at low temperatures, which is also why they’re heated.

    The catalytic converter temperatures shown in the Data List are estimates.
    Not necessarily; some of the conversion reactions are exothermic.
     
    #4 Elektroingenieur, Mar 28, 2020
  5. Mendel Leisk

    Mendel Leisk Senior Member

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    Now I know where I heard that voice before:

    upload_2020-3-28_13-25-54.png
     
    #5 Mendel Leisk, Mar 28, 2020
  6. HerbTheNewPriusOwner

    HerbTheNewPriusOwner New Member

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    Thank you all for your quick and very insightful responses! I've posted on a different thread the experiences I'm having with doing a JDM engine replacement due to a head gasket failure at 219K miles. The end result is it's all finally worked out. In summary, the AF sensor (B1S1) was bad. And when I got under the car and checked the temps with an infrared thermometer, the rears of each unit were infact warmer than the fronts by 100'F. So all's well that ends well but... Those crazy readings on the scan tool make it WAY more difficult to run down the problem. In the end, I decided to go for broke thinking that if I had to replace the cat, I'd also do the sensors at the same time, so why not start with the most culpable sensor first. The bet paid off!
     
    #6 HerbTheNewPriusOwner, Mar 30, 2020
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