Is the rear O2 sensor data used for anything?

Discussion in 'Gen 2 Prius Technical Discussion' started by Hello1024, Jan 6, 2026 at 9:45 AM.

  1. Hello1024

    Hello1024 Junior Member

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    Is it used to adjust fuel trims or anything, or merely to check the cat is working properly?


    I unplugged it, and apart from a light on the dashboard and check engine codes for the sensor and associated heater giving bad data, didn't notice any change in behaviour.
     
    #1 Hello1024, Jan 6, 2026 at 9:45 AM
  2. mr_guy_mann

    mr_guy_mann Senior Member

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    So (AFAIK), on the Gen2 Prius the downstream (S2) O2 sensor is only used to monitor catalyst function (after the fact).

    On many other cars, the S2 sensors are used to adjust fuel trim - under certain conditions. This is known as "downstream fuel control". The idea is to have "closed loop" for cat function, instead of closed loop for engine function.

    This process is poorly documented by most carmakers, which makes it a pain to figure out when there's a problem.

    FYI, Toyota was one of the earliest to use downstream fuel control, on the late 80's V6 Camry (with CA emissions).

    Posted via the PriusChat mobile app.
     
    #2 mr_guy_mann, Jan 7, 2026 at 11:06 PM
    Last edited: Jan 7, 2026 at 11:14 PM
    Brian1954 likes this.
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