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My assumption is that on a 2 bank system, if both catalyst always read the exact same temperature, the values are calculated or inferred from other sensor inputs.

My guinea pig is a 2005 Ford Explorer with a 4.0 liter. In OBDII mode $01, no matter the operating condition, both banks will mirror each other. I even went as far as disconnecting a spark plug wire from cylinder 4, and drove 60 mph to heat up the bank 2 converter. I know that curiosity killed the cat (pun intended), but I haven't found a use case for these values when they are calculated or inferred.

So since the catalyst temperature pid values remained equal during this test drive, why would a technician ever need to monitor them?

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When producing aftermarket tunes, you can run into limiters and fail safes that limit power based on this calculated value.

It’s calculated based on the oxygen sensors and mapping of oxygen capacity of the catalyst.

I don’t know if a tech would ever use them, but that gives you a general idea.

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  • That is an excellent use case. What I don't understand is why inferred catalyst temperature ever made it on the OBDII standard PID list. I could see maybe for freeze frame purposes, as to have an idea of how hard the vehicle was operated prior to a code setting.
    – Milison
    Jun 5, 2018 at 12:57
  • Could have been the idea that if they wanted to do emissions testing they would want to ensure catalyst is up to temperature, or ensure that it gets up to temperature in a certain time.
    – Damon Earl
    Jun 6, 2018 at 1:19

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