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Hey everyone, I've got a question which I've been trying to figure out for a long time. It relates to fuel trims and pre and post cat oxygen sensors.
I've noticed on a lot of makes and models, live data lists not only a short and long term fuel trim, but also a oxygen sensor 2 (post cat) fuel trim.
I've got a good understanding how sensor 1 (pre cat) works to trim fuel rich and lean around stoichiometric in the short term, and store any consistent errors in the long term fuel trim. I also understand the catalyst has a oxygen storage capacity and uses oxygen in its operation, and the rich lean mixture input results in a steady low oxygen output that reads as a rich voltage on sensor 2. And I also get the main purpose of sensor 2 is to verify the catalyst is working.
However, what is the theory behind sensor 2 fuel trim?
Is it calculated when the vehicle is cold and catalyst not working and sensor 2 still swinging rich lean to compare sensor 1 and 2 and create a value to offset sensor 1 which might have a offset because it's old/faulty whatever. Or is it used to keep sensor 2 voltage at a set point, eg to keep the rich/lean percentage of time of each condition 50/50? Which could account for a slowly responding sensor 1? I've noticed sensor 2 voltage is always 750-770mV. Not 650mV and not 850mV but always very close to 750-770mV? Is that the optimum voltage to say the catalyst has a 50/50 rich lean mixture going into it?

Thanks in advance

They are used to optimise the cat efficiency by adjusting the rich/lean swing of the front sensor to maintain a set voltage. The rear sensor being narrowband can determine stoich more accurately.

There is bugger all trimming by the rear sensor though (couple of percent at most), the fuel trims are all pretty much all handled by the front sensor.

In simple terms. It's just to make sure the cat is at the right temperature to burn any excess fuel to lower emissions but not too hot as to reduce the life of the cat. That's my understanding of it?

Part of its function is to confirm the cat is working, as per pman92 post the narrowband O2 voltage wont swing rich/lean post cat if it is working well.

Catalyst temp is modeled/estimated by the PCM calibration.