Mazda Tribute 2003, AJ 3.0, ES-V6 FWD O2 sensors, volts vs %

[Igor1]

My Mazda Tribute has 4 oxygen sensors. Two of them are before catalyst and two after. All are visible by FORScan. FORScan sees them as O2S11, O2S12, O2S21, O2S22.
I was looking at the following parameters:
O2S11 (Volts)
O2S11.STFT.OBDII (%)
O2S12 (Volts)
O2S12.STFT.OBDII (%)
O2S21 (Volts)
O2S21.STFT.OBDII (%)
O2S22 (Volts)
O2S22.STFT.OBDII (%)
and couple of others

O2Test.png (187.46 KiB) Viewed 2819 times

All volts measurements are saw like graphs, but % measurements are straight lines for both O2 sensors that are after catalyst. They are always straight line no matter how long was a trip and how warm the engine.
To my understanding O2S22 (Volts) is measured from the sensor and O2S22.STFT.OBDII (%) is calculated at ECU.
Is this a normal behavior for volts and % to be different shapes for O2 sensors after catalyst? How O2S22.STFT.OBDII (%) values are calculated?

[FORScan]

All volts measurements are saw like graphs, but % measurements are straight lines for both O2 sensors that are after catalyst. They are always straight line no matter how long was a trip and how warm the engine.
To my understanding O2S22 (Volts) is measured from the sensor and O2S22.STFT.OBDII (%) is calculated at ECU.
Is this a normal behavior for volts and % to be different shapes for O2 sensors after catalyst? How O2S22.STFT.OBDII (%) values are calculated?

Yes, Volts are direct measurement and STFT means Short Fuel trim - correction to the fuel trim applied according to the O2 sensor feedback. It directly depends voltage. You may google for keywords Fuel Trim, Short fuel trim, Long fuel trim to get more information.

O2S11 and O2S21 are pre-catalyst sensor, they are directly involved in the fuel trim. O2S12 and O2S22 are after-catalyst and they are not directly involed into the fuel trim, their goal is to control emission. So the graphs you see are normal.