O2 sensor #1Upstream Sensor?

So, let's first look at a normal "narrow band" oxygen (lambda) sensor. It's a zirconium dioxide sensing element (the classic design is a thimble shape) that's coated on both aides with platinum.

The inside of the thimble is exposed to atmospheric air. The outside is exposed to exhaust gases. Add heat and hydrocarbons to the platinum, and you get a catalytic reaction. When that happens with 2 layers of conductors separated by ZiO2, you have a galvanic cell. The output of that cell is dependent on the difference of O2 between the ambient reference air and the exhaust stream.

You need heat to have a reaction so they add a resistive heater.

Now the term "Lambda" refers to stoichiometry or perfect Air-Fuel Ratio, where all of the air reacts with all of the fuel. For 100% straight gasoline, that's 14.7 lbs of air for 1 pound of fuel. That is a Lambda of 1.0, a "lean" ratio is higher than 1.0, while "rich" is less than 1.0.

The actual AFR by weight will be different for different fuels, but Lambda is always 1.0 when you have a stoichiometric mixture (E85 is about 9.8:1, as ethanol has much less heat energy per volume than gasoline)

At 14.7:1, an O2 sensor outputs around 450mV. At 14.6:1, an O2 is near 0V ( less than 100mV), and at 14.8:1 it's near 1.0V (over 900mV). The ECM reads the voltage and uses Short Term Fuel Trim to drive the O2 reading back and forth across 450mV.

That's the limit of this type of sensor - it's only good AT "more than Lambda of 1", or "less than Lambda of 1".

The upstream sensor on a Prius is a "4-wire" Air-Fuel Ratio sensor. 2 are for the heater, and 2 are for the sensing element. Instead of reading voltage, they "flip" the sensor design around and use current. The ECM maintains a 300mV differential across the sensor. Depending on the O2 concentration in the exhaust gases, the sensor will help or hinder current flow.

At Lambda of 1.0 it's 0mA, below 1 (rich) the ECM reads negative mA, and above 1 it reads positive. At normal steady throttle, the AF sensor output is typically 0 +/- 0.1mA (100microamps). It will go just over to +2mA on fuel cut.

So the ECM interprets the AF sensor amperage reading and displays a "upstream O2 sensor Lambda".

The AF sensor CAN read about 12:1 to 18:1. That lets the engineers program the ECM for something other than "either side of stoichiometry". Maybe they want Lambda of 0.95 under WOT, or 1.05 at light cruise. (5 wire AFR sensors can read 10:1 to 20:1)

That's the "commanded" Lambda or AFR (or whatever). It's the target. The upstream S1 A/F sensor lets the ECM see if the result hits that target. STFT is the means to make adjustments to stay on target.

I do not know exactly where they get catalyst temperatures from. It could be interpellated from S1 and S2 heater current readings (increased temperature causes increased resistance = lower current), or it could be algorithms based on RPM and load. There's a bunch of variables that affect what the temperature difference is across the catalyst. If I knew the particulars of how it worked on Toyotas, I might bother to use the data - or not

Now for whatever reason, Toyota engineers decided to put the AF sensor reading as a "made up" voltage instead of current. 3.3V is Lambda of 1.0, lower V is rich, and higher is lean. Typical in-use range is 1.8V to 3.8V, and 5.0V for fuel cut.

If you go to generic OBD2 data, you can get AF sensor current values.

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Close. 1 is what the ECM wants (is programmed to achieve). 2 and 4 are the AF Sensor reporting what is happening (the result of whatever fuel mixture that just went through the engine). 3 (STFT) is a direct reaction to AFS in order for 2 to "match" 1.

So STFT is there to keep AFS on the programmed target. LTFT is a "learned" compensation that is there to keep STFT near 0%. Say STFT goes to +10% "for awhile". LTFT will step in (go toward +10%), and STFT will go back toward 0. (ST + LT gives you total FT).

LTFT has different values for different RPM and load values, and is stored in ECM memory (as long as it has 12V battery power). So LT @ idle is different from LT @ high idle, vs 2500 rpm cruise vs 4000rpm accel, etc.

AFS voltage - above 3.3V is "lean", below 3.3V is "rich".

The B1S2 (downstream) O2 sensor is primarily there to monitor catalyst function. The ECM is programmed to adjust fuel in order to "feed" the cat with a slightly oscillating (rich to lean to rich) mixture. The cat needs CO, HC, AND O2 in order to work.

The cat has cerium in it to "store" the O2, which gets used up to convert the CO and HC to H20 and CO2. So there's "not much" O2 left - the downstream sensor will read (somewhat steady) "about" 700mV when the cat is healthy and lit off. (Lambda doesn't really apply to the downstream reading).

Caveat - some manufacturers DO use the downstream sensor to adjust fuel control (sometimes, and it is poorly documented). Toyota is one of the earliest to do this, they just do not on the Gen2 Prius.

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