So I was reading about EFIE devices, and...

[jakobnev]

Rich exhaust causes oxygen to flow from outside into the exhaust stream, and lean exhaust causes this flow to slow or even stop.

What? (can you draw a picture?)

[t vago]

That's how an O2 sensor works. It works off of the diffusion of oxygen from one side of the sensor to the other. If there is a condition where extra oxygen is demanded and there is none present in the exhaust stream (such as for a rich mixture), oxygen flows from the air outside of the sensor into the exhaust stream. If there's an excess of oxygen in the exhaust stream, that flow slows down, or stops. Higher oxygen diffusion rates cause higher voltages to be generated by the O2 sensor, up to a point.

We're talking about minuscule amounts of oxygen, though.

[cfg83]

t vago -

Do you mean the outside air is "diffusing" through the membrane (shown below)? :

Oxygen sensor - Wikipedia, the free encyclopedia

Zirconia sensor
The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V (200 mV) DC represents a "lean mixture" of fuel and oxygen, where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). An output voltage of 0.8 V (800 mV) DC represents a "rich mixture", one which is high in unburned fuel and low in remaining oxygen. The ideal setpoint is approximately 0.45 V (450 mV) DC. This is where the quantities of air and fuel are in the optimum ratio, which is ~0.5% lean of the stoichiometric point, such that the exhaust output contains minimal carbon monoxide.
The voltage produced by the sensor is nonlinear with respect to oxygen concentration. The sensor is most sensitive near the stoichiometric point and less sensitive when either very lean or very rich.
The engine control unit (ECU) is a control system that uses feedback from the sensor to adjust the fuel/air mixture. As in all control systems, the time constant of the sensor is important; the ability of the ECU to control the fuel-air-ratio depends upon the response time of the sensor. An aging or fouled sensor tends to have a slower response time, which can degrade system performance. The shorter the time period, the higher the so-called "cross count" and the more responsive the system.
The zirconia sensor is of the "narrow band" type, referring to the narrow range of fuel/air ratios to which it responds.

Wideband zirconia sensor
A variation on the zirconia sensor, called the "wideband" sensor, was introduced by Robert Bosch in 1994, and has been used on a lot of cars in order to meet the ever-increasing demands for better fuel economy, lower emissions and better engine performance at the same time. It is based on a planar zirconia element, but also incorporates an electrochemical gas pump. An electronic circuit containing a feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, so that the pump current directly indicates the oxygen content of the exhaust gas. This sensor eliminates the lean-rich cycling inherent in narrow-band sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly. In the automotive industry this sensor is also called a UEGO (for Universal Exhaust Gas Oxygen) sensor. UEGO sensors are also commonly used in aftermarket dyno tuning and high-performance driver air-fuel display equipment. The wideband zirconia sensor is used in stratified fuel injection systems, and can now also be used in diesel engines to satisfy the forthcoming EURO and ULEV emission limits.

CarloSW2

[mwebb]

the "wide band AFR sensors" do cycle
like an 02 sensor

you can not measure it with a low resolution scan tool but i have graphed and logged AFR sensors cycling rapidly
when they stop cycling

they are deader than dog poop , as they are used on newer systems , the system will set a DTC ... well before you need to start checking
unless they loose the zero and start misreporting actual conditions

monitor LTFT and watch for LTFT and STFT to be on opposite sides of zero by upwards of 10 percent
of course
a misreporting rear 02 sensor or an exhaust leak near either the AFR sensor or rear 02 sensor can send fuel trims to opposite extremes.

yes
newer systems with AFR sensor DO trim on the rear 02 sensor as well as the front AFR sensor


alternately
if you are using a HIGH RESOLUTION scan tool with a FAST update rate
you can graph STFT , tighten up the upper and lower limits to bracket the range of cycling
since STFT responds to inputs from the AFR sensor , graphing STFT is almost as good and will show if the AFR sensor flat lines

you can also look at
MODE 6 for system testing to monitor AFR sensor condition s like range and response time and heater condition

if you do not have a hi res scope and a hi res scan tool for your system
best to not be tampering with this