Clarification please on O2 sensors and STFT.

[JohnDee68]

Hi,
I am a little confused as there seems a few people on YT who contradict each other when it comes to explaining the O2 sensors, how they interpret the exhaust gas.

Am I correct in that an O2 sensor returns a voltage based on the Oxygen amount it detects in the exhaust. Where the return of a high voltage reading means more Oxygen detected, therefor the engine is running LEAN ?
Or is it to do with hydrocarbons, so if a high voltage is returned, it means there is un burnt oxygen so it is a RICH condition?

I had one say that the VX (GMH) O2 sensors work opposite to other sensors, presumably in the ECU? So I am confused and that doesn't help in tuning if I am back to front in my logic.

I am pretty sure that I understand STFT & LTFT where when it is a +ve, it is adding more fuel?

Any expert advice is always appreciated of course.

[Charlescrown]

The O2 sensor is a feedback device that responds to the oxygen level in the exhaust compared to whats outside. The voltage swings rapidly from 100 to 900 millivolts and the ECU responds immediately to maintain the air/fuel ratio to 14.7 when certain conditions are met. This sensor can only work under a very narrow band.
Many cars use a wideband which is different where it puts out a voltage is between 1 to around 4 volts (don't quote me) which can determine the air fuel ratio for a wide range of driving conditions such as idle, cruise and wide open throttle.

[vlad01]

Wide bands are basically a narrow band with an oxygen pump, where as the controller uses oxygen pump current to indirectly measure AFR. The pump supplements/subtracts the narrow band until it is at 14.7 on top of the gases outside of the normal narrow range. The controller can calculate the difference from the pump current to determine actual AFR and output as a 0-5V signal or digital in some versions.

That's my understanding anyway.

[JohnDee68]

Ok guys,
I can understand that and as to how they sensors work.
But if the O2 is detecting high levels of oxygen say near the 900mv, does that mean the engine is detecting a lean or rich fuel air burn mixture ?

[vlad01]

nb_sensor_output.jpg (17 KiB) Viewed 3060 times

900 is low oxygen, 100 is high levels.

[Charlescrown]

A high level of oxygen is lean. Look at vlads pic. When the ECU see's this it swings rich then lean then rich again.

[ironduke]

With narrow band I remember it as the greater the difference the greater the voltage..
Running real lean would leave lots of extra O2 in the exhaust, therefor have a low voltage since there isn't much of a difference between outside air and the exhaust.
Running rich burns up all the O2 so there's a large difference in O2 so a high voltage is present..
There is no happy point, it'll either be diving high or diving low..

[Charlescrown]

That sounds pretty good to me. I think your getting confused thinking it's using Oxygen to generate the output voltage. Yes it is called an O2 sensor but the lower the O2 the higher the output voltage. Yes it is confusing.

[JohnDee68]

Ok guys, thanks for the feedback. Yes I am confused, but i want to get this right as it is a vital foundation to knowing what is going on in the car.

Let me see if i am understanding this correctly now. Based on an ideal world and motor, as the perfect burnt exhaust gas hits the O2 sensor, it should read 500mv from the sensor. Therefore not needing to alter fuel amounts for the next combustion.
Then on the next ignition if there is too much fuel (rich), then the O2 sensor will see more fuel (less oxygen) and go to a higher than 500mv reading signaling as a richer than ideal condition.
Where then the adjustment of fuel will be reduced for the next cycle.
Then to complete another swing cycle, if the next burn is too lean (more oxygen), the O2 sensor will return a lower than 500mv reading.

Am I on the mark now guys?
Forgot, so then the STFT will be negatives when the car feels it is running too Rich as it requests to reduce fuel ?

[delcowizzid]

The ecu swings the fueling rich and lean between roughly 0.400 and 0.600 ish volts depending on what's set in the tune if it swings too far either side it will correct it with the short term fuel trims. The trims will never be perfect and will vary day to day a little bit.