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OT - O2 Sensor Function

N

npowell

Member
I understand O2 sensors allow the ECU to know if the engine is running lean by detecting oxygen in the exhaust. Indeed, if the engine is running lean, the ECU can determine how lean based on the magnitude of the O2 sensor signal.

If the air fuel ratio is perfect (stoichiometric) or rich, there theoretically would not be any oxygen in the exhaust for the O2 sensor to detect. So, how the heck does the ECU/O2 sensor distinguish between a little rich and a lot rich, given that both conditions should have little or no O2 in the exhaust?

Sorry for the silly question. It just occurred to me that I don't understand this, even though I've thought I understood O2 sensing for years.

Thanks,

Neil
 
O2 sensors are non-linear devices. In other words they're basically binary. They detect if the mixture is above or below stoichiometry, but don't really detect small changes when the mixture is rich or lean. It's the job of the ECU program to evaluate all the available information, including the O2 sensor output (called lambda) to determine how much fuel to inject into the mixture.
 
There are two types of O2 sensors, narrow band are basically switches, wide band will give you quite a wide air to fuel ratio.

Good comment on lack of straight O2, they look at the O2 that is embedded in CO2 and NO2 and other O2's.
 
Like it's said, the ECU determines the fuel mixture, the 02 sensor senses a slightly rich (not enough 02) or slightly lean, with the threshold around .6 volts, the sensor will cycle the fuel "trim" up to a dozen times a second. As sensors age they become "lazy" and many will only cycle a couple times a second. Coatings build up with mileage and that's the reason they should be replaced around that 75,000 mile point. A sensor can foul itself quite quickly with an upstream exhaust leak, for example, or restricted cat. The basic reason newer vehicles have multiple O2 sensors, 1 right before and 1 after the cat. The ecu will tell you with a code if the cat is not functioning. If you measure the cycling time of the sensor with a scan tool, it tells you what it's doing. Larry
 
The traditional O2 sensor is 0-1 volt in range. If you look at the live data on a typical GM setup you will see the O2 signal is constantly moving up and down that scale very rapidly, kind of like chasing its tail. The O2 sensor doesn't sit at one reading, unless something is wrong. The computer is adjusting fuel trim based on that and other sensor readings. So if it goes rich the computer pulls fuel out till it goes lean and then back again. This all happens very quickly.
 
I've found it is there to turn on the check engine light. LOL. My wives van has the check engine light coming on when it wants and off again. She had the code read and it is the O2 sensor.
 
David .... you mention about the lack of "straight" (I assume you mean pure or molecular) oxygen gas (or O2). You mention detecting oxygen in CO2 and NO2 ..... are you saying those oxygen "atoms" (not molecules) are involved with all of this in measuring the oxygen levels in engine exhaust? I know little of how this all works for exhaust analysis but the oxygen in NO2 and CO2 is not molecular oxygen anymore I don't think (just like the hydrogen in water (H2O) is not molecular hydrogen. I would be very surprised if those atoms were involved with the O2 measurement in exhaust gases. But correct me if I'm wrong or misreading what you said.
 
I don't fully understand it either.

The reading is dependent on everything being correct, any miss fire, vacuum leak, air leak anywhere, upstream exhaust leak will put it out of range and set the check engine light.

Here's a layman's explanation that seems to make sense.
Oxygen Sensor
 
There is plenty of info if you google and wiki
It is basically a voltage generator operating at temp in a hostile environment
and rich or lean apparently some oxygen is present, enough to send the accurate representative response.

I see references to the check engine light.

I believe the more accurate term I have heard used is "money light"
 
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