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Can anyone provide a brief tutorial on O2 sensors?

I really don’t understand the fundamentals of how they work & why they are used.

Here’s what I think I know: O2 sensors assist the engine computer make minor adjustments to the amount of fuel to deliver to achieve the best mixture. The fuel injection system does most of the work, measuring the air intake with a Mass Airflow Sensor, and metering out the fuel to match, and the O2 sensors are used only to make small incremental changes to fuel delivery, based on the O2 reading. If the mixture is too lean, there’s not enough gas to combine with all the O2, and some O2 will come out the exhaust. No good. If the mixture is too rich, no O2 will come out at all, but some unburned gas will be in the exhaust. Again, no good. Ideally, there’d be just enough gas to burn all the O2, but no more. So it seems like the thing for the engine computer to do is monitor the O2 and if there is no O2 coming out, that means there is too much gas, so give it less gas, but only until the O2 first starts to register, which means there is no enough gas, then give it a little more gas, and continue this process indefinitely.

Is this more or less correct? How come cars in the 70’s didn’t have any O2 sensors? What are the physics of how these sensors work? Do they work when the engine is cold or only when it is hot? Why do some cars have only one O2 sensor, and some care have more than one?

All comments appreciated. Thanks.

The O2 sensor produces it’s own voltage between 0-1 volt DC. When the fuel mixture is lean the voltage is low from the sensor. When the fuel mixture is rich the voltage is high from the O2 sensor. This happens very rapidly and is what is called cross-counts.

Vehicles in the seventies didn’t have O2 sensors because the emission requirements weren’t as strict as now. Vehicles of the seventies didn’t have computers and were carburated.

When O2 sensors were first used in vehicles they relied on the exhaust gasses to get them up to temperture before they started working. Todays vehicles use O2 sensors with heaters inside them so they come up to temperature almost immediately.

OBDI vehicles use single O2 sensors for each bank of cylinders because OBDI didn’t monitor the catalyst effeciency. When OBDII came out, it was required that the catalyst effeciency be monitored. So you’ll have an O2 sensor for each bank of cylinders and an O2 sensor for each catalytic converter to monitor it’s effeciency.


Depends on being breif. I just found the best technical explanation I have ever seen online. It was a toyota breif for the technicians It was clear and described in detail issues that you might see on an o2 sensor monitor. The o2 measure is a check first on how much fuel and air are mixed. Then it gets checked after the catalyst to find out two issues. Is the first o2 sensor correct and is the catalyst working. Normally the second sensor sees more o2 than the first. This happens by catalyst reforming NO and CO into normal gases that are less polluting like NO2 and CO2 plus some other stuff that is not brief.

The O2 sensors are galvanic batteries that produce volatge beacause of a differnence in oxygen levels on either side of their surfaces. One surface is in the exhaust stream and the other is exposed to atmosphere. The forward sensors are used strictly for fuel trim and the rearwars sennsod are strictly for catalyst monitoring. The rearward sensors should see a significantly “damped” signal but still track the forward sensors. There should be LESS O2 after the catalyst as the catalytic reaction consumes free oxygen rather than producing it. If the rearward sensors precisely track the forward ones, then the catalyst can be assumed to be missing or ineffective.

More than you will ever want to know

Yeah, I’ll just add that it is one of several sensors that input information to the engine management computer such as the engine temperature sensor and air flow sensor. With that information the computer decides how long the injectors will be open, allowing fuel to flow, called the pulse width. The injectors are under contnuos fuel pressure and the computer just opens and closes them like a water faucet. Like anything, when the limits for adjustment are reached and the computer can no longer correct the condition, ye ole engine light goes on.

Perhaps the attached documents will help you understand how they fit into the fuel control system.

The only thing I will add is that O2 sensors were used in the 70s on European cars. VW and SAAB started using O2 sensors on their 1977 models with the introduction of CIS fuel injection. Prior to that VW used carburetion on their Rabbits, Dashers, etc and SAAB used AFC controlled fuel injection much like the early VW Types 1, 2, 3, and 4.

It just took a few years for everybody else to catch up.

@Tester … lol … I just noticed that photo of the yellow sign you post in your photo. That’s hilarious! “Sharp Edges! Don’t touch the edges of this sign!!”??? Really? That’s a big concern? Someone touching the edges? What the heck are they talking about? Why would anybody touch the edges in the first place? And why are they sharp? Don’t they have smoothing tools in that place they make the sign? It’s really quite funny to see such a sign. Where abouts did you see it? It must be some gov’t or military installation I expect.

Thanks everybody for the great tutorial comments and posted links. I feel like I understand these O2-sensing fellows a little better now. I’ll read up more on the links later. Thanks again.

To add to this; the O2 sensors are used at steady engine speeds, like when you’re cruising along. When you step on the gas, the throttle position sensor calls for more fuel and other sensors may help the computer judge exactly how much is required. When you get off the gas, the fuel is mostly shut off. At steady idle the O2 sensors may have something to say about the exact fuel to air mixture. It is some fun to study some of this stuff.

I believe the sensors detect oxygen ions which pass through the sensor. When enough atomic or subatomic particles have struck the sensor it is said that the sensor is contaminated and it doesn’t work so well any more and must be replaced. Cleaning it will not do any good.

On my truck, both forward and aft sensors are identical but are used for different purposes. The wiring and connector may be different but the sensors are the same. Have fun with all this.