O2 Sensor Basics

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[Chris]

The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust. The signal from the O2 sensor tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors affect the richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. Other sensors monitor these factors too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Problems with the O2 sensor can throw the whole system out of whack.

The computer uses the oxygen sensor's input to fine tune the fuel mixture for the best balance of power, economy and emissions. The engineering term for this type of operation is "closed loop" because the computer is using the O2 sensor's input to adjust the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which helps the catalytic converter operate at its best and keeps the average fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works.

If no signal is received from the O2 sensor, like when a cold engine is first started (more on that in a minute) or the 02 sensor fails, the computer orders a steady, rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to fine tune the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. As you might have guessed, that will set a code and turn on your check engine light.

How does it work? The O2 sensor produces a voltage once it gets hot. The sensor compares how much oxygen is in the exhaust to the oxygen in outside air. The greater the difference, the higher the voltage reading.

If you ever replace an O2 sensor (and if you're a DIY'er this is something you will do eventually), its important to remember that the O2 sensor needs to "breath" outside air to work. So don't put any grease on the sensor because it could block this air flow.

An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen left in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.

When the computer reads a rich signal from the O2 sensor it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs anywhere from 2 to 7 times a second at 2500 rpm on OBDII vehicles, depending on what type of fuel injection system they have.
The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal. Many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly.

[What About Bob?]

Excellent info..

I should add to this

The downstream O2 sensor after or actually on the catalytic converter is basically the same sensor as the upstream, however, it has no effect of fuel mixture. It is only there to monitor catalyst efficiency and nothing more. The live of the downstream is pretty uneventful. With a properly working catalyst, the sensor voltage wont fluctuate much more than maybe .05V.

What will set a code about a downstream is normally 'catalyst efficiency below threshold'. What happens is the failing converter can no longer burn off and/or convert exhaust gases (effectively filtering) and it then ends up with lean and rich mixtures leaving it, much like what exits your exhaust ports as the ECM switches from lean to rich. Of course the downstream voltage starts to fluctuate like the upstream, the ECM picks this up, and then sets a code.

[oldscsc]

Great info. Sounds like my MPG problem (I average about 13), although no codes are set.

Emissions-wise I'm not sure, although it does smell abnormally bad on a cold start if left idling..

[Prospeeder]

alot of OBD1 cars wont set a code for a bad o2 unless its completly just dead

[SofaKingWeToddDid]

im pretty sure my car only has one... i KNOW it doesnt have on on the exhaust pipe under the car, maybe somewhere under the hood... lol but who can see anything on an LQ1 in the car.

[DiscoStudd]

im pretty sure my car only has one... i KNOW it doesnt have on on the exhaust pipe under the car, maybe somewhere under the hood... lol but who can see anything on an LQ1 in the car.

They didn't start installing "downstream" O2 sensors until OBD 1.5 (my 95 Regal was OBD 1.5 and it had a downstream sensor.)  So in 94-95 they began appearing.  All OBD II vehicles have 2 sensors...

[Chris]

Yeah your z34 is OBD1 so you wouldn't have two. What prospeeder said was also correct, if you notice you're getting like 3MPG and your exhaust smells like gas and your cat converter is glowing red hot - you might need a new O2 sensor, but your car will never tell you

[SofaKingWeToddDid]

if ur cat is glowing red its probably plugged.

[Prospeeder]

Yea GMs OBD1 suck that usually dont throw codes for shit unless its WAY WAY WAY out of specs for like certain number of seconds at certain RPMs lol. So throwning codes for some things is like impossible, lol. Sofa your o2 is right off the rear exhaust manifold in the back, have fun with that, lol

[SofaKingWeToddDid]

why? im not touching it. lol it can stay back there.