Here's a question for you techie types. The O2 sensor; exactly how does it work?
I though it reacted with the oxygen in the exhaust gases, but someone else said that it's really only a temperature gauge, and that it determines O2 content by exhaust gas temperature.

So which is it? How does it work? Inquiring minds want to know :-)

Thanks!

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87 GT 3.4 V6 Fiero Automatic

Actually, it does'nt send info to the computer until it gets to a certain temp. If you google, type in (fiero oxygen sensor) and then you'll know. JD

Got no flippin' idea. However, I might point out that if the O2 sensor simply reacts to the exhaust temperature ( I.E. indirect or inferrential measurement) it would be easily defeated by restricted exhaust systems, the temperature of the ambient air, criminally lean mixtures, etc... Not something that the bean-counters at GM would be inclined to trust.

I deduce that the O2 sensor responds directly to the O2 in the exhaust.

[This message has been edited by Jim Gregory (edited 02-16-2003).]

It determines the O2 content through a chemical reaction - this creates a voltage which relates to the O2 content. The ECM reads this voltage to determine if the mixture is rich or lean, and adjusts the pulse width to the injectors to try and keep the mixture at the ideal 14.7:1 ratio.

The center of the O2 sensor has a hollow ceramic body (zirconium dioxide). The inner and outer surfaces of the ceramic body are coated with seperate, thin gas permeable films of platinum. The platinum coatings serve as electrodes. The outer electrode surface is covered with a thin, porous, ceramic layer to protect against contamination from combustion residue. The outer electrode surface contacts the shell which screws into the exhaust pipe. The inner electrode connects to a wire which goes to the ECM. When the zirconium dioxide reaches 570 deg F it becomes oxygen-ion conductive. If the oxygen in the exhaust stream is less than that in the ambient air, a voltage is generated. The greater the difference the greater the voltage. Voltage generated by the O2 sensor will range from .1 to .9 volts.

It is measuring oxygen content by a chemical reaction that generates voltage. Temperature comes into play only because it has to reach a certain temperature before the chemical reaction will start. One wire oxygen sensors like the one on Fieros, and most cars from that era, rely on the exhaust gas to generate that heat. This can take a few minutes and on some cars it will drop back below the minimum temperature during an extended idle. Three or Four wire sensors used on a lot of newer cars have a heating element built in to bring them up to temperature quickly and keep them there.

Virtually all O2 sensors are of a design that's only calibrated at the desired 14.7:1 air fuel ratio equals .45 volts. Less voltage means lean and more voltage means rich, but it's not accurate as to exactly how lean or rich. The ECM continuously adjust the air fuel ratio, overshooting between rich and lean. If you watch the O2 sensor voltage on a Fiero with an ECM scanner it seems to jump all over the place, from about .2 to about .8 volts. This is because the Fiero ECM has such a slow diagnostic connection for outputing data you are only seeing random points in the cycle. If you watch the output on a newer car with a faster diagnostic connection the O2 sensor voltage will be a sine wave continuously and smoothly going above and below .45 volts.

Even though it's from Toyota, here is an excellent reference ... more than you ever wanted to know about oxygen sensors in fewer than six pages:
http://www.autoshop101.com/forms/h37.pdf

quote Originally posted by DRH:If you watch the O2 sensor voltage on a Fiero with an ECM scanner it seems to jump all over the place, from about .2 to about .8 volts. This is because the Fiero ECM has such a slow diagnostic connection for outputing data you are only seeing random points in the cycle.

It's not just a slow ECM (I've been seriously trying to figure out how to overclock one! ).

I've got an A/F meter hooked up on my dash (and lets face it - all they are is a volt meter that reads from .1-.9 volts attached directly to the O2 output wire). It'll sit there and float/swing up/down all day long at idle, until you stand on it and the injectors go nuts.

I can do a short mpeg video if anyone wants to see.

The ECM is more than fast enough to do it's job. Attempting to over clock one is likely to cause problems. (The master clock is used for things besides CPU timing.)

An important note here....

The ECM does read all the sensors in real time however it does not report to ECM Scanners in real time. The data stream to the scanner is only updated once or twice a second.

The C3 ECM isn't built to provide a real time data stream, infact many early ones have minor issues when trying to run with a scanner because of the extra processor load. In many cases you'll notice the idle changes durring scanning. The later P4 ECM's are a little better about scanning but still not real time. I don't think any OBD1 GM ECM provides real time data. (We often say it's real time but there is lag and fast events may not get reported accuratly or at all.)

This important fact is often overlooked by people new to ECM scanners. While an ECM scanner is a valuable tool, you can't always count on just it for reading things the ECM and sensors are doing.

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RIP Crew of Columbia. 01-Feb-03