# Air:Fuel ratios on narrow band oxygen sensors?

What A/F causes an oxygen sensor, producing a normal S wave, to go to “lean” voltage (0.9) output? What A/F will cause an output of 0.1 v. (rich)?

Zero volts output is the value caused by an A/F of 14.7/1, and is widely known.

That wooshing noise was your question going 10,000 feet over my head! I did find an NGK discussion,doubt it helps you
http://home.att.net/~ngksparkplugs/Oxygen_Sensor_Tech_Review.ppt

Don’t you have it backwards? If the 02 sensor voltage is continually high, the air fuel ratio may be to rich or the sensor may be contaminated. When the sensor voltage is continually low, the air/fuel ratio may be lean,the sensor may be defective,or the wire between the sensor and computer may have high resistance.

You don’t want your 02 sensor voltage to be static they should cycle between 0V and 1V in approximately 100ms. If the 02 sensor sits at or close to 0 unplug the sensor. If the voltage signal increases while the sensor is unplugged the sensor is probably shorted to ground.

Some Daimler/Chrysler vehicles use a 5 volt reference signal instead of the typical 0 to 1. It still is a one volt swing but from 5 to 6 inatead of 0 to 1.

Rather than saying you have it backwards (I checked several sources) perhaps the lean low and high rich are switched around for narrow band?

Right, during an oxygen sensor cycle, the voltage goes to 0.9 volts (rich), then to low 0.1 volts (lean). These are the normal values the oxygen sensor produces twice each second during normal operation. But, what are the air/fuel ratios that produce those voltages (O.1 v. and 0.9 v.) on the cycle? The ideal air/fuel ratio, of 14.7 to 1, produces an oxygen sensor 0.450 volts 3 times during each cycle. Clearer?

I think since the voltage is going to be contiuosly changing looking at both long and short term trim codes is of more diagnostic value. The sort term trim is adjusted in response to the amount of time the 02 spends above or below 450mv.

A value of 128 counts indicates no action required to maintain 14.7,below 128 counts indicates rich and above 128 counts indicates lean.(long term trim)

Generally below 10:1 indicates Very Rich at all speeds (This is where you get into high map sensor voltage) 10:1 12:1 low speed only is rich (again high map voltages) above 16:1 very lean at all speeds

I’ve attached a typical O2 sensor voltage plot.

I got some great responses, even though I botched the question.
Loafer, I looked at that graph, and it conveys different ideas than the usual sine wave graph which depicts a flow from one value to the next. I don’t have an O’scope (oscilloscope) to look at an oxygen sensor output. I think that if I did, I would see a square wave output, rather than a sine wave.
From looking at the graph, it answers my poorly phrased question, fairly well. (I’ll have to think of better questions). The answer I see is: the air/fuel ratio of 14.7 to 1 will allow the oxygen sensor to generate an output voltage from 0.2v to 0.8v. Air/fuel ratios 0.1 to 0.2 richer lambda (Or, lower lambda) are all that the narrow band oxygen sensors can handle. Richer, or leaner, than that, just yield a voltage signal of “too rich”, or “too lean” to the engine computer (PCM).
If you want to measure the Air/Fuel Ratio (AFR) directly from the exhaust, before the catalytic converter, or at the tail pipe (car without a cat.) here is an aftermarket instrument manual (LC-1 brand) for your perusal. http://www.innovatemotorsports.com/support/manual/LC-1_Manual.pdf

What causes the air fuel ratio to change so regularly at an (ideal) rate of three crosses per second (three cross points)?

Hellokit
I went to school for several years to learn all of the technical aspects of emissions and controls and I do understand how the PCM works and I’m trying to figure out why you care about having a (perfect at sea level) 14.7:1 A/F mixture or why you are wondering how the A/F ratio affects the O2 cross counts unless this is just a trivia question. If everything is producing and burning as it should, you’re going to be close enough to 14.7:1 where you won’t notice any difference on your gas milege…just wondering.

This is called OBD2 Adaptive Fuel Control Strategy. The PCM by design cycles the mixture from lean to rich this cycling is monitored by the pre cat 02 sensor and this signal is also compared to the post cat signal to get a idea of cat efficiency. The main idea of the cycling is to best maintain a ideal ratio.

Pre cat signal is greater in amplitude and frequency than a post cat signal in amplitude and frequency. This is good, it shows the cat. is storing 02 correctly. When the signals match (or come close) a DTC is set.

I would more correctly say the cycling is not only monitored by the pre cat sensor but caused by the pre cat sensor, hence the name “closed loop”.

Anyone care to add or correct?

Excellent responses, again. I was hoping that this forum could handle the more difficult questions; not, just the easy ones. I’m gratified.
To have an understanding of something, one needs to know the HOW, and the WHY…at least, this one does. One, further needs this knowledge to be able to discern the cause from the effect. You’ll see, many times on this board, in questions and answers, that the two are sometimes confused.
I researched, through numerous Web sites, for some of the HOWS of the oxygen sensor’s operation. There’s not much meat there. There is more information on WHAT the oxygen sensor does.
One example of cause and effect being confused, by not knowing the HOW: some think that the oxygen sensor decides to have cross-counts (so many per second, etc.); rather than the cross-counts being caused by the fuel control system which varies the AFR (Air Fuel Ratio) on the edge of the lamba value of 1.0 (AFR 14.7/1). Thus, good oxygen sensors are committed to an ignoble end, and the larger problem isn’t resolved, thus.
If the oxygen sensor output voltage stays near 0.9v, is that a fault with the oxygen sensor, or of the fuel control system?
Likewise, if the oxygen sensor output voltage stays near 0.1v, is that a fault of the oxygen sensor, or what?
Like a coat of many colors, it takes more than one piece to make a whole.

Rich mixture means low oxygen and higher voltage from sensor. Contamination from oil ,coolant, RTV. When the 02 shows lean the PCM goes rich and vice versa. When a lean exhaust is not caused by an air/fuel problem, the PCM does not know what the true cause is and will enrich the mixture.

Incomplete combustion caused by ignition problems make for more oxygen in the eahaust,also true for lean mixtures or anything else that causes incomplete combustion.

Signal sitting a close to zero can be caused by a short to ground in harness. Signal close to 1 can be cause by a short to the power wire for the heater power feed, also in the harness.

One has to know HOW something is supposed to perform before one can determine if that something is mis-performing, right?
I’m not referring to a faulty oxygen sensor. I’m just asking about the normal manner a regular (“shortband”) oxygen functions.
If the air fuel ratio stays at exactly 14.7 to 1 (stoichiometric value for gasoline) would the oxygen sensor continue to output 0.8 volts? Or?

Ask your question “If the air fuel ratio stays at exacty 14.7 to 1 COULD the oxygen sensor continue to output 0.8 volts”?

There is a continuous measurement /correction going on with closed loop operation. The whole concept of a static air/fuel ratio doesn’t fit with the closed loop measurement/correction technique. The fuel control system is always trying to get to 14.7 but it moves through it and then corrects then moves through it then corrects. You can look at the time the O2 sensor spends high or low as a diagnostic aid.

Others can join in on this.

That is all good information. I have the concepts a little clearer in my mind.
The fuel control (PCM) must be on a clock to vary all of the fuel injections taking place in a second to allow the exhaust to swing from rich to lean, or lean to rich, (en mass) once per second, and have the oxygen sensor measure that swing.
After understanding how a narrow-band oxygen sensor fuel control system operates, it should be a pizza pie to understand wide-band oxygen sensors. Yes?

Rather than narrow or wide band there are 02 sensors that are basically variable resistors, altering a base voltage.

Everyone should be able to test voltage generating 02 sensors off the car. Monitor the voltage created when the sensor is surrounded by the flame of a propane torch. The flame shields the sensor from oxygen causing it to produce a voltage. With flame 1 volt, remove flame goes to zero immeditately.

Trouble is some mechanics don’t want to test just replace.

The wide-band oxygen sensor can help an engine to run more fuel efficiently than narrow-band oxygen sensors can. Toyota uses wide-band oxygen senors on some vehicles. People like the better mpg with those. I wonder why there hasn’t been more application, by more car makers, of the wide-band sensors.
The oxides of nitrogen, it seems, is one concern with the higher cylinder exhaust temperatures with the wide-bands. How much concern, I don’t know. There is a catalytic converter type which reduces nitrogen oxide. This looks like a potential solution to the NOx. Whatcha think?

The narrow band 02 is the switcher. The wide band works without this switching. Variable resistance 02 sensors work at lower temps. I suspect variable resistance 02= wide band 02. Back to homework, complex rational expressions. They are making me and everyone else in this online class sick.

Am I to understand that this forum is only for “How do I fix the car/truck?” questions? Does it make forum readers “sick” to read and/or answer questions which may be somewhat technical in nature? In short, have my questions been offensive in some way?

Not at all. Did you think my getting sick comment was related to the 02 sensor questioning? The comment was related to the homework I was doing at the time I made my last installment to this thread. Its figuring out complex rational fractions that are making me sick. The 02 sensor stuff is a welcome break from that kind of nonsense.

Perhaps the fact that no one else is responding to this 02 thread has made you a little sensitive.