Fine folks, I have had a couple of requests to sort of summarize narrow band vs. wide band oxygen sensors. I will dutifully do my best but please realize I'm a weekend warrior and not an expert at such things. I will link this thread to the threads on Wide Band Sensors (Part I) and (Part II) in the hope it helps mudders who venture to those threads first. Ohh much of this is Toyota Motor Sales' proprietary information - just wanted to admit the piracy at the start! 
First some fundamentals:
- The ECM uses oxygen sensors to ensure that the air/fuel ratio is correct for the catalytic converter/s, so that vehicle efficiency is maximized, and so that vehicle emissions are minimized. Based on the oxygen sensor signals, the ECM will adjust the amount of fuel injected into the intake system's air stream.
- The two most common styles of sensor are ... the "Narrow Range" or "Narrow Band" oxygen sensor, which is the oldest style, and which is simply called the "Oxygen Sensor" in the sense that this implies Narrow Range or Narrow Band ... the other style oxygen sensor is the "Wide Range" or "Wide Band" oxygen sensor, which is the newest style, and which is commonly called "Wide Range", "Wide Band" or even the "Air/Fuel RATIO" oxygen sensor.
- On OBDII (On Board Diagnostics II) vehicles such as the 80's after 95, two oxygen sensors (remember the implied thing of "Narrow Band" here) are required one of which is placed before the catalytic converter/s and one of which is placed after the catalytic converter/s. The oxygen sensor which is placed before the catalytic converter/s is used by the ECM to adjust the air/fuel ratio. This sensor in OBDII terms is referred to as "Sensor 1". On the other hand the oxygen sensor which is placed after the catalytic converter/s is used by the ECM primarily to determine catalytic converter efficiency. This sensor in OBDII terms is referred to as "Sensor 2."
- Old style oxygen sensors (implied "Narrow Band") are made of zirconia, with platinum electrodes and a heater element. The oxygen sensor generates a voltage signal based on the amount of oxygen in the exhaust compared to the amount of oxygen in the atmosphere. The zirconia element has one side exposed to the exhaust stream and the other side is exposed to the atmosphere. Each side of the zirconia element also has a platinum electrode attached. The platinum electrodes conduct the voltage generated. The way this works is when there is less oxygen in the exhaust, there is a large difference in oxygen content when compared to the amount of oxygen in the atmosphere. This in turn produces a higher voltage signal. On the other hand, when there is more oxygen in the exhaust, there is a small difference in oxygen content when compared to the amount of oxygen in the atmosphere. (Please note that this does not mean that there is more or less oxygen in the exhaust than in the atmosphere, it means it is comparing the delta of those two things, large delta = higher voltage signal ... low delta = lower voltage signal ... it is a comparative measurement).
Now, let's look at some actual signals as shown voltages...
With low exhaust oxygen content, we will have high oxygen sensor output which will show as something above .45volts which indicates a rich running condition.
With high exhaust oxygen content, we will have low oxygen sensor output which will show as something below .45 volts which indicates a lead running condition.
From the oxygen content, and the resulting signals, the ECM can determine if the air/fuel ratio is rich or lean and it adjusts the fuel mixture accordingly...
A rich mixture has less oxygen so the voltage signal is high in the range of .6 up to 1.0 volts.
A lean mixture has more oxygen so the voltage signal is low in the range of .4 down to .1 volts.
At the stoichiometric air/fuel ratio of 14.7:1, which is considered ideal for efficiency and emissions, oxygen sensor voltage output is approximately .45 volts which is right in the middle of lean or rich.
Because of the obviously limited range of the old style oxygen sensor ( .1 volt up to .9 volts or so ) the old style oxygen sensor is indeed unable to detect exactly how "lean" or how "rich" the air/fuel mixture is. So essentially the old style oxygen sensor acts as a relatively rapid switch and simply switches from lean to stoich to rich and back and forth and back and forth. This explains the oscillating effect of regular air/fuel gauges which are basically just showing the stoich switch effect.
Also because of the obviously limited range of the old style oxygen sensor, small changes in the air/fuel ratio from the set stoichiometric air/fuel ratio of 14.7:1 will really radically change the voltage signal of the oxygen sensor. Again, the oxygen sensor cannot detect the small subtle changes in the exhaust stream oxygen content produced by changes in the air/fuel mixture. Therefore, in the presence of these really radical changes in voltage signals, the ECM will continuously add and subtract fuel producing a lean/rich cycle that oscillates back and forth over and over.
- Now lets switch to the "Air/Fuel RATIO Sensor" (aka A/F Sensor) (implied "Wide Band"). The A/F Sensor voltage signal is relatively proportional to the exhaust oxygen content. In other words, it is not a comparative measurement between the basic deltas of amount of oxygen in the exhaust vs. amount of oxygen in the atmosphere as in the Narrow Band. Instead it is a signal that shows the proportions of oxygen in the exhaust. The A/F Sensor changes its current in relation to the amount of oxygen in the exhaust. A circuit then completes the translation, detects the direction and strength of the current flow, and puts out a voltage signal relatively proportional to the exhaust oxygen content.
Now let's look at some actual signals as shown voltages...
The A/F Sensor is designed so that at the stoichiometric set point of 14.7:1 there is absolutely no current flow and the resulting voltage signal put out by the circuit is 3.3 volts.
A rich mixture whcih leaves very little oxygen in the exhaust stream, produces a negative current flow and the circuit will produce a voltage signal below 3.3 volts.
A lean mixture which has more oxygen in the exhaust stream, produces a positive current flow and the circuit will produce a voltage signal above 3.3 volts.
It is important to really realize that with the A/F Sensor (Wide Band) the voltage signal IS proportional to the change in the air/fuel mixture. Think of the A/F Sensor as a generator capable of changing polarity. When the fuel mixture is rich (low oxygen content), the A/F Sensor generates current in the negative direction. As the fuel mixture gets lean (high oxygen content) the A/F Sensor generates current in the positive direction. And lastly when the exhaust is at the stoichiometric set point, there is no current generated.
There is a cool chart that I cannot connect to this thread but essentially we have a two axis chart that compares the A/F Sensor voltage signal to the A/F Ratio. Most A/F Sensors have an output of 1 to 4 volts. By eyeball, a 2.4 volt reading on the A/F Sensor equals a A/F Ratio of 12.0 which is really rich. Think of the amount of air to the amount of fuel with 12 parts air to one part fuel being rich. On the other hand a 4.0 volt reading on the A/F Sensor equals a A/F Ratio of almost 20 which is really really really lean. Again, think of 20 parts air to one part fuel being lean.
On our rigs, and with the A/F Sensor and A/F Gauge that I have here, we will typically see ratios somewhere between 10 (Really Rich) to 16 (Really Lean). In general when the rigs are running in "Closed Loop" the ratios are right around 14.7. Sometimes at idle and with hot weather, we will see Idle Ratios of 15.3 or 15.5 but I think that is still "Closed Loop Operation" (still some question about that though). Additionally, in general (at least with my rig) when the rigs are running in "Open Loop" the ratios adjust right away to approximately 12.6 (Rich) and then get richer and richer the longer you stay stuck to the pedal. I have seen ratios of 10.2 after really pushing the performance for a stretch.
So, as I have concluded in the other threads about Wide Band Oxygen Sensor Readings, it seems that on our rigs, which were all equipped with the old style oxygen sensors, the 14.7:1 "ideal" ratio for efficiency and emissions seems to stay impressively steady and indicates that the rig is in Closed Loop most of the time. However when the rig is in Open Loop, toyota clearly plays it pretty "safe" by having a really rich mixture. This leads to lots of other discussions but for the moment, and again at the request of several fine folks here, I'm trying to simply summarize the operation of our oxygen sensors (Narrow Band) as well as the operation of my A/F Sensor (Wide Band). I hope this helps.
First some fundamentals:
- The ECM uses oxygen sensors to ensure that the air/fuel ratio is correct for the catalytic converter/s, so that vehicle efficiency is maximized, and so that vehicle emissions are minimized. Based on the oxygen sensor signals, the ECM will adjust the amount of fuel injected into the intake system's air stream.
- The two most common styles of sensor are ... the "Narrow Range" or "Narrow Band" oxygen sensor, which is the oldest style, and which is simply called the "Oxygen Sensor" in the sense that this implies Narrow Range or Narrow Band ... the other style oxygen sensor is the "Wide Range" or "Wide Band" oxygen sensor, which is the newest style, and which is commonly called "Wide Range", "Wide Band" or even the "Air/Fuel RATIO" oxygen sensor.
- On OBDII (On Board Diagnostics II) vehicles such as the 80's after 95, two oxygen sensors (remember the implied thing of "Narrow Band" here) are required one of which is placed before the catalytic converter/s and one of which is placed after the catalytic converter/s. The oxygen sensor which is placed before the catalytic converter/s is used by the ECM to adjust the air/fuel ratio. This sensor in OBDII terms is referred to as "Sensor 1". On the other hand the oxygen sensor which is placed after the catalytic converter/s is used by the ECM primarily to determine catalytic converter efficiency. This sensor in OBDII terms is referred to as "Sensor 2."
- Old style oxygen sensors (implied "Narrow Band") are made of zirconia, with platinum electrodes and a heater element. The oxygen sensor generates a voltage signal based on the amount of oxygen in the exhaust compared to the amount of oxygen in the atmosphere. The zirconia element has one side exposed to the exhaust stream and the other side is exposed to the atmosphere. Each side of the zirconia element also has a platinum electrode attached. The platinum electrodes conduct the voltage generated. The way this works is when there is less oxygen in the exhaust, there is a large difference in oxygen content when compared to the amount of oxygen in the atmosphere. This in turn produces a higher voltage signal. On the other hand, when there is more oxygen in the exhaust, there is a small difference in oxygen content when compared to the amount of oxygen in the atmosphere. (Please note that this does not mean that there is more or less oxygen in the exhaust than in the atmosphere, it means it is comparing the delta of those two things, large delta = higher voltage signal ... low delta = lower voltage signal ... it is a comparative measurement).
Now, let's look at some actual signals as shown voltages...
With low exhaust oxygen content, we will have high oxygen sensor output which will show as something above .45volts which indicates a rich running condition.
With high exhaust oxygen content, we will have low oxygen sensor output which will show as something below .45 volts which indicates a lead running condition.
From the oxygen content, and the resulting signals, the ECM can determine if the air/fuel ratio is rich or lean and it adjusts the fuel mixture accordingly...
A rich mixture has less oxygen so the voltage signal is high in the range of .6 up to 1.0 volts.
A lean mixture has more oxygen so the voltage signal is low in the range of .4 down to .1 volts.
At the stoichiometric air/fuel ratio of 14.7:1, which is considered ideal for efficiency and emissions, oxygen sensor voltage output is approximately .45 volts which is right in the middle of lean or rich.
Because of the obviously limited range of the old style oxygen sensor ( .1 volt up to .9 volts or so ) the old style oxygen sensor is indeed unable to detect exactly how "lean" or how "rich" the air/fuel mixture is. So essentially the old style oxygen sensor acts as a relatively rapid switch and simply switches from lean to stoich to rich and back and forth and back and forth. This explains the oscillating effect of regular air/fuel gauges which are basically just showing the stoich switch effect.
Also because of the obviously limited range of the old style oxygen sensor, small changes in the air/fuel ratio from the set stoichiometric air/fuel ratio of 14.7:1 will really radically change the voltage signal of the oxygen sensor. Again, the oxygen sensor cannot detect the small subtle changes in the exhaust stream oxygen content produced by changes in the air/fuel mixture. Therefore, in the presence of these really radical changes in voltage signals, the ECM will continuously add and subtract fuel producing a lean/rich cycle that oscillates back and forth over and over.
- Now lets switch to the "Air/Fuel RATIO Sensor" (aka A/F Sensor) (implied "Wide Band"). The A/F Sensor voltage signal is relatively proportional to the exhaust oxygen content. In other words, it is not a comparative measurement between the basic deltas of amount of oxygen in the exhaust vs. amount of oxygen in the atmosphere as in the Narrow Band. Instead it is a signal that shows the proportions of oxygen in the exhaust. The A/F Sensor changes its current in relation to the amount of oxygen in the exhaust. A circuit then completes the translation, detects the direction and strength of the current flow, and puts out a voltage signal relatively proportional to the exhaust oxygen content.
Now let's look at some actual signals as shown voltages...
The A/F Sensor is designed so that at the stoichiometric set point of 14.7:1 there is absolutely no current flow and the resulting voltage signal put out by the circuit is 3.3 volts.
A rich mixture whcih leaves very little oxygen in the exhaust stream, produces a negative current flow and the circuit will produce a voltage signal below 3.3 volts.
A lean mixture which has more oxygen in the exhaust stream, produces a positive current flow and the circuit will produce a voltage signal above 3.3 volts.
It is important to really realize that with the A/F Sensor (Wide Band) the voltage signal IS proportional to the change in the air/fuel mixture. Think of the A/F Sensor as a generator capable of changing polarity. When the fuel mixture is rich (low oxygen content), the A/F Sensor generates current in the negative direction. As the fuel mixture gets lean (high oxygen content) the A/F Sensor generates current in the positive direction. And lastly when the exhaust is at the stoichiometric set point, there is no current generated.
There is a cool chart that I cannot connect to this thread but essentially we have a two axis chart that compares the A/F Sensor voltage signal to the A/F Ratio. Most A/F Sensors have an output of 1 to 4 volts. By eyeball, a 2.4 volt reading on the A/F Sensor equals a A/F Ratio of 12.0 which is really rich. Think of the amount of air to the amount of fuel with 12 parts air to one part fuel being rich. On the other hand a 4.0 volt reading on the A/F Sensor equals a A/F Ratio of almost 20 which is really really really lean. Again, think of 20 parts air to one part fuel being lean.
On our rigs, and with the A/F Sensor and A/F Gauge that I have here, we will typically see ratios somewhere between 10 (Really Rich) to 16 (Really Lean). In general when the rigs are running in "Closed Loop" the ratios are right around 14.7. Sometimes at idle and with hot weather, we will see Idle Ratios of 15.3 or 15.5 but I think that is still "Closed Loop Operation" (still some question about that though). Additionally, in general (at least with my rig) when the rigs are running in "Open Loop" the ratios adjust right away to approximately 12.6 (Rich) and then get richer and richer the longer you stay stuck to the pedal. I have seen ratios of 10.2 after really pushing the performance for a stretch.
So, as I have concluded in the other threads about Wide Band Oxygen Sensor Readings, it seems that on our rigs, which were all equipped with the old style oxygen sensors, the 14.7:1 "ideal" ratio for efficiency and emissions seems to stay impressively steady and indicates that the rig is in Closed Loop most of the time. However when the rig is in Open Loop, toyota clearly plays it pretty "safe" by having a really rich mixture. This leads to lots of other discussions but for the moment, and again at the request of several fine folks here, I'm trying to simply summarize the operation of our oxygen sensors (Narrow Band) as well as the operation of my A/F Sensor (Wide Band). I hope this helps.
