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02 sensors. How they work

O2 sensors are by far the most misunderstood component of modern engine management systems.  In many cases owners are in a hurry to disable this mysterious device when in fact they may be hurting their performance. The original role of the 02 sensor was, and still is, to help in controlling air fuel ratios at or near 14.7:1. 02 sensors actually first appeared in high production during the late 1970s and early 1980s in automobiles. Engineers trying to control carbureted engines used 02 sensors along with primitive ECUs and electronic carburetors. What the 02 sensor did was to give engineers a way to look at what was in the exhaust stream or content of oxygen in the exhaust needed to oxidize that exhaust stream.

With this information they were able to make adjustments "on the fly" at the carburetor and begin controlling what was going on in the engine, thus controlling what was coming out the tail pipe. This simple back and forth adjusting is known as a Closed Loop Feedback System. The 02 sensor sends information about the condition of the exhaust to the ECU, the ECU sends a command to the fuel system and makes an adjustment to correct that condition.  It's the command and condition that needs to be understood to grasp the concept and have an understanding of this device.

Rule number one is to know that the O2 sensor does nothing more than to report a condition. Think of it as a messenger that reliably and honestly reports the condition of the exhaust stream to the ECU so that it can issue a command to the fuel system. Very often, unfortunately, this messenger gets blamed for what is going on when in fact it is just doing it's job of reporting the condition. Many uninformed technicians replace 02 sensors for reporting the truth about the exhaust condition when they should be listening to what it is reporting and correct the problem that is causing the poor condition.

If, for example, I had a leaking fuel injector, the engine would be running rich. The 02 sensor would constantly report this rich condition. In most cases the technician would want to replace the sensor for having a continuous high voltage output when, in fact, that tech should be looking at why it is continuously reading rich. Many 02 sensors that are replaced are still good.

Here is how the sensor actually works:

The 02 sensor contains a compound known as Zirconia. This compound has what is known as high ionic conductivity. What this means is that, in simpler terms, this compound reacts to oxygen and can work for us to report the condition of the exhaust stream. It does this by producing a small varying (analog) voltage. In narrow band sensors this varying voltage is from 0-1volts and in wide band sensors 0-5 volts. With a lot of oxygen present, such as in lean conditions, the output of the sensor is low, around 0 volts. When the condition is rich the output of the sensor is higher up to 1 volt on narrow band and up to 5 volts on wide band. This voltage is constantly varying due to the engine controller attempting to get the air fuel ratio just right. This extrapolation of rich to lean and lean to rich is what generates the varying voltage. Think of the rich/lean/rich/lean as an average over time and you can see that ultimately the air fuel ratio becomes ideal (14.7:1).

Narrow band sensor output

The 02 sensor has part of it's zirconia exposed to the exhaust stream and part of it exposed to the atmosphere. Because the atmosphere (ambient air) has a fixed percentage of oxygen present in it, the voltage on the atmospheric side of the sensor never changes. But because the exhaust stream has a varying percentage of 02 this voltage is constantly changing. It's the difference between these voltages that we see at it's output. For example: if the exhaust stream is very lean (lots of oxygen) and we know the ambient air oxygen is always the same (lots of oxygen) then the difference in what is generated is very small (low voltage). If the exhaust stream is very rich (low or no oxygen) then the difference between the ambient and exhaust stream is high and the output is high (high voltage) 

What is also important to know is that the 02 sensor must be at least 600 degrees F before it's output is reliable. Below this temperature the voltage is always low or zero regardless of 02 content. It's imperative to get the 02 sensor hot before the ECU can start using it's information to make adjustments to the A/F ratio. In the early days of use many of the sensors were located very far downstream of the combustion chamber and they would actually cool off during operation. What this would do would put the engine management system in to an "open loop" status where it would stop adjusting the A/F ratio. This was very undesirable and something needed to be done to rectify this problem and keep the system in "closed loop" status.

Eventually heaters were added to the sensor and this solved the problem. These heaters work off the electrical system of the vehicle using 12volts to warm the sensor quickly. This also solved the problem of getting the vehicle in to "closed loop" during the warm up period of the engine which is known to produce the most pollutants. These miniature heaters can get the sensor to 600 degrees very quickly. The heating element does fail at times and this is probably the number one failure point of the sensor.

Another contributor to 02 sensor failure is contamination. Contamination comes from any thing introduced in to the exhaust stream that is not normally present. For example when an engine develops an internal coolant leak such as a bad head gasket the coolant leaks in to the combustion chamber and ultimately in to the exhaust. The glycol contaminates the sensor by coating it and not allowing exposure to the 02 any longer. This causes the 02 sensor to go dead by not producing any varying voltage. Leaded gasoline can also contaminate 02 sensors as well as using the wrong engine sealers such as some of the aerobic sealers sold today.

Below is a common wiring schematic from a four wire type sensor used in many applications. The "signal" is used by the ECU to make corrections to the A/F ratio. The oscillator signal is used to determine when the sensor is ready for use, or when it is warm enough to start working.  The ground and +12 is the heater wiring from the vehicle's electrical system.

When an 02 sensor is disconnected from the system on a modern closed loop feedback system it puts the system in to a default mode. What this does is cause the ECU to substitute values where it normally used actual values from the 02 sensor. This can mean a loss in performance as well as fuel mileage and emissions. Many times 02 sensors are used on modern motorcycles to achieve top performance and not necessarily lowered emissions. It is dependant on what the engineering of the system is and removing the sensor defeats the objective of the system. :

When catalytic converters are used, 02 sensors play an extremely important role in their performance. The 02 sensor is pivotal in achieving the ideal A/F ratio of 14.7:1 where catalytic converters are most efficient. Without the 02 sensor in place the converter can easily be overheated or contaminated. Although catalytic converters are as much or more misunderstood than 02 sensors, they are extremely effective in reducing CO, HC and NoX. Without them we would probably be walking around with respirators. That would not be fun.

Most modern systems incorporate a self diagnosing ability that can be the largest cause of misdiagnosed components. I'll use the 02 sensor as my example of this. As I stated earlier the 02 sensor reports a condition. If the condition is very rich or very lean and continues in that trend the software will typically turn on a fault light. In your car this is usually the "service engine soon" lamp and on your motorcycle it is typically the "FI" light. With this self diagnosis ability there is usually a code associated with the problem detected. With a continuous rich or lean condition we might see a code such as:  "02 sensor rich" or "02 sensor lean". This is where it all goes down hill quickly. When an unknowledgeable technician sees one of these type codes, they will typically replace the 02 sensor when, in fact, they should be investigating why the sensor is reading continuously high or low.

Lets say the engine develops a large vacuum leak. What will this cause? It will cause the 02 sensor to continuously read LEAN as it should because the exhaust stream is lean due to the added air that is taken in by the engine. Is this the 02 sensor at fault? Of course not. When we see trouble codes indicating any type of continuous fixed output we should always investigate what can cause that and not be in a hurry to replace the 02 sensor.