O2 for You

Oxygen sensors are by far one of the most important sensors used to control the engine. This sensor does just what the name implies, it senses oxygen but specifically it looks at the oxygen content in the exhaust stream. When the computer that controls the fuel injectors needs to see if it is doing a good job managing the air/fuel ratio it will look to the O2 sensor.

The amount of oxygen in the exhaust is directly proportional to how much fuel the injectors are adding to the incoming air. If oxygen content is high it indicates a lean mixture which means that there is not quite enough fuel. If oxygen content is low it indicates a rich mixture or a situation with too much fuel.

The O2 sensor is a voltage generating sensor. An electrode extends from the senor tip into the exhaust stream so that it can pick up oxygen molecules in the exhaust. The other end of this electrode extends out of the exhaust and is actually exposed to ambient air on the outside of the exhaust pipe. This electrode works kind of like a small battery comparing the exhaust to the outside air creates a potential difference electrically speaking. The greater the difference in the oxygen content between the inside and the outside of the pipe the higher the potential difference or the higher the voltage the sensor produces. This voltage is the signal that the computer reads.

O2 sensor cutaway.

In order for the sensor to operate it must be heated to 600° F. On older vehicles the engine and the exhaust had to first warm up in order for the O2 sensor to come online. Most exhaust manifolds will get up to 600° but it wouldn’t happen until the engine had been running for several minutes. During this warm up period the computer would disregard the O2 sensor and function strictly off of the other sensors. This is known as open-loop operation. Once the O2 sensor warms up the computer can begin to look to it for feedback. This is called closed-loop operation. Open-loop is imprecise and the engine loses efficiency when running in this mode. Because of this newer O2 sensors are fitted with a electric heater circuit so that the sensor will heat up faster and get into closed-loop operation sooner.

The voltage signal usually fluctuates between .1 and .9 volts. This seems like a tiny signal but it is enough for the computer that is reading the sensor. The midpoint of the sensor range is .45 volts. Anything above .45 is rich anything below .45 is lean. Usually under normal operation the voltage signal will fluctuate smoothly up to .9 and then back down to .1 volts. The reason this happens is because the computer is purposely switching the mixture between rich and lean. This happens so that there will be some oxygen that makes it through the combustion process. The extra oxygen that makes it through is needed by the catalytic converter in order to allow proper oxidation of some of the harmful pollutants that must be handled by the catalyst.

When the O2 sensor shows a signal that isn’t good the computer will be smart enough to know that there is a problem. Two different problems are possible and sometimes the computer can tell if it’s one or the other. The signal might be legitimate but it indicates a problem with the mixture. This might be because of some malfunction somewhere else in the engine controls causing a rich or a lean mixture. Another sensor might be acting up, the injectors might be leaking or sticking open, there may be a vacuum leak at the intake manifold. Anything that causes the computer to lose the ability to accurately regulate fuel will affect the reading of the O2 sensor.

Normal O2 sensor voltage traced on a digital oscilloscope. Notice how it switch from .2 to about .8 a few times
per second. This is the best way to test an O2 sensor.

Sometimes the O2 sensor signal will be a bad signal because the sensor itself is bad. The signal may get stuck at a high or a low voltage, or the rationality of the signal may be all over the map with the voltage spiking upward and downward in an erratic manner. This is a problem with the sensor and it must be replaced.

This is a biased O2 sensor signal because it is switching but it won't get down to .2 volts like it needs to.

The other thing and perhaps the most common thing that an O2 sensor does when it goes bad is it gets slow. The signal switches back and forth as the air/fuel mixture goes from rich to lean but the signal doesn’t keep up with the actual air/fuel ratio. The signal will only cross the midpoint of .45 1 per second or once every few seconds. This is a bad thing because it does not give good feedback to the computer. The problem with a slow O2 sensor is that the computer quite often cannot tell that the sensor is getting lazy. The fuel economy of the vehicle will plummet and the throttle response of the engine will feel sluggish. This condition sets in very slowly over time so the driver of the vehicle may not realize that there is a problem.

An older style O2 sensor that has just
one wire.

Some manufacturers suggest that the O2 sensor be replaced every 100k miles or something like that. Many times this is not a bad idea. In the early days of fuel injection these replacement intervals were so common that a few cars and trucks actually had a light on the dash that would illuminate when it was time to replace the O2 sensor. Since most manufacturers want to make their cars appear to be inexpensive to maintain, they have cut out a lot of the services that they used recommend. Some of these services really aren’t required anymore but some of them might still be a good idea.

The best way to test an O2 sensor is with a digital oscilloscope. This tool is capable of looking at the voltage from the sensor and plotting it on a graph so that you have a graphic representation of what the voltage is doing. A digital voltmeter will also work but you can’t see the voltage fluctuations as accurately as you can with a scope. While monitoring voltage you can purposely cause the engine to go lean by doing something like pulling off a vacuum line and creating a situation where the engine is drawing in air that it can’t measure. The mixture can be forced rich by using a propane torch or something like that to flow hydrocarbons into the intake. The voltage signal should immediately respond to such manipulations.

Air Fuel Ratio Sensor

Every new vehicle on the road today uses a different type of O2 sensor that is referred to as an air/fuel ratio sensor, sometimes it might be called a wide-band O2 sensor. The regular O2 sensor produces a voltage signal based on oxygen content and the computer can tell if the mixture is rich or lean but it cannot tell how rich or how lean because the O2 sensor works over a very narrow range. A/F sensor can not only tell if the mixture is rich or lean but it can also tell how rich or how lean.

An A/F sensor. This one has 5 wires. This many wires
is a sure sign that it not a normal O2 sensor.

The stoichiometric air/fuel ratio is 14.7:1. The regular O2 sensor can tell you if you are north or south of that ratio and that is all. The A/F sensor can read a mixture as rich 10:1 and as lean as 19:1. This makes it much easier for the computer to be very precise in its control of the air fuel ratio. More precise control leads to more power on less fuel with lower emissions every time.

The A/F sensor operates at about 1200° F so it too will have a built in heater circuit in order to bring it online soon after the engine starts. The voltage and the amperage output from the A/F sensor are important. Voltage above 3.3 volts with positive amperage indicates a rich mixture. Negative amperage and voltage below 3.3 volts indicates a lean condition. Usually the only good way to check A/F sensor output is to use a scan tool to view sensor readings in the data stream.

High performance tuners will usually install a wide-band A/F sensor somewhere in the exhaust stream so that they can monitor the air/fuel ratio on the fly. Even in an old hot rod that is carbureted installation of a sensor like this, hooked up to a display that can always show the reading, is great for making sure that the car is running at its peak. If the A/F mixture were to get too lean then it might cause serious engine damage.

Upstream Downstream

Every new vehicle on the road today will have a minimum of 2 oxygen sensors if it is a 4 cylinder engine, and if it is V engine it will 3, 4 or maybe even 6 O2 sensors. The primary O2 sensors or upstream O2 sensors are located in the exhaust manifolds close to the exhaust ports in the cylinder heads. These are the sensors providing feedback concerning the A/F ratio. The secondary or downstream O2 sensors are located much further down the exhaust pipe after the catalytic converter. These sensors are there simply to monitor the function of the catalyst.

After the exhaust passes through the catalytic converter any oxygen that is in the exhaust should be used up in the chemical reactions that take place in the cat. This will mean that the voltage readings from the monitoring O2 sensor will be flat and will not switch up and down like the primary sensors do. If the signal from the downstream sensor comes to resemble the signal of the upstream sensor then the computer knows that there must be a problem with the cat, and it will set a trouble code P0420 for catalyst inefficiency and illuminate the check engine light.

Upstream sensor (left), downstream sensor (right). They are not always this close together.

The O2 sensor is a critical piece of control equipment for any fuel injected vehicle. A good sensor can keep the vehicle running right for a very long time. If your check engine light comes on, don’t ignore it. Any problem with any sensor can compromise the way the vehicle is running, but a bad O2 sensor will cause emissions to go up, and power and fuel economy to go down. What could be worse?