How does an Oxygen Sensor Work
The function of an automotive oxygen sensor is to send real-time data to the Engine Control Module regarding the fuel and air mixture being combusted in the engine. This data allows the Engine Control Module, or ECM, to effectively regulate the fuel injectors thereby maintaining an optimum fuel to air ratio at all times. This is simpler than it sounds. The following is an explanation of how the oxygen sensor performs its function.
An oxygen, or O2 sensor, is manufactured from the material zirconium oxide plated with platinum and is placed in the stream of exhaust gases produced by the engine. When heated to more than several hundred degrees Fahrenheit, this combination of materials is capable of generating a small amount of electricity referred to as a micro-voltage. The sensor compares the amount of oxygen in the exhaust with that of ambient air. The difference between these two generates the electricity. The higher the difference, the higher the voltage. By monitoring this voltage, the ECM can determine the efficiency of the combustion in the engine.
For an internal combustion engine to function, it must have the correct amount of fuel and air. Combustion is most efficient when the amount of air is 14.7 parts to 1 part fuel, or a ratio of 14.7 to 1. When the O2 sensor is exposed to the exhaust produced by this ratio, it generates approximately 0.5 volts.
If the ratio dips down below 14.7 to say 13.9 to 1, there is not enough air to efficiently burn the fuel and the engine is said to be running "rich". Likewise, if it increases to 15.0 or more, there is not enough fuel for complete combustion and the engine is running "lean".
How does the O2 sensor respond to this? If the engine is rich, the difference between the oxygen content in the exhaust and that of the ambient air increases and the voltage it generates will increase proportionally. Depending on how rich the mixture, the voltage may climb to around 1.0 volt. If the engine is lean, the difference decreases and with it the voltage. It may drop as low as 0.2 volts.
The ECM is constantly monitoring the voltage from the O2 sensor. If it is 0.5 volts, the ECM continues operating normally. If the voltage drops however, the ECM opens the fuel injectors for a longer period to dispense more fuel into the engine, thereby enriching the mixture. If the voltage rises above 0.5, the ECM will shorten the injection time to lean out the mixture. This constant monitoring and adjustment keeps the engine running efficiently under varying conditions.
An O2 sensor is built into a threaded metal fitting that can be removed from the vehicle similar to a spark plug. Newer vehicles may contain up to three sensors. One is located in the exhaust manifold. With engines of a V configuration, each exhaust manifold may have its own. There may be another mounted directly after the catalytic converter.
Each sensor has connected to it one to four wires. In a single wire unit, that wire carries the data. The circuit is completed through the ground of the vehicle. The second wire in a two wire unit is a ground wire and is used instead of the vehicle ground to completely isolate the circuit from the rest of the electrical system. In a three or four wire unit, the other two wires are used to power a heating element built into the sensor.
Why a heating element? O2 sensors do not generate a voltage until they are heated to more than several hundred degrees. Until this occurs, they provide no data and the ECM must rely on an internal software program to control the engine. This is referred to as "open loop mode" and is less efficient. While the exhaust will eventually heat the sensor, it can be heated sooner with the help of an element, thus sending data to the ECM more quickly and allowing much more efficient engine control.
O2 sensors must be replaced occasionally. Failure can be caused by several means. Physical damage can occur when servicing other parts of the engine. A buildup of carbon can plug up the sensor and insulate it from the exhaust. The use of leaded fuel will ruin the sensor. Any fuel additive or other substance containing silicone will instantly destroy it. Make sure any chemical additive, gasket sealer, etc. is labeled "O2 sensor safe".
If your sensor fails, your "check engine" light may illuminate or you will notice a drop in fuel mileage. If either of these occurs, have the sensor checked and replaced as necessary.
