How does something in the path of the exhaust stream generate electrical voltage and help the ECM/PCM to adjust the air/fuel mixture? The core of a typical oxygen sensor is a thimble shaped element made of a special material called zirconium dioxide (ZrO2). The inner and outer surfaces of this element each have an electrode made of a thin,porous layer of platinum. The inner surface of the element is open to the outside air, while the outer surface, which also has a porous ceramic coating, is open to the exhaust stream. While there is still some oxygen in the burnt exhaust gases, there is obviously more oxygen in the atmosphere. Because the proportions of oxygen differ between the inner and outer surfaces of the element and because of the special properties of ZrO2, a voltage is generated between the two electrodes. Keeping in mind the amount of oxygen in the atmosphere is relatively constant the voltage output will change as the amount of oxygen in the exhaust stream changes. When the air/fuel mixture is lean, less voltage is generated because of the higher oxygen content in the exhaust stream. And conversely, when the air/fuel mixture is rich, more voltage is generated because of the lower oxygen content in the exhaust stream. By monitoring this voltage, the ECM/PCM recognizes how rich or how lean the air/fuel mixture is and adjusts the mixture accordingly. So what causes an oxygen sensor to fail? Logically, you would think it is the carbon from the exhaust stream, but in reality, it is not. The number one enemy of an oxygen sensor is silicone. When the exhaust side of the element comes in contact with silicone, the pores of the protective ceramic coating get clogged. Once that happens, the exhaust side gets less oxygen. As a result, the oxygen sensor generates more voltage than it normally would for a given air/fuel mixture. This could cause the ECM/PCM to make the mixture lean enough to cause driveability problems or cause the MIL to come on. If the outside air surface of the element is contaminated with silicone, the porous electrode gets clogged, reducing the oxygen on that side. The oxygen sensor generates less voltage, so the ECM/PCM tries to compensate by making the mixture richer. This could also cause driveability problems or cause the MIL to come on. Sometimes the source of the silicone is fuel, but the more likely sources are the silicone sprays, greases, and adhesives commonly used for servicing. When you use these products, avoid getting them in the engine’s air intake tract, the exhaust system (upstream of the oxygen sensor), or the oxygen sensor vents. Handling Oxygen Sensors Always handle oxygen sensors with care when ever you are carrying out repairs. Although oxygen sensors are designed to work in a hostile environment, they are very fragile and can be easily damaged by oil, brake cleaners, silicone, and other chemicals. If any foreign material (chemicals, oil, etc.) gets into the sampling ports, the residue (smoke) can alter the oxygen levels, which can cause the ECM/PCM to make a miscalculation and set a DTC. Other chemicals can permanently damage the sampling part of the sensor. So, when you are carrying out repairs with the oxygen sensors, it is good practice to wrap the sensor in a clean shop towel until you are done.