Bad Oxygen Sensor
When Oxygen Sensors Go Bad -- it's not a reality TV show, but a real-life occurrence which you don't want to experience any time soon. That's because the oxygen sensor, also known as the O2 sensor, acts as a canary in the coal mine for your vehicle.
When your engine is burning fuel as it's supposed to, without any contaminants creeping in and becoming part of the exhaust gases, the O2 sensor is content as can be. But when too much other stuff starts getting burnt as well, the O2 sensor catches wind of it (so to speak). If the contamination is bad enough, the sensor will activate the vehicle's Check Engine light.
The O2 sensor typically screws into the vehicle's exhaust system, either in the exhaust manifold or a pipe downstream. If it's contaminated with oil (a sign that oil is leaking into the combustion chamber and being burned), the sensor tip will have dark brown deposits.
Oil deposits or any other deposits on the sensor tip are bad news because they can cause a "false lean" reading. In other words, this fouling of the sensor tip tells the engine computer that the fuel mixture is too lean and needs to be made richer. That, in turn, can cause problems including surging (unwanted bursts of power), poor fuel economy and higher carbon monoxide emissions.