What is an O2 sensor? 

An O2 sensor, also known as an oxygen sensor (sometimes called an air/fuel ratio sensor), helps your vehicle keep exhaust and emissions levels within  acceptable vehicle limits. Your vehicle requires fuel, air, and spark for combustion to keep the engine running. An O2 sensor monitors your vehicle's exhaust gases to determine how much oxygen is in them and informs the engine's computer, known as the engine control unit, to determine the ideal air/fuel mixture for maximum 
  engine performance. If there is too much air and not enough fuel, the sensor informs the ECU and reduces timing or adds extra fuel to achieve ideal combustion.

A bad O2 sensor not only causes poor engine performance and reduced fuel consumption, but also has a significant impact on the environment by releasing more pollutants into the air. Issues that can cost thousands of dollars to fix. Various O2 sensors are used on newer cars to provide the engine computer with two additional readings and data. Before and after the catalytic converter in the tailpipe are upstream and downstream O2 sensors, commonly known as sensor 1 and sensor 2, respectively. The upstream sensor monitors combustion efficiency, allowing the ECU to calculate the ideal air/fuel ratio to provide the most power and ensure the engine is running efficiently. The ECU then compares the downstream readings to the upstream readings. If the combustion readings are too similar, a diagnostic trouble code will indicate that the catalytic converter is not working properly. Engine Performance Owners should replace them approximately every 60,000 to 90,000 miles. Knowing the signs and symptoms of a bad O2 sensor can help you determine when it needs to be replaced. A check engine light or onboard diagnostic trouble code will illuminate with a misfire caused by a failed O2 sensor. If these lights come on, it is best to run a diagnostic test to determine what the ECU thinks the problem is. When your car starts to run rough, you notice a lack of power when accelerating, the engine stalls, or the fuel consumption is extremely low; It might be time to replace your O2 sensor. If you smell exhaust fumes in your car when you first start the engine, this could also indicate a problem with your O2 sensor. Also, there is a rotten egg smell from burnt sulfur as a result of a faulty sensor causing a poor air/fuel mixture which can damage the catalytic converter. One of the most common reasons for failing an emissions test is faulty O2 sensors. Save time and money by having a mechanic examine the sensors before performing an emissions test. Replacing the O2 Sensor If your vehicle has encountered these problems, you are welcome to replace the O2 sensor. You extend the life of your vehicle and contribute to a cleaner environment. If you have basic vehicle and engine knowledge and the right tools, you can replace the sensor yourself. Replacing the O2 sensor is a fairly routine maintenance item for an experienced mechanic or service center. It's a good idea to have the O2 sensors checked when the car isn't running properly and when doing routine maintenance like changing the oil.