What is an oxygen sensor?

This is my first of many posts hopefully. This post will be about the basic use of cars and trucks oxygen sensors, what they do and the difference between upstream and down stream application.

1. Oxygen sensors are an essential part to your vehicle running properly. The sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in the air. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess. They help determine, in real time, if the air to fuel ratio of a combustion engine is rich or lean. Because the oxygen sensor is located in the exhaust manifold (upstream) it does not directly measure the air or the fuel ratio being put into the engine. It instead works with other systems to tell you indirectly how your engine is functioning, if it is functioning improperly you should receive a check engine code. In addition to enabling electronic fuel injection to work efficiently, this emissions control technique can reduce the amounts of both unburnt fuel and oxides of nitrogen entering the atmosphere. So it not only tells you that your engine is functioning properly but also tells you if your releasing more than normal levels of combustion left overs into the air.
2. There are two types of oxygen sensors. The first being upstream which is located directly in the exhaust manifold of your engine. The second being down stream located lower in the exhaust system past the catalytic converter. Taking another reading and letting you know a better reading of what exactly is happening in the emissions. 
3. These sensors are generally easy to replace and remove. Making it easy for the fix it yourself type people to simply purchase the part and replace it using a crescent wrench.

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