Name

• An oxygen molecule consists of two atoms of oxygen bound together. Thus, O2 refers to the chemical formula for pure oxygen.
  
  

Task

• O2 sensors monitor how much unburned oxygen is present in the exhaust from an automobile engine. The amount of oxygen indicates how efficiently the engine is burning fuel.
  
  

Effect

• The car's computer uses readings from the oxygen sensor to adjust the mixture of air and fuel in the engine's combustion chambers for maximum efficiency.
  
  

Location

• There is an O2 sensor inside the exhaust manifold. Vehicles produced after 1995 also have a "downstream" sensor to measure the exhaust as it leaves the catalytic converter, to ensure that the converter is working properly, too.
  
  

Malfunction

• Symptoms of a malfunctioning O2 sensor include declining gas mileage, rough idling and engine surges or hesitation. A vehicle's "check engine" light will often come one.