A sensor is a device that monitors different aspects of a vehicle like temperature, coolant system, oil pressure, emission levels, etc. and make the necessary changes to the engine

Few examples of car sensors and their role are .

Oxygen sensor to measure the amount of oxygen in air and exhaust and compare it to determine the fuel to air ratio. This fuel ratio aids in a car performing effeciently

Mass Air Flow sensor monitors the amount of air flowing into the engine, failure of the Mass Air Flow sensor illuminates the check engine light .

Manifold Absolute Pressure sensor measures the density of the air and adjust the amount of fuel used in combustion process .

Engine Speed Sensor measures the speed of the crankshaft in rotations per minute and determines the car's performance .

The list of sensors used in a car is long and each sensor play it's role to help in the vehicle's overall performance.