The speed sensor is measuring the rotational speed of the wheels.

The measured data is used by the speedometer to determine the vehicle speed.

It is also used for the cruise control system.

The speed sensor is usually placed in the rear axle or around the transmission.

The sensory unit is based on the Hall-effect or has a magneto resistive sensor system.

If the speed sensor fails, the engine management will issue a diagnostic trouble code and illuminate the check engine light.

Also the driver might experience problems with the speedometer or the cruise control system.

The main reasons for a defective speed sensor are corrosion and mechanical damage of the sensor or of the sensor gear wheel.