An actuator is a mechanical or electrical device that moves a part when it receives a command signal. Cars use dozens of actuators to run automatic systems. They act as the muscles of the vehicle, taking commands from the computer and converting them into physical pushes, pulls, or rotations.
You interact with actuators every day. When you click the lock button on your key fob, an electric door lock actuator slides a metal rod to lock the door. Inside the dashboard, blend door actuators use electric motors and plastic gears to open and close vents, routing warm or cold air to the cabin based on your climate control settings.
Actuators run on electric motors, vacuum pressure, or hydraulics. If an actuator fails, the system it controls stops working, even if the computer and wiring are perfect. A common failure is stripped plastic gears inside the actuator housing, which causes a clicking noise under the dashboard when you change the A/C settings.