Onboard diagnostics of an active air dam assembly

A vehicle includes a body and an active air dam assembly. The body has a first end facing an ambient airflow and a second end facing away from the ambient airflow. The assembly is disposed at the first or second end and controls the ambient airflow between the body and a road surface. The assembly includes an adjustable flap, an actuator, sensors, and a controller. The flap shifts between different stowed and deployed positions. The actuator moves the flap, e.g., via a shaft. Each sensor measures a separate performance parameter of the actuator. The controller executes a method to receive the measured performance parameters, identify a fault mode of the air dam assembly from among different possible fault modes using the performance parameters, and execute a control action corresponding to the identified fault mode.