An ambient light sensor testing module running on an information handling system verifies that an ambient light sensor managing brightness of an information handling system display is active and responding correctly. The testing module sets the display at first and second manually-selectable brightness levels and compares the brightness set from the first and second manually-selectable brightness levels when the ambient light sensor is engaged to set display brightness. An excessive difference between the brightness set by the ambient light sensor from the first and second manually-selectable brightness levels indicates a faulty ambient light sensor.