What could indicate that a vehicle's air suspension system is defective?

Leaking shock absorbers are a clear indicator of a potential problem within a vehicle's air suspension system. The air suspension system relies on shock absorbers to help maintain stability and comfort while driving. When these shock absorbers begin to leak, they can no longer effectively dampen the motion of the vehicle, leading to an unstable ride, excessive bouncing, or uneven vehicle height. This degradation in performance jeopardizes not only the comfort of the ride but also the safety of the vehicle when carrying loads.

In the context of the other options, excessive load capacity does not directly signify a defective air suspension system; it simply refers to the maximum weight the vehicle can handle. Visible tire tread, while important for traction and safety, does not provide direct evidence about the air suspension's functionality. Similarly, operational lights relate to electrical systems rather than directly indicating the condition of the suspension. Thus, the presence of leaking shock absorbers distinctly points to issues within the air suspension system itself, making it the correct indication of a defect.