What is a sign of a potentially dangerous defect in leaf springs?

A cracked spring hanger is a clear indicator of a potentially dangerous defect in leaf springs. Leaf springs are crucial components in a vehicle's suspension system, as they help support the weight of the vehicle and play a key role in maintaining stability, especially when carrying heavy loads. If the spring hangers that attach the leaf springs to the frame of the vehicle are cracked, it can lead to improper alignment, instability during driving, and possibly the failure of the suspension system. This can create a serious safety hazard, particularly when braking or turning.

The other options, while they may indicate issues with the vehicle, do not directly relate to the structural integrity or functionality of the leaf spring system. For instance, missing brake lights, a loose gas cap, and dirty spark plugs are all maintenance-related concerns that can affect safety or performance in other ways, but do not pose the same immediate risks to the suspension and vehicle handling as a cracked spring hanger would.