A customer reports that the rear suspension of their vehicle sags when there are passengers in the rear seats. What is the most likely cause?

The issue described, where the rear suspension sags when there are passengers in the rear seats, is commonly attributed to weak springs. The springs in a vehicle's suspension system are designed to support the weight of the vehicle and absorb road shocks. When the springs become weak due to age, fatigue, or damage, they lose their ability to effectively support weight, leading to a sagging appearance, especially when additional load is applied, such as passengers in the rear seats.

Weak shocks, while they can affect the ride quality and comfort, do not directly cause the sagging of the rear suspension under load. Rather, they primarily control the motion of the springs and dampen oscillations. A worn sway bar is related to body roll and stability during cornering, but it does not impact the vertical sagging of the suspension under weight. Similarly, worn jounce bumpers can contribute to ride quality when the suspension compresses, but their primary purpose is to provide a cushion at the limit of travel and do not usually cause sagging when weight is added.

Thus, weak springs are the most direct cause of the inability to support the weight, leading to the sagging described by the customer.