This paper proposes a descriptor Nonlinear Parameter Varying (NLPV) observerfor damper fault estimation in vehicle semi-active suspension systems, here considering thehighly practical case of a varying sprung mass, which is an open problem. This mass and thecontrol input are treated as known scheduling parameters for the design and implementationof the observers, thus making the structure adaptive. This novel modeling leads to parameter-dependent dynamics and output matrices in the nonlinear parameter-varying formulation. To solve this problem, we extend an existing reduced-order observer design method (following descriptor system modeling) to this new case. Both polytopic and grid-based methods are then considered to compute and implement the problem solutions. The designs are then assessed using frequency-domain analysis and realistic time-domain simulations.