The areas of analog circuit fault simulation and test generation have not witnessed the same degree of success as their digital counterparts. This is due mainly to the complexity of analog behavior and the lack of a fault mode. We present a new functional fault modeling technique called the perturbation fault model. The perturbation fault model is based on an estimation of the distance between the responses of the faulty and fault-free circuit and their distributions. The model allows fault abstraction of physical defects through structural fault modeling and perturbation estimation. The fault simulation technique uses a linear estimation of the faulty and fault-free output circuits. Techniques for fault observation and propagation are presented in order to build a hierarchical analog fault simulator.