This paper deals with the problem of fault detection and isolation for nonlinear Hessenberg systems. In particular, the first part of the FDI problem associated with the disturbance decoupling transformation is considered. The approach is based on a relation between the relative degree from the unknown input to the output variables for nonlinear Hessenberg systems. The major advantage for this class of systems is that a decoupled system can be obtained analytical without solving partial difference equations. Moreover this transformation can be used to define the structure of a reduced observer for the system where different observer design procedures can be used, since the subspace is uniform observable.