The detection and diagnosis of faults occurring in local subsystems are considered as abrupt changes of unknown magnitudes at unknown times, using the local information alone. Because of limited information the application of the centralized fault detection methods in general is not straight forward to decentralized fault detection and requires a new approach. Here we show that, using a projection operator to suppress the information due to the unknown interactions of other subsystems on the residuals of the local systems, the application of the centralized generalized likelihood ratio test for fault detection is straight forward.