In this paper, we present a method for detecting,faults that can appear in some parts of a chemical plant. This method is based on statistical information generated by the Extended Kalman Filter (EKF) technique and is designed to reveal any drift from the normal behavior of the process. Originally developed for linear systems, our contribution to this method consists in extending it to unsteady state nonlinear systems, such as stirred reactors in the presence of exothermic chemical reactions. We examined the abnormal behavior of a chemical reactor due to one fault in its control parameters. An abrupt variation of cooling flow rate has been studied experimentally. The objective of this experimental study is to detect the presence of such faults online, and to pinpoint the moment one occurs.