A complex industrial plant, such as a nuclear power plant, is monitored thanks to a number of sensors. The instrumentation may be itself a complex system liable to failures. We propose a model-based sensor diagnosis system which relies on the topological description of the plant and on a set of component models. This model implicitly conceals relations involving only sensor data. Such relations must always be verified if components behave normally; thus, the detection task consists of verifying these relations. So, this work is a first step in extending the scope of model-based diagnosis, since we question here the information stemming from the plant and normally considered as safe. As further studies, we wish to monitor this detection system itself; i.e., whenever the instrumentation is supposed to behave correctly, non-verified constraints point out to errors in the plant model.