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In this paper, a method for on-line fault diagnosis of analogic channels of the VVER1000/440 monitoring systems is proposed. The method is based on the analysis of the amplitude fluctuations of electrical signals at the output of analogic channels. The advantage of this method is the possibility to perform on-line fault diagnosis of the monitoring system during the normal operation of the nuclear power plant. The method is also considered to be simple enough for practical implementation and use. The paper presents the practical results of the tests carried out to verify the method with the aim of demonstrating that the shape of the histograms of the amplitude fluctuations characterizes different types of sensor faults and component faults in the analogic channels, such as drifts, frozen or noisy signals. The parameters of the histograms of the amplitude fluctuations are used to construct a fault recognition system which is based on the Pearson's chi-square criterion for verifying the probability hypothesis of the discrete random variable.