An appliance operation signal processing system comprising: an input (11, 15) for receiving an appliance operation signal, the appliance operation signal comprising information relating to operation of an appliance; an output (13, 17) for outputting information relating to degradation of the appliance; and a processor configured to determine a probability that the appliance or a component of the appliance is in the or each of one or more degradation states, by applying a classifier to a feature vector comprising information relating to frequency data extracted from the appliance operation signal; characterised in that the processor is further configured to: monitor an input appliance operation signal for a change in magnitude; if a change is detected: extract a first section of the input signal, the first section being a section of the signal from a location prior to the detected change to a location after the detected change; extract a pre-event portion of the first section, the pre-event portion being at least a part of the first section of the signal prior to the change; extract a post-event portion of the first section, the post-event portion being at least a part of the first section of the signal after the change; transform each of the pre-event portion and the post-event portion into the frequency domain; subtract one of the frequency domain pre-event portion and the frequency domain post-event portion from the other of the frequency domain pre-event portion and the frequency domain post-event portion to give resultant frequency data; wherein the frequency data extracted from the appliance operation signal is the resultant frequency data. an input (11, 15) for receiving an appliance operation signal, the appliance operation signal comprising information relating to operation of an appliance; an output (13, 17) for outputting information relating to degradation of the appliance; and a processor configured to determine a probability that the appliance or a component of the appliance is in the or each of one or more degradation states, by applying a classifier to a feature vector comprising information relating to frequency data extracted from the appliance operation signal; monitor an input appliance operation signal for a change in magnitude; if a change is detected: extract a first section of the input signal, the first section being a section of the signal from a location prior to the detected change to a location after the detected change; extract a pre-event portion of the first section, the pre-event portion being at least a part of the first section of the signal prior to the change; extract a post-event portion of the first section, the post-event portion being at least a part of the first section of the signal after the change; transform each of the pre-event portion and the post-event portion into the frequency domain; subtract one of the frequency domain pre-event portion and the frequency domain post-event portion from the other of the frequency domain pre-event portion and the frequency domain post-event portion to give resultant frequency data; extract a first section of the input signal, the first section being a section of the signal from a location prior to the detected change to a location after the detected change; extract a pre-event portion of the first section, the pre-event portion being at least a part of the first section of the signal prior to the change; extract a post-event portion of the first section, the post-event portion being at least a part of the first section of the signal after the change; transform each of the pre-event portion and the post-event portion into the frequency domain; subtract one of the frequency domain pre-event portion and the frequency domain post-event portion from the other of the frequency domain pre-event portion and the frequency domain post-event portion to give resultant frequency data;