The invention concerns an asset monitoring system (10) having a monitoring unit 8 and a plurality of sensors (4) onboard the asset. The sensors (4) are arranged to measure a plurality of operational variables for the asset in use and the monitoring unit (8) is arranged to receive operational variable measurements output from the sensors. The monitoring unit is also arranged to determine a current value of a sensor characteristic (ZA1; ZB1) for each sensor and to compare the current sensor characteristic value with a predetermined sensor characteristic value in order to determine automatically whether a change of one or more sensors has occurred between instances of operation.