1. An apparatus comprising: an input interface configured to receive multiple input signals, each input signal associated with a respective stage of a rotating or reciprocating system; a processing unit configured to identify a fault in the rotating or reciprocating system using the input signals, wherein the processing unit is configured to identify the fault by: generating a first pressure-volume diagram associated with a first stage of the rotating or reciprocating system and a second pressure-volume diagram associated with a second stage of the rotating or reciprocating system from a fusion of the input signals; and comparing the first and second pressure-volume diagrams to determine whether a problem exists in at least one of the stages; an output interface configured to provide an indicator identifying the fault.