Abstract: A method and apparatus detects degradation in performance of an engine by comparing signals such as those obtained from magnetostrictive sensors which are related to, but not necessarily accurately correlated with, stress/strain states of a torque/load transmitting member. A first signal is obtained relating to the stress in the member produced by a first loading event or series of events, typically from the firings of a cylinder in an internal combustion engine. A second signal is obtained which relates to the stress in the member produced by a second loading event or series of events from the same or other cylinder in which the torque/load was intentionally inhibited. The first and second signals are then compared to detect degradation in output of the engine.