Method and apparatus for monitoring a controllable valve

The method involves measuring pressure oscillations of the valve (2) and generating an output signal. A frequency analysis is performed on the signal to determine amplitude for the signal at an oscillation frequency. The oscillations amplitude is compared to expected amplitude for the frequency. An error signal is generated if the difference between the calculated and expected amplitudes exceeds a preset limit. An independent claim is also included for an arrangement for monitoring operational status of a cyclically operated valve.