There is provided mechanisms for deciding whether a bearing is faulty or not. A method is performed by a controller. The method comprises obtaining, as a measurement signal, metal particles counting data, where the metal particles counting data indicates number of metal particles present in lubrication oil of the bearing per time unit. The method comprises differentiating the measurement signal, resulting in a differentiated measurement signal. The method comprises deciding, depending on how large share of a probability density estimate of the differentiated measurement signal is above a threshold value, whether the bearing is faulty or not.