Method for preventive maintenance of an elevator and an elevator system

A method for preventive maintenance of an elevator speed sensor system includes at least a first and a second sensor, which are independent of each other. The method includes: determining a reference distance for an elevator car travel between a first door zone and a second door zone, during the elevator car travel, defining continuously a first elevator car speed information from the first sensor and a second elevator car speed information from the second sensor, calculating a cumulative sensor system error by integrating the difference between the first elevator car speed information and the second elevator car speed information, and dividing the cumulative sensor system error with the reference distance to obtain a sensor system performance indicator.