The invention relates to a method for maintenance of a frequency converter for driving an electric motor of a transportation device, said frequency converter being supplied with AC mains power via a contactor, and including a rectifier circuit for providing a DC supply voltage, a capacitive intermediate circuit for leveling DC supply voltage, and an inverter circuit for providing power to the electric motor. In the method, after the transportation device has been vacant for a predetermined time period, said contactor is opened to disable power flow from mains supply to the frequency converter, and a test load is connected to the intermediate circuit, parallel to a capacity of the intermediate circuit. The DC supply voltage Udc(t) is detected, and the detected DC supply voltage Udc(t) is utilized for establishing a maintenance information indicating that a maintenance should be done on the frequency converter. Another aspect of the invention is a software program realizing the method when executed on a computer.