It is usually either the power supply or the motor unit or both. None of those items are considered do-it-yourself repairs. For information on sending the unit in for service you can check with the manufacturer at www.maytronicsus.com or try us at www.arrowpools.net. If the unit is several years old and in need of repair and also the replacement of wear items such as brushes and/or tracks, it can be cost prohibitive to fix. It is helpful if you get the serial number from the units casing when you contact us or Maytronics so we have an idea of how old it is.