The boy, from Telford, Shropshire, denied having the electronic grabber in a car park. But Telford Youth Court found him guilty and remanded him on bail for pre-sentence reports. It also ordered that the grabber be forfeited.
It is the first prosecution in relation to the device, which records the signal sent to a car's central locking or alarm system and is then used to gain entry.
The boy told the court that he knew what the device was when he bought it for pounds 50 from a unnamed man in a van two days before being arrested.
He said he signed a receipt saying that he would not use it to commit a criminal offence and intended using it legitimately to repossess hire purchase cars.