One rotor runs at full power

Hi, I have a Syma X8C quad copter, one of the rotors (rear counter clock wise), runs at full power from power on. At initial power up, sometimes I get a normal response, sometimes the rotor spins up to full power and stays on. In every case, once synced to controller, it runs at full power irrespective of controller commands. The other three work as normal. It started as an occasional problem (it would spin and stay at full power for a moment or two). Now it runs constantly.

Question: is the likely culprit the controller or the copters main board?