The engine P0121 Code is available in different meanings and accurate meaning of the engine code can be found from the car engine manufacturer. You need to confirm that you have found right meaning of the code. However, you need to verify the car engine for understanding the real meaning of the engine problem. The short definition of the engine code is Throttle Pedal Position Sensor/Switch (TPS) A Circuit Range Performance Problem. Based on symptoms, you can understand the problem of the car engine. There are several symptoms available what you need to follow to identify the car engine problem and reason.