What does p0121 code mean on the chevy tahoe?

A trouble code of p0121 on the Chevy Tahoe means there is an issue with the throttle pedal position sensor switch. The throttle position sensor is located on the throttle body of the intake manifold. In short, this sensor is designed to take the input from Your pedal and translate it to the engine. This makes the car able to tell how much air to let into the engine, thus controlling the acceleration of the vehicle. If the sensor is bad then it will not know how much air to allow in, and may not open at all. This will either make it so the engine dies, or the amount of speed does not match the deal inputs You use. This may also lead to surging where the car will suddenly speed up when You are not pushing the pedal.