Ok, I think I help shed some light on this for you. P0121 is for the throttle position sensor. They also said it was for a pedal position sensor but that only applies to cars that come with ETC (Electronic Throttle Control). ETC is also referred to as drive by wire. Newer cars use an electric motor to open and close the throttle plate in the throttle body as opposed to earlier or non-ETC cars that use a cable that connects the gas pedal to the throttle. ETC uses a throttle position sensor built into the ETC throttle body assembly and a pedal position sensor to tell the computer how far to open the throttle. In your case, I don't think you have ETC but you need to look at the throttle body and see if there is a cable connected to it that goes to the gas pedal or if there is an electrical connector on the throttle body that has more than 3 wires. If you have a non-ETC engine, you still have a problem with the TPS sensor. Since you say you already replaced the TPS sensor, that tells me you have a non-ETC engine. The TPS should have 3 wires going to it. One is the signal wire that goes to the computer. The other two wires are the 5v supply and the 5v return, or sensor ground. You need to check to see what the sensor reading is at the computer and if you have good 5v to the sensor. You would do most of this testing with a digital volt ohm meter. You would also need to check the wires for continuity and for proper resistance in the wires. It sounds like you may have a trip to a qualified service repair center in your future.
Former ASE master certified gm dealer service/diagnostics technician,
Engineering technician for Delphi Powertrain Systems, I have been installing and testing ETC systems for several years, mostly on Hyundai cars.