Hello hotel2zero, thanks for your question in the autoaid Community.
Your described fault Code means the following:
P0120 - Throttle/Pedal Position Sensor/Switch 'A' Circuit
This fault could be a faulty Throttle Position Sensor, Throttle Position Sensor circuit is open or shortedor Throttle Position Sensor poor electrical connection
If you have a circuit plan you should check the cables from the throttle pedal to the ECM, to check a fault from the electrical side.
You could also check the actual values in the ecm, maybe you could see there a fault.