P2138 GMC -how do I know which part to replace

Question

surely they aren't both bad at the same time

Answer 1

P2138 Accelerator pedal position (APP) sensor 1 - 2 correlation

The accelerator pedal position houses APP1 and APP2 , this is the gas pedal,when this fails it commands the TAC (throttle actuator control) to reduce engine power, this is where there is limited accleration.