02 sierra 2500HD 8.1

[Cudahemi]

So out of the blue I replaced my alternator and started up the truck and my gas pedal is hard can’t push it down to accelerate.did a computer scan and had hard codes of p1221 and p1120 which says tps ?

[Formulabruce]

Drive by Cable throttle, check out where the cable meets the throttle body.

[carkhz316]

The 8.1 is actually drive by wire despite it being pre-'2003. Since your gas pedal isn't directly connected to the engine, I would suspect that something failed inside the sensor portion of the pedal to cause it to mechanically bind up. As far as the TPS codes, it could very likely be related, but I'd start with the pedal/ APP sensor first.