09 Silverado 3500HD Diesel engine power reduced code p2123.

Question

slow to take off but will speed up after about 30-40 seconds when first start shows engine power reduced

Answer 1

P2123 Accelerator pedal position sensor high circuit high voltage

Circuit high , you   have a poor mating connection of the APP.  Chevy has a TSB for this code condition #PIP3783D titled  SES light reduce engine power mode and APP sensor DTC. what they want is the inspection of the harness of the APP for any water intrusion and even recrimp the wire at the APP.