i have a u1000 code on a trailblazer and need help troulble shooting

Question

there are actually 6 codes that make up the u1000
im trying to trouble shoot the following 4 PO 1221 PO 1275 PO 1280 PO 1635
i'm trying to find out what is causing theses codes and how to fix it?

the truck is at the mechanics now and dont want to be taken for a ride, can anyone help or point me in the right direction on where, what site or how to trouble shoot these codes.
would appreciate any help i can get..
i can be reached at this number as well if the typing is to long for answers.516 250 4262
thank you