pmdcable.com has you covered

 

Blue 24, I hope this helps you out: the code 26 is accelerater pedal position (app)2
circuit low, 88 is accelerater pedal position fault and 99 is accelerater pedal position (app)5v ref fault. Sounds like your pedal position module is bad.

Big Joe

 

here maybe some more help to but have zoom in on it to read it

 

well we replaced the whole throttle pedal assembly unit with a gm oe one about a year ago because it went bad

 

check the plugs at it make sure there clean from dirt and plug in good and check all the grounds and clean