Truck Repair: what do I have to do?, light flashes, cummins

Question
My n-14 cummins 500 starts good but, after 20 mins. in the conslote, appears check engine and the red light flashes and then shuts off. When I restart it it doesn't accelerate, but if I shift to third or reverse it runs.

Answer
Turn key to ON position.Turn cruise switch to ON,Hit set switch then count flashes.The fault codes are 3 digit codes with a 1 sec pause between digits and a 2-3 sec pause between faults & they keep repeating the faults until you key off.Get those & I can help more.