Its a black box located under the intake on top of the motor. The box contriols the injection pump. Usually the stock box fails due to to much heat. Now you can buy a new one that relocates it to the top of the motor to keep it cool. Im actually not exactly sure how to test it heres the symtoms on another site.
"Symptoms of failure are stalling, hard starting, surging, poor fuel mileage, erratic idle and finally no-start. When they go it usually will not give any codes with the SES (Service Engine Soon) light."
Mine went bad and the truck would shut off while going down the highway
