engine dash light stays on-mechanics say nothing wrong

WHEN IT FIRST HAPPENED WE DID NOT DRIVE IT UNTIL IT WAS TOWED TO MECHANIC, PAID THEM, DROVE IT A WEEK OR TWO AND IT HAPPENED AGAIN 4 OR 5 MORE TIMES AND FINALLY THE MECHANIC SAID TRUCK IS FINE AND THEY CANNOT FIGURE OUT WHY IT KEEPS COMING ON, THEY CHECKED IT ALL OUT AND THIS WAS NO SHADE TREE MECHANIC---ITS A GARAGE THAT DOES ALL WORK.