If the technicians can't find a problem then there is an intermittent problem. When this happens the vehicle will have to be diagnosed when it's acting up. Patience on your part is needed when your vehicle won't cooperate. Once the vehicle cooperates then the problem can be diagnosed. It would help if you kept a log of exactly what happened when the MIL illuminated with details of how the vehicle was running, engine speed, engine temp, vehicle speed and any other details you can think of. It really helps if you can duplicate it consistently. The more information you can give the techs the greater chance of diagnosing the problem.