Connect your Star system to the car via the OBDII port. Select MB as the brand and then the model car and the sub model. Click on the Continue button is the bottom right of the screen and you car with the VIN should come up. On the left margin click on the stethoscope (diagnostic function), a couple of screens and you will have in the bottom of the screen F1 and F2. F1 takes you back and F2 takes you forward. You will go through a couple more screens and eventually get the the Main Group screen. The first item is Quick Test. That will query every control module in the car and report all fault including CELs. Go through the list of faults to determine what has set you CEL. Double click on any fault and it will take you to more details.Hello all today my CEL came on and I own a MBSTAR diag system , but I dont know how to troubleshoot CEL can someone walk me through the process thanks !