Hook up a code reader to your OBD II port and read the code. If you don't have a reader, a $20 or less. Some will communicate with a smart phone, others are stand alone. If you don't want to buy one, go to an auto parts store, most will scan your vehicle for free.
The code could be as simple as a loose gas cap.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI