If your DEF light is on but the tank is full, it likely means there’s an issue with the DEF level sensor. This sensor usually fails due to clogs from old fluid or sediment build-up inside of it. It could also be damaged if you frequently fill up your tank too quickly, which can cause air bubbles to form in the line and prevent accurate readings. The best way to fix this issue is by having a mechanic clean and replace the DEF level sensor. They may need to access the bottom of the vehicle for this task, so make sure they have all necessary safety equipment before starting work. Once they’ve installed a new DEF level sensor, they should reset any associated codes that are triggered by low levels or poor performance of this component. If all goes well, you should see your check engine light go off shortly after!