If you have an avia dvd, put that in and run the speaker setup, that will play a sine sweep on all your speakers, and that will definitely tell you if your sub is firing or not. If you hear something inside rattling around, and your sub isn't working, then something is obviously wrong, it could just be what you hear are the speaker wires going to the sub have come loose, but it could be something more than that. Just call def tech and ask, they have great customer service and are very friendly.