I have a Mj18 4ohm being pushed by a fosgate amp around 500w. yesterday it just quit working. i took it out and hooked it up to another amp it worked fine. ive also disconnected and reconnected every wire and changed all the fuses blown or not. It is still not working. any clue on what it could be?

If you thoroughly checked all the connections and tested the sub on another amp (and the sub was fine), it might be a bad amp.

When you tested the sub on another amp, did you swap the new amp with your old amp, or did you take the sub out of your car and move it to the new amp?

i took it out put it in another truck. when i unhooked the speaker wires from the box they touched and sparked, would it do that if the amp was bad?

Fuses perhaps?

Try swapping the good amp in place of the bad amp. If it works, we can rule out the wiring in your car. If the good amp doesn't work, then the problem is somewhere in your car.

ive already changed the fuses. Ill try swapping them in my truck.