Q: What effects could bad or failing mega fuse cause and what would be the symptoms on a failing mega fuse?

MEGA fuses are bolt-down style fuses that are used to protect high current circuits. If a MEGA fuse blew, the car likely would not operate at all because these fuses protect critical, high current circuits, ones important to keeping the car’s electrical system operating. If a fuse (of any type) fails, that causes the circuit to instantly open and then no power flows. There is no "in between" or partial failure of a fuse excepting if the external, in this case bolt-on style, contacts to the fuse have high resistance or an intermittent contact in the fuse holder. A bad or loose connection is not likely if the fuse has never been touched, or loosened, as the fuse is in a closed plastic compartment. If you are having car trouble, but your car does run, it is not likely that the poor operation is due to a fuse failure. However, if your car won’t start, a blown fuse is a possibility but, of course, fuses don’t blow for no reason so you might still need to perform some diagnostics or enlist the aid of a professional to find out why the fuse blew. If you need assistance, please request service and the responding certified mechanic will address the issue that you are having with the car. If you have any further questions, do not hesitate to re-contact YourMechanic as we are always here to help you.