Most common failure is the horn diaphram out of adjustment, so instead of making and breaking connection..."fails to move" or once moving "hangs" in between the two excursion zones. If the electro-magnet is too weak, cannot pull the diaphram, or once it moves to maximum extent, cannot break the electrical circuit to allow it to return. The adjustment screw has limited range between those two extremes. Cheap plastic parts, corrosion or temperature makes this adjustment "touchy". Have had to replace the horn several times for those two failure modes. Later model horns add a relay which adds to the complexity and troubleshooting "exercise". 