My F.L is older, 120 hp, so the injector pump is no doubt different, but..
My solenoid suddenly stopped working one time. It turned out to be a grounding wire which was an easy fix once I found it.
However.. My shut off is achieved by pulling the rail forward, not pushing it back. In fact for the two or three days I used the boat without an electrical shut down I had to lift the hatch and pull the rail forward by hand. This became a pain so I ran a cable to the rail up through the sole to a convenient location where I could give it a tug to shut the engine down.
Turns out I liked the idea of having a positive mechanical shut down so I have left the cable in place and labeled. Not sure it would work in case of an engine "runaway" but in theory it should. BTW, I store a coffee can-like contraption near the air intake in case I ever have to "choke it off.
pete