1996 FLD120 what is this

The lights will flash on the light bar. Your problem doesn't sound like a problem that would be caused by a coolant level sensor.

Without being able to flash the faults or have another way to check codes, you don't know what you're looking for. You're just chasing your tail.

I suspect you're losing ignition power to the ECM. Until you can check faults, focus your attention inside the battery box. Clean all your cables, and pay extra attention to the small wires, their fuse holders, and terminal ends.

The bulbs in the light bar may be burnt out BTW. They are replaceable. Push a piece of small vacuum line over them to remove

 

So it’s dead now. Do the dash lights cel yellow and red ( along with others) all light up (test mode) when key is turned to the ON position? If not, the ECM isn’t getting power from the battery. That’s what the small wires at battery are for. There’s also a small inline fuse that could be corroded. I had the same problem twice, only caused by moisture. Shut down when wet. Unplug coolant sensor, it ran till dry, Shut down again. Plug in sensor. Ran fine till it got wet. Usually after a Truck wash, a couple times during a heavy rain. The problem was a broken coolant sensor wire. They commonly break along the valve cover. Had the problem for years. Kept replacing sensors, till a Mechanic told me about the harness. Last time the same problem returned, I replaced the harness again, cutting original harness shorter. However it turned out being the actual sensor itself. It drove me nuts for years. Shutting down sometimes in dry weather. To complicate things the battery in my ECM is bad. That means it won’t store old codes. I finally caught the problem in action, double checked the problem by swapping parts. Very aggravating.