In the old days, if a car came into the shop with a blown fuse, the first thing to do was to replace the fuse and see what happened. If the fuse still didn’t blow, the car was driven until it did. If the fuse didn’t blow, we chalked it up to a weak fuse. Case closed. If the fuse went out again, then some basic troubleshooting took place by changing parts until the fuse quit blowing. Each blown fuse cost from 10 to 25 cents, so not much was lost until the problem was corrected.