You need to remove one of the legs from the board to get an accurate reading. Measuring the resistance with the fuse installed then also measures the resistance of the rest of the circuits that the resistor in in parallel with. If it is really 18 ohms then you would need to remove and replace it anyway so you can start by removing one leg and then measure it again. If it is still 18 ohms then it really needs to be replaced but at 18 ohms, I think that it would remove itself.