Stray current is basically a ‘short’ or electrical fault in one of the vehicles electric circuits causing voltage to be present in the radiator coolant. In other words, the engine is trying to ‘earth’ to the body though the coolant as this is more conductive than the earths on the vehicle.
This stray current will begin to erode the cooling system including cylinder head(s), water pump, thermostat housing and of course the radiator. Often the radiator will show the first signs of this erosion as it is the lightest / thinnest material in the cooling system. Please see our warranty card instructions for more details.