During driving Ford Explorer you might need fog lights sometimes. You shouldn’t buy other voltage than 12 V and make sure the power of bulb is 21 W. While replacing the whole fog lamp in Explorer 4.0 made in 2001, look at shape of your purchase. Only recommended by Ford shape specially for 4.0 engine cars is suitable. Purchase BA15S kind of base if you want to change Explorer fog lamps.
Less or more voltage than 12 V can destroy reflectors in Explorer fog lights. Fog lights of Ford Explorer may use huge amount of bulb types. So, in output you can get more than 21 W of power but keeping the base at BA15S.
Real capacity of BA15S base bulb easily might go over 21 W. You must prudently place 12 V bulb into Ford’s fog optics. To ensure safe drive on Ford Explorer 4.0 buy high quality lamps for fog-light.