Quick Fix to Your BMW 318i Headlight Problems
Ideally, the BMW 318i headlight assembly and bulbs are replaced every after two or three years. But when a problem appears earlier than expected, you have to address it right away. Here are the common problems that car owners encounter involving headlights:
Both headlights don't work
The cause may be any or some of these: bad headlight fuse, wiring, headlight switch, bulbs, relay, or control module. Identify what causes the problem. Start by checking the fuse. (Refer to your manual for the location of the fuse.) If the fuse is busted replace the fuse. If not, check if it has power using a 12-volt test light or volt meter. No power means there something wrong with the wiring. But if there's power, check the relay. Shake the relay. If anything inside rattles, replace it with a new one. If the relay is intact, check the control module. If there's voltage in the module when the headlight switch is on, but the headlights don't turn on, you probably have a bad module. If there's no voltage in the module when the headlight switch is on and the headlights don't turn on, you have a problem with the switch.
One headlight does not work
Possible problems are corroded bulb socket, loose bulb, bad wiring and ground connection, or busted headlight bulb. Replace the headlight bulb that is busted or leaking. If the headlight is intact, use the good bulb on the other headlight and test it to the side that's not working. If the good bulb works, then buy a replacement bulb. If it doesn't work, then take the good ignitor module on the other headlight and test it to the side that's not working. If the headlight works, the problem is not the bulb, but the ignitor module. If the headlight still doesn't work, check the wiring. Don't suspect for other problems. Fuse, relays and other parts discussed above don't affect one headlight.
The headlight only works on high beam
If the headlight works on high beam and not on low beam or if it work on low beam and not on high beam, the problem could be the dimmer switch. Replace the entire switch to solve the problem.