The complexity of modern automobiles can make even basic malfunctions difficult to diagnose and repair. The E46 generation BMW M3 is no exception to this rule, as the combination of high-intensity discharge lamps coupled with self-levelling motors can result in multiple causes for inoperable headlights.
The first possibility for an inoperable headlight is that the bulb has burnt out. All bulbs, even high-intensity discharge (HID) bulbs, can burn out over time. In addition to age, bulb lifetime can be dramatically reduced if the headlight housing has been damaged, allowing water into the housing. Another common bulb failure causation is if a bulb has been handled by an exposed hand. When this occurs, the oils in the fingerprints will eventually cause the bulb to shatter. Finally, if the power wiring to the bulb has come loose, the bulb will not receive sufficient energy to ignite.
BMW has issued a service bulletin (SI B63-04-08) in relation to water condensation in headlight housings. The condensation may be caused by leaking automatic headlight washers in vehicles equipped with the headlight washer option.
HID bulbs rely on ballasts to ignite and maintain power in the system. A ballast can fail, particularly if it has been damaged or the unit's seal compromised. In addition, if the wiring from the ballast to the HID bulb has been damaged, the bulb may have insufficient current to ignite. BMW refers to ballasts as Xenon Control Units, and they are a common failure point. Failing Xenon Control Units can result in lights flickering.
A common failure point in E46 BMWs is the ignition unit. The ignition unit is built into the headlight assembly, and a failure in the igniter will result in sporadic headlight operation. When an ignition unit is failing, it will cause the lights to flicker and occasionally shut off then reignite. Note that when ignition units are faulty, ballast and bulb replacements will not correct the problem.
While corner lights should not affect headlight operation, on the BMW E46 the headlight failure indicators can come on due to problems with corner bulbs. Typically the problem is the contact point on the bottom of the bulb has oxidised or become fouled so that a clean connection no longer exists between the bulb and the lighting sensor. Removing and cleaning the bulbs (be sure not to touch the bulbs with bare hands) may clear this problem.
The self-levelling motors for HID headlights in the E46 BMWs rely on a sensor mounted on the suspension arms, which reads levels and corrects the headlight angle. If this sensor fails, the headlights on vehicles equipped with this option will act erratically and will fail to properly aim.
A dying battery can result in power dips, which can cause light flickering. In addition, all wiring should be examined as a break, damaged insulation or cut in any wire will result in compromised headlight performance. In addition, check all grounds to insure that they are still clean and free of oxidation, as an insufficient ground will result in a system short, which can cause headlight malfunctions.