Your BMW 540i tail light is one of the main safety features of your car, and is used as a communication tool to other drivers especially when driving under low-visibility conditions. With this component, you are less likely to get crashed into by another car behind you. Because of its importance, some states require all cars to have properly working tail lights. Driving around without this component can earn you a ticket and a fine. Avoid these inconveniences by repairing or replacing your tail light immediately. If you don't know what's wrong with it, you can always troubleshoot it by looking for these three common problems.
The first and probably the most obvious cause of tail light failure is a loose or broken bulb. Because bulbs are considered wear-and-tear items, it is normal for them to become loose over time. To check for any loose or broken bulb, remove the tail light lens using a screw driver or a ratchet and socket. Then inspect the filament in the bulb to see if it's loose or broken. If it's broken, replace the bulb with a new one. If it's loose, remove the bulb and reinstall it, then check if it's correctly placed. If it's still not properly installed, then the bulb might be incompatible with the assembly and you will have to buy the correct one.
If there's nothing wrong with the bulb, then there's probably something wrong with the wires connecting the tail light to other components such as the brake pedal, the modules, and the battery. Inspect all the connection between the wires and the sockets; know which one connects to which part. If you find a bad connection, don't repair it by yourself unless you're an electrician. Instead, take your car to a professional and have it repaired.
Because every electrical component is powered through a fuse, a blown fuse in your car can cause the object wired to it to fail. So if the entire tail light is busted instead of just one bulb, then chances are the failure is caused by an electrical problem that led the fuse to blow up. You can find the tail light's fuse in the power distribution center, also known as the fuse box. You can refer to your owner's manual to see where the tail light's fuse is exactly located in that box. You'll need a connect test light to check whether the fuse is working. If it's already faulty, then simply replace the fuse.