Your catalytic converter, or cat, is one of the most important parts of the OEM exhaust system in your BMW 740il. While your exhaust's job is to get rid of noxious fumes in your engine cylinders and combustion chamber, the job of your cat is to convert those harmful substances into water vapor and carbon dioxide using chemical reactants. From there, your exhaust manifold pulls the resulting gases through the exhaust pipe, sending them out tail pipe and into the air. When to replace your catalytic converter depends on both its quality and the conditions you drive in. However, they typically last about 100,000 miles. Just look for the signs your catalytic converter is bad and needs replacing. They range from plummeting fuel efficiency to engine misfires or the inability to start at all. You may not be able to accelerate correctly. You might see dark exhaust coming from your tailpipe, indicating a clogged cat, or you could smell rotten eggs, indicating fuel sulfates aren't getting removed properly. You could simply see your Check Engine light come on or fail an emissions test. You might even hear rattling beneath your vehicle; in this case, your catalytic converter might be damaged because its honeycomb mesh interior has collapsed or broken. The rattle you may be hearing is from pieces vibrating when you start up. If you're knowledgeable about vehicles, try temporarily removing your sedan's oxygen sensors from its exhaust system. If its performance gets worse, your cat is likely going bad. Luckily, if you need to change or replace your factory cat, our auto parts department can help. With us, ordering a genuine BMW 740il catalytic converter online is as easy as a few clicks. Browse our wide selection, buy now, and save both time and hassle!