The E46 6-cylinder engine is equipped with two exhaust manifolds (bank 1 = cylinders 1-3, bank 2 = cylinders 4-6). Each manifold ducts the exhaust to a built-in catalytic converter. The catalytic converter's job is to clean up the exhaust leaving the engine to meet emission standards. In a catalyst (cat for short) exhaust gases are forced to pass through a fine-meshed ceramic matrix impregnated with platinum-iridium alloy. These metals bring unburned CO and hydrocarbon molecules together with excess oxygen in the exhaust and accelerate oxidation. This chemical reaction creates heat and the heat improves oxidation efficiency; therefore cats need to run at a high temperature for maximum efficiency. Over time the high heat as well as contaminants in the exhaust melt or otherwise damage the ceramic matrix in the cat so that it requires replacement.
A damaged catalytic converter can rarely be diagnosed visually. Catalyst failure usually sets fault codes in the engine control module (ECM). Before you remove the exhaust system, use a BMW scan tool or equivalent to download fault codes and determine if there is any reason to suspect the failure of these major emissions control components or of the oxygen sensors. The oxygen sensors, particularly those downstream of the catalysts, may be easier to remove once the exhaust system is off the car.
The most common fault codes that show your catalytic converters are faulty include: P0420 Catalyst System Efficiency Below Threshold (Bank 1) or P0430 Catalyst System Efficiency Below Threshold (Bank 2). Bank 1 refers to the catalyst attached to the front three cylinders (1-3). Bank 2 refers to the catalyst attached to the rear three cylinders (4-6). You do not have to replace both cats at the same time. Replacing only the faulty one can save quite a bit of money. However, keep in mind, if one is faulty the other may not be far behind. When dealing with the previously mentioned fault codes, check that the engine is running well and there are no other fault codes stored before condemning the cats.
Exhaust manifold and catalyst replacement is a big job and requires removing the complete exhaust system and many other components. Read through the procedure thoroughly before beginning. Plan for this job to take a day, if all goes as planned.
When removing the exhaust system, have new fasteners, gaskets and rubber insulators on hand to help complete the repair. When removing the exhaust system and particularly if you are going to store it while other repairs are taking place, be sure to protect the oxygen sensors and their electrical leads. Do not drag the exhaust system on the ground carelessly.
Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you are working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.
Vehicle models change and evolve, as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.
Raise and support front of the vehicle on jack stands. See our Pelican Parts technical article on Jacking Up Your BMW
Remove all four oxygen sensors from the exhaust manifold. This will help you avoid accidentally damaging the sensors. See our Pelican Parts technical article on Replacing Your Oxygen Sensors.
Working at the front of the exhaust system, remove the nuts that connect the exhaust system to exhaust the manifold (green arrows). Be careful when loosening these nuts. I like to spray the studs with penetrating oil and clean the end of stud with a wire brush before removing. Depending on your region, these can seize up. If they break, donÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ't worry. You can remove the studs by hammering them out. Support the engine from below using a hydraulic floor jack with a block of wood between the jack and the engine. Next, working at the right front of the cylinder head, remove the secondary air valve from the engine.
Working in the engine bay, locate the top of the right engine mount. This photo shows the mount looking down past the exhaust manifold. Remove the nut from motor mount (green arrow). Remove the right side engine mount bracket fasteners and remove the bracket from the engine. The bracket has four fasteners and bolts to the side of the engine block. You can access it from below the vehicle.
Remove the exhaust manifold fasteners (green arrows). There are eight fasteners on each manifold. Remove the front manifold fasteners first, then the rear manifold fasteners. This photo shows the manifolds with the engine removed for clarity. Once all fasteners have been removed, pull the exhaust manifold away from your BMW cylinder head and remove it from the engine compartment. Remove the front manifold, cylinders 1-3 and remove the rear manifold, cylinders 4-6. Do not drop your exhaust manifolds when removing. The catalytic converters are fragile and can be damaged from an impact.