N54 engines feature a separate pre-catalytic converter damped to each
Turbocharger, and to replace the N54 pre-catalyst, begin by removing the lower engine cover and the exhaust system. Next, at the side of the transmission, remove the holder for the
Oxygen Sensor wiring harness and then the oxygen sensors. For the pre-catalytic converter for cylinders 1 - 3, remove the nuts and bolts from the catalytic converter bracket, support the catalytic converter while removing the clamp, and lower the converter for removal. During installation, clean the sealing surfaces and replace the seal between the converter and turbocharger, hold the catalytic converter in position using BMW special tool 18 0 110 or an equivalent, and install the clamp while checking the exhaust system for leaks. For the pre-catalytic converter for cylinders 4 - 6, first remove the pre-catalytic converter for cylinders 1 - 3 and the oxygen sensors, then support the catalytic converter, remove the clamp, and lower the converter for removal. The installation process is the same as for cylinders 1 - 3, ensuring to clean sealing surfaces, replace the seal, hold the converter in position, install the clamp, and check for leaks.