Annually or every 20,000 km.
The check should be done with a cold engine, so it is best to do it before the first trip of the day or no sooner than three hours after turning off the engine. Check the condition of the entire exhaust system, starting from the engine and ending with the end of the exhaust pipe. Ideally, this check should be done with the car lifted on a winch, when it is freely accessible from below. If a winch is not available, jack up the car and securely support it on axle supports.
Check pipes and pipe connections for signs of leaks, severe corrosion and damage. Check the condition and reliability of all brackets and hangers of the system.
Inspect the rubber exhaust system mounts for cracks.
At the same time, inspect the underbody of the car for holes, corrosion, loose seams, and other defects that could allow exhaust gases to enter the car's interior. Seal all openings with a suitable sealant.
The exhaust system, especially its mounts and hangers, is often the source of rattling and other unpleasant sounds. Wiggle and tug the pipes, mufflers and catalytic converter (if provided). If these components come into contact with body or suspension components, replace the exhaust system fasteners.
Inspection of the inside of the exhaust pipe allows you to determine the current operating condition of the engine. The composition of the deposits in the pipe indicates the quality of the engine settings. If the pipe is black and sooty on the inside, this may indicate that it is necessary to thoroughly check the fuel system.
Information obtained from this online resource: BMWman.ru