Assuming you have a catless downpipe:
Emissions testing can't be done successfully on the n55, period. The current downpipe fix available involve clearing the code (and cat readiness at the same time). You can't get readiness.
To date there is no available (and working) known downpipe fix like on the n54 that can get you pass emission with a catless downpipe.
On map 0 you will get the cat error code (obviously as there is none actually), map 1/2/3/4/5 will auto-clear the code so it won't show on the dash.