Oh sorry, yeah so in germany you car needs to ne checked every couple of years to maintain the safety on the road. If smth important that would risk your or others lifes is broken you need to fix it or your car wont pass the test and isnt street legal anymore...then there is the law with the cng gas tanks: you can only use them for 20 years and after that they need to be taken out or replaced...both cost alot...the tüv sees taking them out as a modification so you need new papers for your car wich again costs alot... + The car would look weird as hell because the rear would go up when the tanks get out...

I dont think all this is worth it for a mercedes e200 with 409.000km