Just purchased 2004 W220 a week ago. New to MB and this site. Car has 197K miles but is in primo condition - with one exception. CEL is on dropping code P0454. I also get the loose gas cap message intermittently. I've checked the cap and it appears ok, but I don't know. I've read what the code means but it looks like that could cover a number of things. I want to have a little more information before I take it to the shop. What can cause this code? Will it be expensive to repair? I appreciate any help.