Try to have the codes pulled for free, or even find what a Mercedes independent or Mercedes dealer wants.The AMM will be aroud $250 - check the price at FASTLANE on top of this forum page. Unless you want to add the cost of a OBD II code reader device on top of that

You can also do a search under Steve Brotherton ( sp? ) cause he has written some detailed post on check engine light diagnostic. He writes about fuel trim codes and the ultimate way to diagnose the situation.
Many people take a chance if the code is the P0170 and go and buy the AMM.