I suppose if you have a way to scan dynamic engine characteristics such as HGO, on/off states, live maf, coolan temp etc then a dealer isn't needed. Most folks don't have access to interrogate live data from mercedes computers. You are right in that the dealer should be last resort but I try to shy away from just throwing parts at something and hoping to fix it. When o2 sensors start failing they usually throw codes that are non specific in many cases.What are you using to map all the live data from your crossfire, I want one but don't want to spend 3k on a used analyzer. Are the cell phone apps sophisticated enough to perform high level diagnostics now?

BTW, I have multilple OBDII scanners and a couple can-bus scanners, problem with the can-bus scanners is they are for VW's. I can read codes as well as anyone but interpreting the codes without live data can get expensive.