What scanner are you using please ? The reason I ask is that P0300 is a generic code that means there is a misfire issue but not precisely what. If a DTC only scanner like Torque Pro, it does not do in-depth scanning required for a Mercedes (Crossfire is really a Merc SLK320 or SLK32 (SRT) that has been rebodied.

To read the underlying code you either need a Dealer scanner (DAS/STAR) or a handheld that has a MB module (like an ) or something equivalent.