2005 C240 Wagon Check engine light on

The reason your light is on is because the system detected a problem related to the engine(emmissions system). Usually benz recommends premium grade fuel. It is possible that the plugs have fouled or some misfiring occurred and the system illuminated your light. Do you remember seeing the light flash at all before it remained constant. When the gas cap is left loose or the seal is worn out the M.E. module sets a p0440 codes(miner leak in evap) or a p0442(major leak in evap. The first thing I would check(assuming the car is running o.k.. is the gas cap tightness. Either way you will have to take the vehicle in and have the codes read and erased. If the module notices a problem during a key cycle within 17 minutes of vehicle operation it stores a fault. One the 2nd key cycle if the module notices the same problem with in 17 minutes it turns the light on. Hopefully I have been helpful