Q: I put in regular unleadead by mistake my car takes premium

Your vehicle will start and run well on both regular and premium fuel unleaded. Premium fuel is stated as "required" by Mercedes Benz simply because premium will maximize the performance, fuel economy, and possibly service life of the engine. But, an occasional tank of regular fuel is harmless, particularly if the knock sensor in the engine is working properly. In all events regardless of what fuel you use the knock sensor adjusts the timing on your engine so that varying octane fuels (e.g., regular versus premium) can, in practice, be used. Consequently, the hard starting condition you are experiencing is caused by a fault in either the car’s starting system, the ignition system, the fuel system (fuel pressure and amount), and/or the air induction system. If you request a hard starting diagnostic the responding certified mechanic will identify exactly where the problem is and will give you a repair estimate.