Hello, 

I have a 2011 Nissan Sentra 2.0 automatic transmission with about 88,889 miles on it. A check engine light came on and I used my OBD|| scanner. It came back as P0300. The car is currently experiencing rough idling, there is a rattling sound under the engine and on the muffler, engine seemed to be shaking more than usual, the muffler sound is abnormal, and the rpm tends to dance and/or unstable. I replaced the spark plugs, ignition coils, o2 sensor behind the engine, fuel injectors, battery, pcv valve, checked hoses for any sign of cracks and I cleaned the throttle body, but it's still the same. I drove it around the neighborhood, the check engine light is on and it seemed fine running, but the car would shake at some point, and the check engine light would start flashing and would come back to solid light. When I put it back to Park, and get off the car, there's this burn/rotten/sulfur smell that came from the exhaust. I don't know what else to check at this point, need advice before I take it to a mechanic. Thank you for reading this...