The OBD 2 engine code P0302 means that the ECU has recorded number two cylinder misfiring. Possible causes are the spark plug, spark plug wire, ignition coil, fuel injector, injector electrical harness, valve, valve spring, piston ring etc. The most common causes of this code on a Nissan Maxima with under 100,000 miles are spark plugs and ignition coils. A compression test can be performed to check the engine's mechanical integrity, using a compression tester like one available at DenLorsTools.com. Compression should not vary more than about 10 percent between the cylinders, any more than that can cause a rough idle.