A P0300 diagnostic code on a Nissan Quest is caused by an engine misfire. This means that the on-board computer has detected that not all of the engine’s cylinders are firing properly.
One of the most common reasons for an engine misfire is a problem with the ignition system. The ignition system consists of spark plugs, coil packs, ignition cables, a distributor cap and a rotor. Over time, the ignition components wear and lose their ability to effectively transfer the spark needed to ignite the air/fuel mixture within the combustion chamber. Eventually the combustion process can be interrupted completely, triggering the P0300 diagnostic code. Most vehicle manufacturers recommend a tune-up, which includes replacing all of the spark plugs, every 100,000 miles to ensure optimal ignition system performance.
Another common reason for an engine misfire is an imbalanced air-to-fuel ratio, called a lean misfire. A vehicle needs a richer air and fuel mixture to idle smoothly. Therefore, a rough or inconsistent idle could indicate a lean misfire. A lean misfire can be caused by a faulty EGR valve, a leaking intake manifold gasket, a detective mass air flow sensor, a failing fuel pump or a clogged fuel filter. Less common reasons for an engine misfire include a faulty fuel injector, an intake air leak, insufficient cylinder pressure and incorrect fuel pressure. In rare cases, a P0300 diagnostic code can be triggered by bad or compromised gasoline.