A P0300 diagnostic code indicates a "random" or "multiple" misfire.
Unfortunately, P0300 doesn't tell you specifically which cylinders are mis-firing, nor why (code P0301 would indicate cylinder #1; P0302 would indicate cylinder # 2 etc). This means there could be an issue with any number of components/circuits relating to the ignition/combustion circuits.
I would recommend you check for the following:
Sometimes the underlying cause of a random misfire is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold, unmetered air getting past the airflow sensor, or even an EGR valve which is stuck open.
Hope this helps...
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