Diagnostic code PO300 indicates a misfire of a cylinder in a vehicle’s engine, but it does not indicate the cylinder in question or the reason for the misfire. When the code features a digit other than zero, the digit indicates the misfiring cylinder number; for instance, if the code is PO302, it indicates misfiring of the number two cylinder.
Some symptoms of a misfiring cylinder include hesitation or stumbling in the engine or an engine that is harder to start than usual. The code may also indicate faulty spark plugs or wires, a faulty coil pack or oxygen sensors, a malfunctioning fuel injector or a burned exhaust valve. Other possibilities include a leaking, blocked or stuck EGR valve or passages or a defective computer. A faulty camshaft position sensor may also be to blame.
If the car experiences symptoms of hesitation or stumbling in the engine, the owner should check the wiring and connections to ensure proper installation. If the catalytic converter is bad, the exhaust may smell like rotten eggs, which indicates the part likely requires replacement.
It is also possible that a vacuum leak in the intake manifold, a stuck EG valve, or air bypassing the airflow sensor causes random misfires that trigger the PO300 code. Repairing these conditions corrects the code.