What does it mean?
This diagnostic code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996 and newer), although specific repair steps may vary slightly by model. Therefore, this engine code article applies to Chevrolet, Ford, Nissan, Toyota, Dodge, Honda, GMC, etc.
Basically, this P0300 code means that the car’s computer has detected that not all of the engine’s cylinders are functioning properly.
A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that misfired. A P0302 code, for example, would tell you that cylinder number two is misfiring. Unfortunately, a P0300 does not specifically tell you which cylinder(s) is/are failing, nor why.
P0300 Mechanic
Symptoms
Symptoms may include:
the engine may be harder to start
the engine may stumble/stall and/or hesitate
other symptoms may also be present
Causes
A P0300 code can mean that one or more of the following has occurred:
Faulty spark plugs or spark plug wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Stuck/blocked/leaking EGR valve/passages
Faulty camshaft position sensor
Faulty computer
Possible Solutions
If there are no symptoms, the simplest thing to do is to reset the code and see if it returns.
If there are symptoms such as the engine stumbling or hesitating, check all the wires and connectors leading to the cylinders (i.e., spark plugs). Depending on the lifespan of the ignition components in the car, it may be wise to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap and rotor (if applicable). Otherwise, check the coils (e.g., coil packs). In some cases, the catalytic converter has failed. If you smell rotten eggs in the exhaust, your catalytic converter needs to be replaced. I have also heard in other cases that the problems were faulty fuel injectors.
Random misfires that jump from one cylinder to another (read: P030x codes) will also set a P0300 code. The underlying cause is often a lean fuel condition, which can be due to a vacuum leak in the intake manifold or unmetered air getting past the mass airflow sensor, or an EGR valve that is stuck open.
It is helpful to have access to a factory service manual and an advanced scan tool to properly diagnose a random misfire DTC.