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P0304 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


The P0304 trouble code indicates that the cylinder #4 in your vehicle is misfiring.


A P0304 trouble code tells you that your cylinder 4 is experiencing some kind of misfiring. A cylinder misfire occurs when an inadequate or insufficient amount of fuel is burning in a cylinder. As you know, your vehicle needs to burn fuel efficiently for the engine to be able to operate properly. Operation of your engine is dependent on fuel combustion, as fuel combustion is the primary power source of the motor.

If one or more of your cylinders are misfiring, this can be caused by a faulty ignition system, internal engine failure, or a problem with your fuel system. When a P0304 trouble code occurs, this is something that should be fixed as soon as possible. Long term driving with this trouble code can lead to more instances of that cylinder and other cylinders misfiring, which can result in damaging the engine. This trouble code commonly occurs when you have worn out spark plugs, faulty ignition coils, or worn out spark plug wires in your vehicle.


Though this trouble code indicates that there is an issue with cylinder 4 misfiring, P0304 trouble code P0304 can be caused by several other issues with your vehicle including:

  • Having worn out or faulty spark plugs.
  • Having faulty spark plug coils or wires
  • Distributor failure.
  • Having a faulty fuel injector.
  • Having a leak in the vacuum.
  • Low fuel pressure.
  • Having a defective camshaft sensor.
  • Having a defective crankshaft sensor.
  • That your engine timing is off.
  • Having a leaky head gasket.
  • Low engine compression.
  • Poor fuel quality.


There are some common symptoms that the mechanic and/or driver will notice when encountering a P0304 trouble code.

  • The check engine light will come on.
  • The check engine light will flash.
  • The engine may run rough or shaky.
  • There is a lack of power from the engine.
  • You will smell fuel from the exhaust.
  • The vehicle may hesitate or jerk when accelerating.


There are several steps that a mechanic should work through when diagnosing a a P0304 trouble code:

  • The mechanic should start by hooking up their OBD2 scan tool to the DLC port to see which codes are present.
  • The freeze frame data will reflect every code found and should be noted for use on the test drive.
  • The mechanic will then clear the codes and road test the vehicle to verify the trouble code data that appeared on the freeze frame.
  • They will then perform a visual inspection of the vehicle to look for worn out or broken components, damaged wires, and visible leaks.
  • Then, they will use the scan tool to monitor the misfire counter for that cylinder.
  • The scan tool will then be used to analyze long-term fuel trims to determine if fuel issues are present.
  • They will also check the intake air pressure to see if there are any vacuum leaks present.

Common mistakes

The most common mistakes are when certain components are left untested, or when unnecessary or unrelated components are replaced. If the above diagnosis steps are followed accordingly, it will minimize the chances of making an error when correcting this trouble code. Keep in mind that the entire system must be tested before determining what needs to be repaired.

How serious is this?

A P0304 trouble code should not prevent the vehicle from driving it to a safe location, but you should drive it to a safe location or mechanic as soon as you notice symptoms to prevent further damage from happening. For that reason, it is strongly advised that you have this repaired as soon as possible to ensure the safe operation of the vehicle.

What repairs can fix the code?

Here are the most common repairs that will fix the code:

  • Replacing the spark plugs.
  • Replacing the cap and rotor, coil pack, or plug wires (depending on the vehicle).
  • Fixing any air leaks.
  • Repairing the fueling faults.

Related codes

None listed.


Though a P0304 trouble code may not be immediately detrimental to a vehicle, it is advised that you take the vehicle to a mechanic as soon as you start noticing these signs and symptoms.

P0304 Code – What Does It Mean & How To Fix It