All about P0304 CODE: Cylinder 4 Misfire

P0304_code

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The diagnostic trouble code P0304 signifies that a misfire in the engine was detected by the vehicle’s computer system.

To be more specific, this misfire happens in the fourth cylinder.

Are you trying to get rid of code P0304? We’re here to help! 

In this article, we’ll explore what the code P0304 means, its symptoms, causes, and how it can be fixed.


What Does the P0304 Code Mean?

Nowadays, vehicles are equipped with smart computers that can detect cylinder misfires. Detection monitors can specifically spot the cylinder with the problem.

When the engine experiences a misfire, the Check Engine Light will be turned on and you’ll be presented with a number. The last number of the code indicates the cylinder with the error.

So, for instance, if the code shown is P0301, this means the misfire is in cylinder number 1. In our case here, P0304 refers to a misfire in the fourth cylinder. This is a particularly common error.

A misfire means that the cylinder isn’t firing as it should.

Combustion takes place in the cylinder and powers the vehicle. That’s why when an engine misfires, the vehicle loses power. If an engine has four cylinders, and one of them misfires, this means that there will be a 25% drop in the power of the car.

The P0304 code shows that there’s a misfire, but it doesn’t show the reason behind it. That’s why a full diagnosis is crucial.

Repairing P0304 immediately is a must. Failing to check the error right away could result in unsafe conditions while driving the vehicle. It could also lead to ignition failure and damage in the catalytic converter. 


What Are the Symptoms of the P0304?

Along with shown code and the Check Engine light that flashes, these are some other symptoms that you may notice.

  • The car jerks during acceleration
  • The engine may be more difficult to start
  • A drop in the engine’s power
  • A fuel smell from the vehicle’s exhaust
  • A decrease in the fuel’s economy

What Are the Possible Causes of the P0304?

There are many possible causes behind the P0304 code. The most common causes include defective or worn-out spark plugs, defective spark plug wires, and faulty distributor caps.

Other causes may be:

  • Malfunctioning fuel injector
  • Low fuel pressure
  • Poor quality of fuel
  • Faulty O2 sensor
  • Faulty Mass Airflow Sensor
  • Leaking head gasket
  • Vacuum leak

How is the P0304 Diagnosed and Fixed?

Here are a few simple steps to diagnose and repair cylinder misfire.

Find Out if Other Codes Exist

This is a common first step in many diagnosis processes. You’ll need to check if there are other codes using a scanning tool.

If that’s the case, you’ll need to address the other codes first and take note of freeze frame data.

Examine the Wires and Coils

The next step is to visually inspect the wires for any damage. Also, be sure to check any worn-out parts or leaks, and find any loose connectors at the ignition coils and tighten them if necessary.

Inspect the Spark Plugs and Spark Plug Wires

Checking if the spark plug and spark plug wires are the problem require moving things around a bit. To test the spark plug, move the cylinder 4 plug to cylinder 1, for instance.

If the misfire indicates code P0301, then you’ll know that the plugs are defective. Replace the spark plugs and spark plug wires and check if the code is still present.

The same applies to individual coil packs. Remove the cylinder 4 coil and exchange it with the cylinder 1 coil. If the code P0301 is shown, you’ll know that the coil pack is damaged and must be replaced.

Check the Fuel Pressure

If there are no issues with the previous steps, it’s time to check your fuel system. You need to make sure that the engine is getting an adequate amount of fuel because inadequate or low fuel pressure could cause misfires. 

Take a look at the fuel injectors and the wiring to check that they’re not damaged and functioning properly.

If none of the above-mentioned problems exist, the issue might be with the mechanical system of your vehicle. In that case, you'll need to perform  down and compression tests.



What are the Common Mistakes When Diagnosing P0304?

If you skip a step in the diagnosing process, this could mean that you’ll replace a component that doesn’t necessarily need to be replaced.

It’s best to follow all the steps to determine what exactly you need to repair.


About the Author JamesL Davis

James is a certified auto technician specializing in commercial vehicles. With 30 years of experience under his belt, James has encountered almost every type of automotive issue there is! Besides his day job at the repair shop, he is also an amateur race car driver.

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