P0304 Code – Meaning, Causes, Symptoms (& How To Fix)

If you have scanned your engine computer to see what’s causing trouble and found the P0304 code, you might wonder what is going on and how to fix it. Thankfully, it doesn’t take a lot of troubleshooting to get to the bottom of this issue.

We will be discussing the meanings of P0304's trouble code in this guide. This guide will also explain what could happen if this happens and how you can fix it. 

Definition of code P0304

P0304 – Cylinder #4 Misfire Detected

What is the P0304 Code?

P0304 DTC means that there’s an engine misfire in cylinder 4. The Powertrain Controller Module (PCM), which detects a misfire in the fourth cylinder of the car motor and alerts it to issue this code, will send a message to the engine. It’s a generic code, applying to all OBD-II equipped models.

The P0304 trouble codes can be used to diagnose general problems, but repairs will depend on what make and model of vehicle you have. Multiple misfires could result in the appearance of P0304 along with related codes. This code indicates problems with other types of cylinders. 

Trouble code P0304 symptoms

Performance issues can be noticed if an engine cylinder fails to fire. The Check Engine Light will also be on when you set the code, alerting to any problems. 

Here are some of the top symptoms you’ll notice with the P0304 DTC.

The P0304 Code: Causes

A variety of issues can cause a car to stop working. In some cases, the issues could stem from the ignition system, but it’s also possible that something has failed with the fuel system.

Below are the most frequent causes for the P0304 code trouble code.

What is the P0304 Code and How Serious?

Extreme – The P0304 DTC is considered one that should be taken seriously. A misfired engine can make it dangerous to drive, particularly if it causes the vehicle to stop in the middle. You could be in danger if the engine suddenly stops.

It’s important to have the P0304 trouble code examined as soon as possible. Other engine problems can be caused by misfiring engines, which will increase the cost of repairs. A tow truck may be required if your vehicle breaks down. 

Which Repairs Are Possible to Fix the P0304 Code

Before you make any repairs, it is important to do a thorough diagnosis. Otherwise, you could spend unnecessary money on parts that aren’t needed.

Here are some possible solutions based on the most common failures.

• Replace spark plug
• Change the ignition coil/wires
• Replace your fuel injector
• Fix leaking valve
• Repair vacuum leak
• Change the piston ring
• Repair leaking head gasket
• Replace computer module

Common errors in P0304 diagnosis

Most often, misfires are caused by spark plugs. However, you should never assume that’s the problem without first performing the diagnostic steps we outline. It is important to inspect the wires, plugs, coils, as well as coil packs before you replace them. 

You can then move on to parts that are not working properly. Also, you should perform a compression test. Additionally, if there are any other trouble codes present, these might help you narrow down what’s going on. 

How to diagnose P0304 Trouble code

When diagnosing the P0304 code as a professional mechanic you will want to use a structured plan. Gathering the code from the scanner is just the first step of figuring out what’s wrong. You will be able to find the solution by following the correct steps.

Below are the main diagnostic steps of the P0304 DTC.

1. Collect the trouble codes with your OBD-II scanner and use freeze frame data to see what’s going on. Use the OBD-II scanner to collect trouble codes and then use freeze frame data to troubleshoot based on any additional codes.
2. Set the code back to default.
3. You can test drive your vehicle and see which codes return.
4. Check the spark plug and ignition coil as well as any coil packs on cylinder 4, to determine whether they are damaged or worn. Replace if necessary. You can try to move over the spark plug and ignition coil to another cylinder to see if you get a trouble code on the other cylinder instead – if so replace the faulty spark plugs and ignition coil.
5. Vacuum leaks can be found in the intake.
6. If this isn’t the problem or the code doesn’t go away, you will need to inspect the fuel injectors. 
7. Older cars may also require you to check the distributor cap or the ignition cables. 
8. A compression test on cylinder 4 is required. 
9. Inspect the timing gear and camshaft, and if needed inspect the timing shaft.
10. If you can't find anything else with diagnostics, it is possible that the PCM needs to be replaced. However, with this problem, you’ll likely see other false codes or problems. 

The estimated cost of repair

Depending on what’s causing the problem, you are looking at a varying amount of repair costs. You will pay less to repair some problems than others. Here are some of the most popular repairs, along with the average labor and parts costs.

• Replace spark plug – $40-$350
• Replace ignition coils/wires – $75-$400
• Install a new fuel injector – $200-$500
• Fix leaking valve – $500-$2,500
• Replace piston ring – $1,000-$5,000
• Repair leaking head gasket – $1,500-$3,000
• Replace computer module – $750-$3,500

The P0304 Code: Mechanics Tips

It is possible to replace the plug if you discover that the cylinder 4 ignition spark plug is causing the code. However, it’s always a better idea to replace all of the plugs at the same time. One plug is likely to fail, and the rest will soon follow. It is easy to stick to a maintenance plan if you replace all the spark plugs at once. 

Also, ensure that spark plugs have a proper gap. It is possible to have a misfire if the gap between spark plugs and metal is not right. Use a spark plug gap tool to ensure the space matches the manufacturer’s specs.

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