When your check engine light comes on and a scan shows the P0304 code, the engine is pointing to a specific problem: cylinder 4 is not firing correctly.
Compared to a general misfire like P0300, which means the issue is happening randomly across different cylinders, P0304 narrows it down to one cylinder. That’s helpful, because it gives you a clear direction for diagnosis.
Even so, a single-cylinder misfire is not something to ignore. When one cylinder stops doing its job properly, the engine becomes unbalanced. Over time, that can affect performance, increase fuel consumption, and in more serious cases, damage components like the catalytic converter.
In this guide, I’ll walk you through what this code really means, what usually causes it, how it shows up while driving, and how to fix it before it turns into a bigger problem.
What Does the P0304 Code Mean?
To understand the P0304 code, it helps to picture what your engine is doing while you’re driving.
Inside the engine, each cylinder is constantly repeating the same process. Air and fuel enter, the piston compresses that mixture, and then the spark plug ignites it. That ignition creates the force that keeps the engine turning.
For the engine to run smoothly, each cylinder needs to fire at the right moment and produce the same amount of power. When one cylinder fails to do that, the engine starts to run unevenly.
That’s what we call a misfire.
Your car’s computer monitors this process by tracking the rotation of the crankshaft. When a cylinder fires properly, the crankshaft speeds up slightly. If a cylinder misfires, that small boost of energy is missing, and the crankshaft slows down for a brief moment.
When this happens repeatedly in cylinder 4, the ECU detects the pattern and stores the engine misfire code P0304.
In simple terms, the system is saying:
“Cylinder 4 is not contributing power consistently.”
P0304 Symptoms Drivers May Notice
A misfire in cylinder 4 doesn’t always feel dramatic at first, but it usually becomes more noticeable as the issue develops.
One of the first signs is a rough idle. When the car is stopped, the engine may feel slightly uneven, almost like it’s struggling to maintain a steady rhythm. This happens because one cylinder isn’t producing power consistently.
As the misfire becomes more frequent, you may begin to feel vibration through the vehicle. This is often noticeable through the steering wheel or the seat, especially at low speeds.
When accelerating, the car may feel weaker than usual. Since cylinder 4 isn’t contributing properly, the engine has less power available, which becomes more noticeable under load.
You may also experience hesitation during acceleration, where the engine briefly stumbles before gaining speed. This can be particularly noticeable when trying to accelerate quickly or merge into traffic.
Fuel consumption can also increase. When combustion isn’t happening efficiently, the engine may use more fuel than necessary, leading to poor fuel economy.
And of course, the check engine light will usually be present. In many cases, it stays solid. But if the misfire becomes severe, it may start flashing. A flashing light is a sign that the issue could damage the catalytic converter if you continue driving.
If you want to better understand how these symptoms develop and how they compare across different situations, this guide on engine misfire symptoms explains it in more detail.
Most Common Causes of the P0304 Code
Because the P0304 code points directly to cylinder 4, the cause is usually something that affects that specific cylinder.
In many cases, the issue starts with the spark plug. Over time, spark plugs wear out or become fouled with carbon deposits or oil. When that happens, they may not produce a strong enough spark to ignite the air-fuel mixture properly.
Another common cause is the ignition coil. The coil provides the high voltage needed to create the spark. If the coil serving cylinder 4 begins to fail, it may produce a weak or inconsistent spark, leading to repeated misfires.
Fuel delivery can also be the problem. The fuel injector for cylinder 4 must deliver the correct amount of fuel at the right time. If it becomes clogged or starts malfunctioning, the cylinder may not receive enough fuel, which disrupts combustion.
Air leaks are another possibility. A vacuum leak near cylinder 4 can allow extra air into the intake system. This throws off the air-fuel mixture and can make combustion unstable, especially at idle.
In some cases, the issue is mechanical. Cylinder 4 needs to compress the air-fuel mixture properly before ignition. If there’s a loss of compression—caused by worn piston rings, leaking valves, or a head gasket issue—the mixture may not ignite correctly.
When that happens, the ECU detects the imbalance and logs the P0304 code.
P0304 vs P0300 Misfire Code
Understanding the difference between misfire codes can make diagnosis much easier.
The P0304 code points specifically to cylinder 4. This allows you to focus on the components related to that cylinder, which often makes troubleshooting more straightforward.
The P0300 code, on the other hand, indicates misfires that occur randomly across multiple cylinders. This type of issue usually points to something affecting the entire engine, such as fuel pressure problems or air intake issues.
If you want to understand how random misfires differ and why they are more complex to diagnose, take a look at our guide on the P0300 code — random engine misfire explained.
Is It Safe to Drive With a P0304 Code?
Whether you can continue driving depends on how severe the misfire is.
If the symptoms are mild—such as a slightly rough idle and a steady check engine light—you can usually drive short distances carefully. However, it’s still important to address the issue soon.
If the engine starts shaking more noticeably or feels weak during acceleration, it’s a sign that the misfire is becoming more serious. At that point, driving should be limited.
A flashing check engine light is a clear warning that the situation is urgent. Severe misfires can allow unburned fuel to enter the exhaust system, which can overheat and damage the catalytic converter.
If you’re unsure how serious your situation is, this guide on check engine light meaning can help you interpret the warning signs correctly.
How Mechanics Diagnose a P0304 Code
Diagnosing a cylinder-specific misfire usually follows a logical process.
The first step is confirming the code using an OBD2 scanner. This ensures that the issue is indeed related to cylinder 4.
Next, the technician inspects the spark plug and ignition coil for that cylinder. These are the most common causes and are relatively easy to test.
One of the most effective techniques is to swap components between cylinders. For example, moving the ignition coil from cylinder 4 to another cylinder. If the misfire moves with the component, the faulty part has been identified.
If the ignition system appears to be working correctly, attention shifts to the fuel injector. The injector may be tested or cleaned to ensure it is delivering fuel properly.
If needed, a compression test is performed to evaluate the internal condition of cylinder 4. This helps determine whether the issue is mechanical.
If you want to learn how to approach some of these checks yourself, this guide on how to diagnose car problems at home explains the process step by step.
How to Fix the P0304 Code
The repair depends on what’s causing the misfire, but many cases are straightforward.
If the spark plug is worn or fouled, replacing it is often enough to fix the issue. This is one of the most common and least expensive repairs.
If the ignition coil is faulty, replacing it will restore proper spark to cylinder 4.
When the issue comes from the fuel injector, cleaning it may restore proper fuel delivery. In some cases, the injector may need to be replaced.
If a vacuum leak is found, repairing the affected hose or gasket will restore the correct air-fuel mixture.
Mechanical problems, such as low compression, are less common but more complex and may require more extensive engine work.
How to Prevent Cylinder Misfires
Preventing misfires comes down to regular maintenance and paying attention to early warning signs.
Replacing spark plugs at the recommended intervals helps maintain a strong and consistent spark. Using good-quality fuel helps keep injectors clean and functioning properly.
Keeping the intake system in good condition ensures that the engine receives the correct air-fuel mixture, which is essential for smooth combustion.
Simple maintenance habits can go a long way in preventing problems like the P0304 code.
Frequently Asked Questions
What does P0304 code mean?
The P0304 code indicates that the engine has detected a misfire in cylinder 4.
Can bad spark plugs cause P0304?
Yes, a worn or fouled spark plug in cylinder 4 is one of the most common causes.
Can I drive with P0304?
Short drives may be possible if symptoms are mild, but the issue should be addressed quickly.
Is P0304 serious?
It can become serious if ignored, especially if it leads to catalytic converter damage.
What causes cylinder 4 misfire?
Common causes include spark plug issues, ignition coil failure, fuel injector problems, vacuum leaks, or low compression.
How do you fix P0304?
The fix depends on the cause but often involves replacing spark plugs, ignition coils, or servicing the fuel injector.
Final Advice
The P0304 code is a clear sign that cylinder 4 is not operating properly. While the issue may start small, it’s not something that resolves on its own.
The good news is that most causes are relatively easy to diagnose and fix when addressed early. Taking action quickly can help you avoid more expensive repairs later.
If your check engine light has led you here, think of it as a useful warning. With the right approach, this is a problem that can usually be resolved without major complications.