What is OBD-II Code P0304 – Cylinder 4 Misfire Detected

What is OBD-II Code P0304 – Cylinder 4 Misfire Detected

As a mechanic, I understand how frustrating it can be for vehicle owners to see the check engine light suddenly turn on. In most cases, the check engine light indicates that something is wrong with the vehicle’s engine, but it doesn’t tell us what specifically is causing the issue. This is where OBD-II codes come into play. These codes are numerical and alphanumeric codes that provide specific information related to the malfunctioning of a vehicle’s engine. One such code is P0304, which indicates that cylinder 4 misfire has been detected.

If you’re not familiar with what cylinder 4 is, consider this: the engine in most modern vehicles is divided into a particular number of cylinders, usually four, six, or eight. Each cylinder is essentially a miniature combustion engine, responsible for burning fuel and air to generate the power that drives your car. Cylinder 4 is just one of these cylinders.

Now, let’s dive into OBD-II Code P0304 and what it means for your vehicle.

Symptoms of a Misfiring Cylinder 4

A cylinder misfire occurs when there is a lack of combustion in one or more of the engine cylinders. In the case of OBD-II Code P0304, the misfire is happening in cylinder 4. Common symptoms of a misfiring cylinder include:

1. Rough idling: If your vehicle shakes or vibrates while at a stoplight or when in park, this could be due to cylinder misfire.

2. Lack of power: A misfiring cylinder means that one of the engine cylinders isn’t working correctly, which can lead to poor performance, including loss of power.

3. Engine surging: An engine that is surging or struggling to maintain its speed may be experiencing a cylinder misfire.

4. Stalling: If the engine stalls while driving, there may be a cylinder misfire issue that needs to be addressed.

Causes of a Misfiring Cylinder 4

A misfiring cylinder can occur due to several reasons, here are some of the most common causes:

1. Failed ignition system: The ignition system is responsible for sending an electrical spark to ignite the fuel and air mixture in the engine’s cylinders. A faulty ignition system can cause a misfire.

2. Fuel delivery problems: A malfunctioning fuel pump, clogged fuel injectors, or a fuel filter issue can cause misfires.

3. Vacuum leaks: A vacuum hose that has become disconnected or a leak in a vacuum line can cause a lean fuel mixture, which can lead to misfires.

4. Mechanical problems: Worn piston rings or cylinder walls, a damaged or worn camshaft, or a broken valve spring can all lead to misfires.

Diagnosing Cylinder 4 Misfire

When faced with a misfiring cylinder, a mechanic’s first step is to diagnose the problem. Here’s what you can expect during this process:

1. Use an OBD-II scanner to retrieve the code: A mechanic will use an OBD-II scanner to retrieve the code and determine that the issue is, in fact, a misfire in cylinder 4.

2. Check the spark plugs: Worn, damaged, or poorly gapped spark plugs can cause a cylinder misfire. A mechanic may remove and inspect these components to determine if they need to be replaced.

3. Check the ignition coil: If the spark plugs appear to be in good condition, then it’s possible that the ignition coil is malfunctioning.

4. Check the fuel system: A mechanic will want to ensure that the fuel system is delivering fuel correctly to the engine. They may inspect the fuel injectors, fuel pump, or fuel filter.

5. Check other engine components: If none of the above issues appear to be the cause of the cylinder misfire, a mechanic may take a closer look at other engine components, such as the camshaft or valves.

Repairing Cylinder 4 Misfire

Once a mechanic has diagnosed the cause of the cylinder misfire, they can begin to make repairs. Here are a few examples of what a repair job might entail:

1. Replacing spark plugs: If the spark plugs are worn or damaged, then they will be replaced with new ones.

2. Replacing ignition coils: If the ignition coils are malfunctioning, a mechanic may replace them.

3. Repairing the fuel system: Depending on the cause of the fuel system issue, this could involve replacing a fuel filter, repairing a fuel pump, or cleaning fuel injectors.

4. Replacing engine components: If other engine components are the root cause of the cylinder misfire, then they will need to be repaired or replaced.

Frequently Asked Questions

1. How long can I drive with a misfiring cylinder?

Driving with a misfiring cylinder can lead to additional issues, including damaging the catalytic converter or engine components. It’s essential to have the problem diagnosed and repaired as soon as possible.

2. Why is cylinder 4 misfiring?

There are several possible reasons for a misfiring cylinder, including problems with the fuel system, ignition system, or engine components.

3. Can I drive my vehicle while it has the P0304 misfire code?

While it’s possible to drive your vehicle with a P0304 misfire code, it’s not recommended. The issue can lead to additional problems down the line, and it’s best to have it repaired as soon as possible.

4. Can I fix cylinder 4 misfire myself?

While some issues, such as replacing spark plugs, can be done by a knowledgeable vehicle owner, most cylinder misfire issues require a mechanic’s expertise.

5. How much should it cost to repair cylinder 4 misfire?

Costs vary based on the root cause of the issue and your vehicle’s make and model. In general, repairs can range from $100 to $1,000 or more.


A cylinder misfire, specifically cylinder 4 misfire detected by OBD-II Code P0304, can be a frustrating issue for vehicle owners. Fortunately, with the help of a skilled mechanic, this issue can be identified and repaired. If you’re experiencing any of the symptoms of a misfiring cylinder, we recommend having your vehicle checked out by a professional mechanic as soon as possible.

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