P0304 Code – Cylinder 4 Misfire Detected

What Does the P0304 Code Mean

The P0304 code indicates that the engine control unit (ECU) has detected a misfire in cylinder 4. A misfire occurs when the air-fuel mixture inside the cylinder fails to ignite correctly during the combustion process.

Modern vehicles use sensors to monitor crankshaft speed and combustion stability. When the ECU detects irregular combustion repeatedly in cylinder 4, it stores the Diagnostic Trouble Code P0304 and activates the Check Engine Light.

Misfires can significantly affect engine performance, fuel economy and emissions.

Symptoms of the P0304 Code

Drivers may experience several symptoms when the P0304 code appears.

Common symptoms include:

• Check Engine Light illuminated
• Engine vibration or shaking
• Rough idle
• Reduced engine power
• Poor acceleration
• Increased fuel consumption

If the misfire becomes severe, the Check Engine Light may flash, warning that the catalytic converter may be damaged.

Common Causes of the P0304 Code

Several electrical or mechanical problems may cause cylinder 4 to misfire.

Faulty Spark Plug

A worn or fouled spark plug in cylinder 4 may prevent proper ignition of the air-fuel mixture.

Spark plug wear is one of the most common causes of misfires.

Faulty Ignition Coil

Many modern engines use coil-on-plug ignition systems. If the ignition coil responsible for cylinder 4 fails, the spark may become weak or disappear entirely.

Fuel Injector Problems

A malfunctioning fuel injector may deliver too little fuel or none at all.

Possible issues include:

• clogged injector
• injector electrical fault
• injector stuck closed

Vacuum Leaks

Vacuum leaks around the intake manifold may allow excess air into the engine, creating an incorrect air-fuel mixture.

This imbalance can lead to misfires.

Engine Compression Problems

Mechanical engine faults can also cause cylinder misfires.

Examples include:

• worn piston rings
• damaged valves
• head gasket failure

Low compression prevents proper combustion inside the cylinder.

How to Diagnose the P0304 Code

Diagnosing a misfire requires checking several engine systems.

Typical diagnostic steps include:

1. Connect an OBD2 scanner to confirm the P0304 code.

2. Inspect the spark plug in cylinder 4.

3. Swap ignition coils between cylinders to see if the misfire follows the coil.

4. Check the fuel injector operation.

5. Perform a compression test if necessary.

These steps help determine whether the problem is electrical, fuel-related or mechanical.

Can You Drive With the P0304 Code

Driving with a misfire is possible for short distances, but it should not be ignored.

A persistent misfire may cause:

• catalytic converter damage
• increased emissions
• reduced engine performance
• higher fuel consumption

If the Check Engine Light begins flashing, the vehicle should be repaired immediately.

How to Fix the P0304 Code

The correct repair depends on the root cause of the misfire.

Common repairs include:

• replacing worn spark plugs
• replacing a faulty ignition coil
• cleaning or replacing the fuel injector
• repairing vacuum leaks
• repairing engine compression problems

After repairs are completed, the diagnostic code should be cleared and the vehicle should be tested to ensure normal operation.

Conclusion

The P0304 code indicates a misfire in cylinder 4. This issue is often caused by faulty spark plugs, ignition coils or fuel injectors, but mechanical engine problems may also be responsible.

Timely diagnosis and repair help restore smooth engine performance and prevent further damage.