P0304 — Cylinder 4 Misfire Detected

P0304 indicates that the engine control system has detected a misfire on cylinder 4.

A misfire occurs when cylinder 4 does not properly burn the air-fuel mixture during combustion. The problem can be related to ignition, fuel delivery, air intake, compression, engine timing, or an electrical fault.

The code identifies cylinder 4 as the cylinder where the misfire was detected, but it does not automatically identify which component has failed.

Start by confirming the trouble code with a diagnostic scanner and checking for additional codes. Other cylinder-specific misfire codes and fuel-trim codes can provide useful diagnostic clues.

Confirm the cylinder numbering for the specific engine before testing. Cylinder numbering can vary between engine designs and manufacturers.

If the vehicle uses interchangeable ignition coils, a technician may swap the cylinder 4 coil with another cylinder and monitor the misfire data.

If the misfire moves to the cylinder receiving the suspected coil, the ignition coil becomes a strong suspect. This type of component testing should be performed according to the vehicle manufacturer's diagnostic procedure.

A faulty fuel injector can cause cylinder 4 to misfire by delivering too much fuel, too little fuel, or no fuel.

Check the injector's electrical circuit, connector, control signal, and operation before replacing it. Fuel pressure and overall fuel delivery should also be considered when necessary.

If the ignition and fuel systems appear to be functioning correctly, perform a compression test and compare cylinder 4 with the other cylinders.

Low compression may indicate a valve problem, piston or piston-ring problem, head-gasket failure, or another internal engine fault.

An air leak near cylinder 4 can create an incorrect air-fuel mixture and contribute to a misfire.

Inspect the intake manifold, gaskets, vacuum hoses, PCV system, and other nearby intake components.

P0304 identifies cylinder 4 as the cylinder where the misfire was detected. It does not automatically prove that the spark plug, ignition coil, injector, or another component has failed.

A proper diagnosis should check spark, fuel, air, compression, timing, and electrical circuits before replacing expensive components.

P0304 can be serious when the misfire is continuous or severe. A severe misfire can allow unburned fuel to enter the exhaust system and may cause excessive heat and catalytic-converter damage.

If the check engine light is flashing or the engine is running very badly, the vehicle should be diagnosed promptly.

Avoid prolonged driving while the engine is actively misfiring.

If the engine is shaking severely, losing significant power, stalling, or the check engine light is flashing, stop driving when it is safe to do so and have the vehicle properly diagnosed.

P0304 is a diagnostic starting point, not a parts-replacement command.

Before replacing components, systematically check SPARK, FUEL, AIR, COMPRESSION, TIMING, and the related electrical circuits.