The most common cause of a P0304 is a misfire in cylinder 4, usually tied to the ignition system on that cylinder—most often a worn or fouled spark plug or a failing ignition coil. Replacing the suspect plug or swapping the coil for another cylinder can quickly reveal the source, though a proper diagnostic check is recommended to rule out fuel, vacuum, and mechanical issues.
Understanding P0304
P0304 is an OBD-II diagnostic trouble code that signals cylinder 4 misfire. It means the engine control module detected that cylinder 4 did not ignite or burned improperly during combustion. Symptoms can include rough idle, reduced acceleration, and lower fuel economy, and the check engine light may come on. P0304 may appear alone or alongside related misfire codes for other cylinders (P0300 or P0301–P0308).
Most Common Cause
In most vehicles, a cylinder-specific misfire is caused by ignition components on that cylinder. The following items are the usual suspects for cylinder 4:
- Worn, fouled, or damaged spark plug in cylinder 4
- Weak or failed ignition coil for cylinder 4
- Damaged spark plug boot or ignition coil wiring around cylinder 4
- Clogged or malfunctioning fuel injector on cylinder 4
The ignition-related causes are by far the most common. If the plug and coil test OK, technicians proceed to check the fuel system, air intake, and compression.
Why ignition problems are the leading culprit
Spark plugs wear over mileage, coils can degrade with time, and cylinder 4 can be more prone to certain loads depending on engine design. Because ignition components are relatively quick and affordable to test and replace, they remain the primary focus when diagnosing P0304.
How to diagnose and fix
Diagnosing a P0304 involves inspection of ignition components, fuel delivery, and engine compression. A systematic approach helps prevent unnecessary part replacements and protects the catalytic converter from unburned fuel. The following steps outline a practical method:
- Scan the vehicle for codes and review misfire data, including any related codes (P0300, P0301–P0308) and relevant freeze-frame data.
- Inspect the spark plug on cylinder 4 for wear, fouling, oil fouling, or damage; replace if necessary.
- Test or swap the cylinder 4 ignition coil with another cylinder’s coil to see if the misfire follows the coil (coil swap test).
- Check the spark plug boot and wiring for cracks, corrosion, or a poor connection; repair or replace as needed.
- Inspect the fuel injector for cylinder 4—verify operation, cleanliness, and proper spray; test fuel pressure and consider replacing the injector if faulty.
- Look for vacuum leaks around the intake manifold that could affect cylinder 4; repair any leaks found.
- Measure compression in cylinder 4 and compare with the other cylinders to rule out mechanical issues (sticking valve, worn rings, or head gasket problems).
- Clear codes and perform a road test; re-scan to confirm the misfire is resolved and that no new codes appear.
If the misfire persists after addressing ignition components, proceed to the fuel delivery and compression checks, as the root cause may lie there. Timely diagnosis helps prevent catalyst damage and reduces unnecessary repairs.
Summary
A P0304 code points to a cylinder 4 misfire. The most common culprit is an ignition problem on that cylinder—typically a worn or fouled spark plug or a failing ignition coil—followed by potential fuel or mechanical issues if ignition tests pass. A methodical diagnostic approach provides the most reliable path to a fix and helps protect the engine and exhaust system.