What is the code P0304 on a Honda Accord?

P0304 indicates a misfire in cylinder 4.

On most Honda Accords, the engine uses sequential ignition with a separate ignition coil for each cylinder. When cylinder 4 fails to ignite properly, the engine control unit logs P0304 and may illuminate the check engine light. A persistent misfire can cause rough idle, reduced power, higher fuel consumption, and potentially damage to the catalytic converter if not addressed.

What P0304 Means for a Honda Accord

P0304 is one of the cylinder-specific misfire codes in the OBD-II system. It is commonly accompanied by other misfire codes or P0300 (random/multiple misfires) and it points to cylinder 4 as the source of the problem. Diagnosing requires confirming the misfire and then identifying whether the source is ignition, fuel, air, or engine mechanical in nature.

Common causes of P0304 on a Honda Accord

Before you start replacing parts, consider the typical culprits that trigger a cylinder 4 misfire:

• Worn, fouled, or misgapped spark plug in cylinder 4


• Faulty ignition coil or coil connector for cylinder 4


• Clogged, dirty, or leaking fuel injector on cylinder 4


• Low fuel pressure or a malfunctioning fuel delivery system affecting cylinder 4


• Vacuum leaks or intake manifold gasket issues around cylinder 4


• Compression problems in cylinder 4 (bent valve, worn piston rings, head gasket issue)

Starting with the spark plug and ignition coil in cylinder 4 is a practical and common starting point because those parts are easy to inspect and test and account for many misfires.

Diagnosing P0304 on a Honda Accord: a practical approach

Here is a typical diagnostic workflow to verify the source of the misfire and avoid unnecessary part replacement:

1. Use an OBD-II scanner to confirm P0304 and check for related codes (P0300, P0301–P0303) and freeze-frame data.


2. Inspect the spark plug on cylinder 4 for wear, fouling, or damage; replace if necessary and ensure correct gap.


3. Test the ignition coil or coil-on-plug for cylinder 4. If possible, swap the coil with another cylinder to see if the misfire follows the coil.


4. Check the wiring and connectors to the ignition coil and spark plug for corrosion, cracks, or looseness.


5. Inspect the fuel injector for cylinder 4: listen for injector pulse with a stethoscope, check resistance spec, and consider injector cleaning or replacement if faulty.


6. Measure fuel pressure to ensure the engine is receiving adequate fuel; test for a failing fuel pump or clogged filter.


7. Check for vacuum leaks or cracked intake hoses around cylinder 4; use a smoke test if available.


8. Conduct a compression test on cylinder 4 and compare with adjacent cylinders; low compression suggests mechanical issues needing further repair.

If the misfire persists after these checks, professional diagnostics may be needed to explore deeper mechanical problems or ECU-related faults.

Common fixes for P0304 on a Honda Accord

Once you’ve identified the root cause, these fixes are commonly employed to resolve a P0304:

• Replace the cylinder 4 spark plug and verify correct torque and gap


• Replace or swap the cylinder 4 ignition coil or fix any damaged coil boot/connector


• Clean or replace the cylinder 4 fuel injector and ensure proper sealing


• Repair or replace a faulty fuel pump, fuel filter, or address low fuel pressure


• Repair vacuum leaks or replace intake manifold gaskets as needed


• Address mechanical issues such as worn valves, piston rings, or head gasket problems if compression is low

Note: Driving with an active misfire can harm the catalytic converter and increase emissions, so address the issue promptly and have the vehicle inspected if symptoms worsen.

What to do next

If you’re not comfortable performing mechanical diagnostics, book an appointment with a trusted mechanic. If you have access to basic tools and a test light or scan tool, you can perform initial checks (spark plug and coil) to narrow down the source before committing to more expensive repairs.

Summary

P0304 on a Honda Accord denotes a misfire in cylinder 4. The common culprits include ignition components (spark plug, coil), fuel delivery (injector or pressure), vacuum leaks, or compression problems. A systematic diagnostic approach—confirmed with a scan tool, physical inspection, component swapping, and compression testing—helps identify the root cause and guide proper repairs. Addressing a misfire promptly protects the engine and catalytic converter while restoring performance.