Yes—P0304 indicates a misfire in cylinder 4 and should be investigated promptly. While a single, occasional hiccup may be tolerable, repeated misfires can damage the catalytic converter, reduce engine efficiency, and increase emissions.
What P0304 Means
P0304 is a diagnostic trouble code stored by the vehicle's engine computer (PCM) when it detects that the spark has failed to ignite in cylinder 4. This can affect engine smoothness and power and is commonly accompanied by other misfire codes for other cylinders or a P0300 code for random misfires.
Typical symptoms
Symptoms you might notice include rough idle, hesitation or stumble during acceleration, reduced power, engine vibration, and higher fuel consumption. In some cases the check engine light may come on, and in severe situations the engine may run very poorly.
- Rough or unstable idle for cylinder 4
- Engine misfire during acceleration or cruising
- Noticeable loss of power or shuddering
- Increased fuel consumption
- Possible check engine light or blinking MIL
Persistent misfires should be treated seriously, especially if the engine's efficiency drops or if the warning light remains on after a brief drive.
Common Causes
Several issues can cause a cylinder 4 misfire. The following categories cover the main possibilities, spanning ignition, fuel, and mechanical health.
- Worn or fouled spark plug for cylinder 4
- Defective ignition coil or coil-on-plug on cylinder 4
- Clogged or leaking fuel injector serving cylinder 4
- Vacuum leak near cylinder 4 or intake manifold gasket issue
- Low fuel pressure or faulty fuel pump
- Compression problems in cylinder 4 (bad valve, piston, or head gasket)
- Faulty wiring or connector to the spark plug/coil on cylinder 4
Because misfires can stem from several systems, proper diagnosis often requires scanning for stored misfire data, inspecting ignition components, and sometimes performing compression tests.
What to Do If You See P0304
If your vehicle shows P0304, follow a practical, step-by-step diagnostic approach. The following actions help identify the root cause without risking further damage.
- Check for other trouble codes and review freeze-frame data to understand operating conditions when the misfire occurred.
- Inspect the spark plug on cylinder 4 for wear, fouling, or damage; replace if needed.
- Test or swap the ignition coil for cylinder 4 to see if the misfire follows the coil.
- Check fuel delivery to cylinder 4, including injector operation and fuel pressure with the manufacturer's spec.
- Look for vacuum leaks around the intake manifold or hoses near cylinder 4; fix any leaks.
- Perform a compression test on cylinder 4 to assess mechanical health if ignition and fuel checks are inconclusive.
After completing these steps, you may be able to address the issue or determine that professional diagnostics or repair is required, especially if the problem involves internal engine wear or exhaust system damage.
When to Seek Professional Help
If you cannot quickly identify and fix the problem, or if you notice persistent rough running, loss of power, or misfires under load, seek a professional mechanic. A shop can perform advanced diagnostics, including fuel trim analysis, coil and injector testing with scope equipment, and, if needed, a compression check or leak-down test.
Summary
P0304 signals a misfire in cylinder 4. It is not always immediately dangerous, but it can lead to engine damage, decreased performance, and costly repairs if ignored. Addressing the root cause—whether ignition, fuel delivery, or mechanical issues—early minimizes risk and helps restore engine smoothness and efficiency.