P0304 means your engine's computer has identified consistent misfires in cylinder 4. On most inline-4 engines, cylinder 4 is at the rear near the firewall, which can make access slightly harder. The fault is isolated to that cylinder's ignition, fuel, or compression system.
P0304 means your engine's computer has identified consistent misfires in cylinder 4. On most inline-4 engines, cylinder 4 is at the rear near the firewall, which can make access slightly harder. The fault is isolated to that cylinder's ignition, fuel, or compression system.
P0304 is a misfire-related diagnostic trouble code classified as high severity. When your vehicle's ECU detects this condition, it stores P0304 and illuminates the check engine light.
If your vehicle has triggered P0304, you may notice one or more of these symptoms:
Here are the most common causes of P0304, ranked by how likely they are to be the culprit:
No - address this immediately. A persistent misfire on cylinder 4 will damage the catalytic converter and may cause sudden power loss. Do not ignore a flashing check engine light. Have the vehicle diagnosed as soon as possible.
Follow these steps to pinpoint the root cause of P0304:
Repair costs for P0304 vary depending on the root cause and your vehicle. Here are typical estimates:
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0304 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:
These codes are often seen alongside P0304 or indicate related issues:
P0304 tells you that the engine control module has detected repeated misfires on cylinder 4. The ECU counts misfires by monitoring tiny variations in crankshaft speed and flags the code when cylinder 4 misfires exceed the allowed threshold.
On inline-4 engines, cylinder 4 is typically closest to the firewall at the rear of the engine bay. Access can be tighter, especially for spark plugs and coils. On some vehicles you may need to remove an engine cover or intake components.
The misfire itself does not usually damage the engine block, but unburned fuel washing the cylinder walls can dilute oil and accelerate ring wear over time. The bigger immediate risk is catalytic converter damage from overheating.
This is usually caused by an incorrectly gapped plug, the wrong plug type for your engine, a plug not seated properly, or a damaged ignition coil boot. Remove the cylinder 4 plug, verify the gap and part number, and re-seat it.
It is not strictly necessary, but if the vehicle has high mileage (over 100,000 km) and the other coils are original, replacing the full set is a good preventive measure. Coils of the same age tend to fail in sequence.
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