P0304 - Cylinder 4 Misfire Detected

P0304 means cylinder 4 is misfiring often enough for the ECM to detect it. The repair path usually becomes clear once you confirm whether the fault follows ignition parts, stays with fuel delivery, or points to a mechanical issue.

Treat the code as the starting point: compare symptoms, likely causes, and repair paths before pricing parts or moving into an estimate.

Cylinder 4 Misfire Diagnostic Path

Treat P0304 as a focused cylinder 4 fault until spark, fuel, air, and compression evidence points to the repair.

Inspection Priority

• Inspect the spark plug before replacing the coil
• Swap coil or plug only when the test can prove whether the misfire moves
• Check injector command, fuel delivery, and compression if the fault stays on the same cylinder

Estimate Guidance

• Quote ignition parts after plug or coil testing supports the fault.
• Use injector or compression diagnosis when swap testing does not move the misfire.
• Check catalyst-risk history before pricing downstream converter work.

Use the blueprint links to verify the likely system, then continue the estimate with the same vehicle and code context.

Code Overview

P0304 means cylinder 4 is misfiring often enough for the ECM to detect it. The repair path usually becomes clear once you confirm whether the fault follows ignition parts, stays with fuel delivery, or points to a mechanical issue.

Common Causes

• Ignition coil breakdown under load or heat soak
• Spark plug fouling, worn electrodes, incorrect gap, or oil contamination
• Injector leakage causing cold-start misfire or fuel-washed plug tips
• Low compression, valve sealing problems, or worn valvetrain components
• Small head gasket seep causing overnight coolant intrusion into one cylinder
• Vacuum leaks, intake runner leaks, or PCV leaks affecting cylinder balance

Symptoms

• Rough idle or steady vibration
• Hesitation or weak pull during acceleration
• Flashing MIL if the misfire is strong enough to threaten the catalyst

Diagnostic Steps

1. If the misfire happens only on cold start, inspect for injector leakage, coolant seep, or valve sealing issues before replacing coils only.
2. If the misfire appears under acceleration or load, inspect ignition coil output, plug gap, plug condition, and coil boots for breakdown.
3. If the same-cylinder misfire stays after a coil swap, move toward injector testing, compression testing, and leak-down testing.
4. If coolant loss appears with an overnight rough start, head gasket seep or coolant intrusion suspicion increases.
5. If the misfire repeats with clean plugs and no obvious ignition fault, compression and leak-down testing become important.
6. If multiple cylinders misfire with positive fuel trims, inspect vacuum leaks, intake leaks, PCV leaks, and shared fuel delivery.

Diagnostic Insight

P0304 usually becomes clear once cylinder 4 is tested by condition, swap movement, injector behavior, and mechanical sealing.

• A misfire that moves with the coil or plug usually confirms ignition; a fixed cylinder 4 misfire needs injector, compression, and leak-down checks.
• Cold-start-only cylinder 4 misfire can come from injector leakage, coolant seep, or valve sealing that changes as the engine warms.
• Misfire under load points toward coil breakdown, plug gap, plug fouling, or boot tracking before replacing unrelated parts.
• Coolant loss, a washed-clean plug, or overnight rough start raises suspicion for small head gasket seep.

Repair Difficulty

Moderate

General difficulty estimate for the most common repair path.

Likely Repairs & Cost Guides

Use symptoms, scan data, and quick checks to confirm the likely repair path before pricing parts. The estimator helps compare repair paths before replacing anything unnecessarily.

Related OBD Codes

Browse nearby code-family pages when the same fault pattern overlaps across systems, such as misfire, lean fuel trim, EVAP sealing, cooling, or charging faults.

Next Steps

Move from code lookup to diagnosis, then estimate the likely repair only after symptoms, causes, and checks point to the same path.