P0304 – Cylinder 4 Misfire Detected means the engine control unit has identified that cylinder 4 is not firing properly. A single-cylinder misfire can cause rough idle, hesitation, loss of power, and increased fuel consumption. If ignored, misfires can also damage the catalytic converter due to unburned fuel entering the exhaust. This guide explains what P0304 means, the symptoms, causes, diagnosis steps, and the most effective repair solutions to restore smooth engine operation.
What Does P0304 Mean?
P0304 is a generic OBD-II trouble code that indicates the ECU has detected a misfire event occurring specifically in cylinder 4. The ECU monitors crankshaft speed, ignition timing, and fuel delivery to determine when a cylinder produces insufficient combustion power. When cylinder 4 repeatedly misfires or contributes less torque than expected, P0304 is stored.
Timely diagnosis is crucial because ongoing misfires can overheat the catalytic converter, leading to costly failures.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Ignition / Fuel / Air / Mechanical
- Difficulty Level: Easy–Moderate
- Estimated Repair Cost: €50–€350+
- Last Updated: 5 December 2025
Real-World Example / Field Notes
A compact sedan arrived with a noticeably rough idle and reduced acceleration. Misfire counters on the scan tool confirmed repeated misfires on cylinder 4. Pulling the spark plug revealed heavy carbon buildup and worn electrodes. After replacing the spark plug and ignition coil, the engine ran smoothly and P0304 cleared immediately.
Another case involved intermittent misfires on rainy days. Moisture had seeped into the coil boot on cylinder 4, causing spark leakage. Replacing the boot and sealing the coil eliminated the misfire permanently.
Symptoms of P0304 – Cylinder 4 Misfire Detected
- Rough idle: Noticeable shaking or vibration from the engine.
- Loss of power: Hesitation or stumbling under acceleration.
- Poor fuel economy: Misfires waste fuel and reduce efficiency.
- Check engine light: May flash during severe misfires.
- Strong fuel smell: Unburned fuel may reach the exhaust.
- Stalling: Severe cases may cause intermittent stalling.
Common Causes of P0304
Most Common Causes
- Worn or fouled spark plug in cylinder 4
- Faulty ignition coil or coil boot
- Clogged or leaking fuel injector (cylinder 4)
- Vacuum leak near the intake runner for cylinder 4
- Dirty or failing mass airflow (MAF) sensor
Less Common Causes
- Low cylinder compression from valve or piston wear
- Camshaft or crankshaft position sensor faults
- Fuel pressure too low due to pump or regulator problems
- ECU driver malfunction (rare)
- Carbon buildup affecting cylinder 4 combustion
Diagnosis: Step-By-Step Guide
Diagnosing P0304 requires systematic inspection of ignition components, fuel delivery, intake system integrity, and mechanical condition.
Tools You’ll Need
OBD-II scanner with misfire counters, spark tester, fuel pressure gauge, multimeter, compression tester, smoke machine, and basic hand tools.
- Check freeze-frame data: Identify RPM, engine load, and temperature when the misfire occurred.
- Inspect the spark plug: Look for wear, fouling, cracks, or oil deposits.
- Swap ignition coils: Move coil 4 to another cylinder to see if the misfire follows the coil.
- Check fuel injector: Perform an injector balance test or swap injectors to identify flow issues.
- Check for vacuum leaks: Smoke-test the intake area near cylinder 4.
- Clean or test the MAF sensor: Under-reported airflow can cause lean misfires.
- Review fuel trims: Lean trims indicate air leaks; rich trims may point to injector problems.
- Test fuel pressure: Confirm pump and regulator are providing proper pressure.
- Perform compression test: Low compression means a mechanical issue affecting cylinder 4.
- Verify cam/crank timing: Timing faults can cause intermittent misfires.
Pro Tip: If the misfire occurs mainly under load or at higher RPM, suspect the ignition coil — weak coils frequently fail under high voltage demand.
Possible Fixes & Repair Costs
- Replace spark plug (cylinder 4): €20–€60
- Replace ignition coil: €50–€120
- Clean or replace fuel injector: €120–€250
- Repair vacuum leak: €50–€150
- Clean or replace MAF sensor: €20–€150
- Repair mechanical engine issues: €300–€900+
Can I Still Drive With P0304?
Driving with P0304 is not recommended. Misfires can cause severe catalytic converter damage due to unburned fuel reaching the exhaust. If the check engine light is flashing, stop driving immediately and diagnose the issue before further damage occurs.
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Related Cylinder Misfire Codes
Compare nearby cylinder misfire trouble codes with similar definitions, fault patterns, and diagnostic paths.
Key Takeaways
- P0304 indicates a misfire specific to cylinder 4.
- Common causes include spark plug, ignition coil, injector, or vacuum leak problems.
- A flashing MIL means severe catalyst-damaging misfires.
- Most fixes are inexpensive when diagnosed early.
FAQ
What is the most common cause of P0304?
Ignition coil and spark plug failures are the most common sources of a cylinder 4 misfire.
Can a vacuum leak only affect cylinder 4?
Yes. A leak near the intake runner for cylinder 4 can create a localized lean condition.
Is it safe to drive with a cylinder 4 misfire?
No. Driving with a misfire can damage the catalytic converter and reduce drivability.
How do I know if the injector is bad?
A balance test or swapping injectors between cylinders will show whether the injector is the cause.
Should I replace all spark plugs or just one?
It is recommended to replace all spark plugs if they are worn or the vehicle has high mileage.