The P0304 code engine indicates a misfire in cylinder 4, where the air-fuel mixture fails to ignite properly. Ignoring this check engine light warning can lead to poor acceleration, increased emissions, and even catalytic converter failure. At CarAutomobileHub, we’ll guide you through diagnosing the issue with an OBD II scanner, troubleshooting common causes like faulty ignition coils or clogged fuel injectors, and restoring your engine’s performance.
For general misfire issues, check our P0300 Code Guide.
Symptoms of a P0304 Code
- Flashing check engine light (severe misfire detected).
- Rough idling or engine vibrations.
- Reduced power during acceleration.
- Increased fuel consumption.
- Engine stalling or hesitation.
Pro Tip: Use an OBD II scanner to confirm the code and monitor live misfire data.
Top Causes of a P0304 Cylinder 4 Misfire
1. Ignition System Failures
- Faulty ignition coil or spark plug: A cracked coil or worn plug in cylinder 4 disrupts spark generation.
- Damaged spark plug wires: Corrosion or breaks prevent electricity from reaching the plug.
Fix: Swap the ignition coil with another cylinder. If the misfire moves, replace the coil.
2. Fuel Delivery Issues
- Clogged fuel injector: Deposits block fuel flow to cylinder 4.
- Weak fuel pump: Fails to maintain adequate pressure (test with a fuel pressure gauge).
Diagnosis Tip: Listen for injector clicking or use a noid light.
3. Mechanical Problems
- Low compression: Worn piston rings or leaking valves in cylinder 4.
- Vacuum leaks: Unmetered air enters through cracked hoses near cylinder 4.
Perform a compression test (healthy engines: 125–200 PSI per cylinder).
How to Diagnose a P0304 Code
- Scan the code using an OBD II scanner.
- Inspect spark plugs: Check for fouling, cracks, or incorrect gaps in cylinder 4.
- Test ignition coils: Swap coil #4 with another cylinder. If the misfire moves, replace the coil.
- Check fuel injectors: Clean or test injector resistance with a multimeter.
- Look for vacuum leaks: Use a smoke machine or listen for hissing sounds.
Learn about cylinder-specific fixes in our P0301 Code Guide.
Fixing the P0304 Code
- Replace spark plugs: Use OEM-recommended iridium or platinum plugs (e.g., NGK or Bosch).
- Install new ignition coils: High-quality coils ensure consistent spark delivery.
- Clean fuel injectors: Use additives like Chevron Techron or seek professional cleaning.
- Repair vacuum leaks: Replace cracked hoses and reseal intake manifold gaskets.
Cost Breakdown:
- Spark plugs: 20–20–80
- Ignition coil: 75–75–250
- Fuel injector cleaning: 50–50–200
Preventing Future P0304 Codes
- Replace spark plugs every 30,000–100,000 miles (check your manual).
- Use synthetic oil to reduce engine wear.
- Inspect vacuum lines annually for cracks.
FAQs About the P0304 Code
Can I drive with a P0304 code?
Avoid prolonged driving—misfires can damage the catalytic converter.
What causes cylinder 4 misfire?
Common causes include bad ignition coils, worn spark plugs, or clogged fuel injectors.
Will a P0304 code clear itself?
Temporary misfires might reset, but persistent codes require diagnosis.
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A P0304 code isn’t just a minor inconvenience—it’s a critical warning that demands immediate attention. Ignoring cylinder 4 misfires can cascade into severe and expensive consequences. For instance, unburned fuel entering the exhaust system can overwork and destroy the catalytic converter, a repair costing upwards of $1,000. Persistent misfires also strain engine components like piston rings and valves, accelerating wear and tear that shortens your engine’s lifespan. Even your fuel efficiency takes a hit, as the engine control module (ECM) compensates for the imbalance by dumping more fuel into the combustion chamber, wasting gas and increasing emissions.
By addressing the P0304 code proactively—replacing worn spark plugs, testing ignition coils with an OBD II scanner, or repairing vacuum leaks—you’re not just silencing a warning light. You’re investing in your vehicle’s longevity, ensuring smoother acceleration, better fuel economy, and peace of mind on the road. Think of it as preventive care for your car: a small effort now avoids major headaches (and bills) later.
🚗💡 Pro Tip: Share this guide with fellow car enthusiasts or save it for your next garage session. Knowledge is power—empower others to tackle P0304 codes confidently and keep their engines running strong!
