What is P0304?
The P0304 diagnostic trouble code indicates that your vehicle’s engine control unit (ECU) has detected a misfire condition specifically in cylinder 4. A misfire occurs when the fuel-air mixture in that cylinder fails to ignite properly or burns incompletely during the combustion cycle. This is a high-severity code that should be addressed promptly, as it can damage your catalytic converter and reduce engine performance significantly.
What Does P0304 Mean?
P0304 stands for “Cylinder 4 Misfire Detected.” The “P” indicates a powertrain code, “03” refers to the ignition system, and “04” specifies cylinder number 4. When this code is triggered, the ECU has detected one or more combustion events where cylinder 4 failed to produce the expected power output.
Modern vehicles monitor ignition timing and crankshaft acceleration between each cylinder’s firing. When the ECU detects that cylinder 4 is not contributing its expected power—either consistently or intermittently—it logs this code and illuminates the check engine light. The severity depends on how frequently the misfire occurs and how much it affects overall engine operation.
Common Symptoms
- Check Engine Light: The primary indicator, usually appearing as a steady or blinking light on the dashboard
- Rough Idle: Engine vibrates or shakes noticeably when stopped at traffic lights or in park
- Loss of Power: Noticeable reduction in acceleration and overall engine performance
- Engine Hesitation: Delay or stumbling when pressing the accelerator
- Fuel Smell from Exhaust: Unburned fuel escaping through the tailpipe, often accompanied by a rotten egg smell
- Excessive Vibration: Shaking felt throughout the vehicle, especially during acceleration
- Poor Fuel Economy: Increased fuel consumption due to incomplete combustion
- Difficulty Starting: Engine cranks longer than normal or struggles to start
- Blinking Check Engine Light: Indicates a severe misfire condition requiring immediate attention
Possible Causes
P0304 misfires can stem from ignition system failures, fuel delivery problems, or mechanical issues. Listed below are the most common causes, ranked by frequency:
1. Faulty Spark Plug (Most Common)
Worn, fouled, or damaged spark plugs are the leading cause of cylinder misfires. Over time, spark plugs accumulate carbon deposits, gap erosion, or oil fouling that prevents proper ignition. This is especially common if spark plugs haven’t been replaced according to manufacturer intervals.
2. Defective Ignition Coil
The ignition coil for cylinder 4 may be failing, producing insufficient voltage to fire the spark plug. Coils can degrade due to heat, moisture, or electrical stress. Many modern vehicles use coil-on-plug designs where each cylinder has its own coil pack.
3. Fuel Injector Issues
A clogged, leaking, or malfunctioning fuel injector for cylinder 4 can prevent proper fuel delivery, creating a lean mixture that won’t ignite reliably. Injector problems often result from carbon buildup or electrical failures.
4. Vacuum Leak
Unmetered air entering the engine through cracked hoses, gasket leaks, or intake manifold problems creates an excessively lean fuel mixture that misfires. Vacuum leaks affect the entire engine but may be more noticeable on specific cylinders.
5. Low Fuel Pressure
A failing fuel pump, clogged fuel filter, or faulty fuel pressure regulator can reduce fuel pressure below the engine’s requirements, causing lean misfires across one or more cylinders.
6. Compression Issues
Low compression in cylinder 4 due to worn piston rings, valve leakage, or a blown head gasket prevents proper combustion. A compression test can diagnose this condition.
7. Carbon Buildup
Excessive carbon deposits on intake valves, piston tops, or spark plugs can interfere with combustion in cylinder 4, particularly during cold starts or low-load conditions.
8. Bad Engine Control Unit (ECU)
In rare cases, a faulty ECU may incorrectly report misfires. This is typically a last-resort diagnosis after all other causes have been ruled out.
Diagnostic Steps
Step 1: Retrieve Full Diagnostic Data
Use an OBD-II scanner to pull all stored and pending codes. Note whether P0304 appears alone or alongside other misfire codes (P0300-P0308). Multiple cylinder misfires suggest fuel system or ignition timing issues, while a single cylinder points to that cylinder’s specific components.
Step 2: Inspect Spark Plugs
Remove the spark plug from cylinder 4 and examine it for:
- Excessive gap (should match manufacturer specs, typically 0.028″-0.060″)
- Black, sooty deposits (fouling)
- Oil coating (indicates oil burning)
- Electrode erosion or damage
- Age (replace if over 30,000 miles or manufacturer interval)
Replace the spark plug if any issues are found, then clear the code and test drive to see if the problem returns.
Step 3: Test the Ignition Coil
Using a multimeter or dedicated coil tester, check the ignition coil’s primary and secondary resistance values against manufacturer specifications. A faulty coil will show out-of-spec readings. Swap the coil from cylinder 4 with another cylinder’s coil; if the misfire moves to the other cylinder, the coil is bad.
Step 4: Check Fuel Injector Operation
Listen for a clicking sound from cylinder 4’s fuel injector while the engine runs. Use a fuel injector tester to verify proper electrical signal and spray pattern. A clogged injector may require cleaning or replacement.
Step 5: Perform a Compression Test
Disable the fuel pump and ignition, then use a compression gauge to test cylinder 4’s compression. Compare the reading to manufacturer specifications and other cylinders. Low compression (typically below 100 psi) indicates internal engine damage.
Step 6: Check for Vacuum Leaks
Inspect all vacuum hoses for cracks, splits, or loose connections. Use a smoke test machine to identify leaks by introducing smoke into the intake system and observing where it escapes. Listen for hissing sounds around the intake manifold.
Step 7: Test Fuel Pressure
Connect a fuel pressure gauge to the fuel rail and measure pressure at idle and under load. Compare readings to manufacturer specifications (typically 35-45 psi for most vehicles). Low pressure indicates a failing fuel pump or clogged filter.
Step 8: Clear the Code and Monitor
After addressing the suspected cause, clear the code using your scanner and perform a test drive under various conditions (idle, acceleration, highway speeds). If the code doesn’t return within 100+ miles, the repair was successful.
Repair Cost Estimates
The cost to repair a P0304 misfire varies widely depending on the underlying cause:
- Spark Plug Replacement: $50-$200 per plug (including labor)
- Ignition Coil Replacement: $150-$400 per coil
- Fuel Injector Cleaning: $50-$100
- Fuel Injector Replacement: $300-$900
- Fuel Pump Replacement: $400-$1,200
- Vacuum Leak Repair: $100-$500 (depending on location)
- Compression Repair (Internal Engine Work): $1,500-$5,000+
- Diagnostic Service: $75-$150 (many shops waive this if you proceed with repairs)
Most P0304 cases are resolved with spark plug or ignition coil replacement, typically costing $150-$400 total.
Can I Still Drive?
Severity: HIGH – A blinking check engine light with P0304 indicates a severe misfire that requires immediate attention. Driving with a severe misfire can cause serious damage:
- Catalytic Converter Damage: Unburned fuel entering the exhaust can overheat and permanently damage the catalytic converter, a $500-$2,000+ repair
- Engine Damage: Prolonged misfires can cause internal engine damage, leading to costly repairs
- Reduced Performance: Power loss and rough running make the vehicle unsafe in traffic
- Emissions Violations: A misfire increases harmful emissions and may cause inspection failures
Recommendation: If the check engine light is blinking, do not drive the vehicle except directly to a repair shop. If the light is steady, limit driving to essential trips only and schedule a diagnostic appointment immediately. Addressing the issue quickly prevents more expensive damage.
Frequently Asked Questions
What’s the difference between P0304 and other misfire codes?
P0304 specifically indicates cylinder 4 is misfiring. Other codes like P0300 (random misfire), P0301 (cylinder 1), P0302 (cylinder 2), etc., point to different cylinders or a pattern of misfires across multiple cylinders. Multiple misfire codes often suggest fuel system, ignition timing, or vacuum leak issues affecting the entire engine, while a single cylinder code usually indicates a problem specific to that cylinder’s spark plug, coil, or injector.
Can a bad spark plug cause P0304?
Yes, absolutely. A worn, fouled, or gapped spark plug is the most common cause of P0304. Spark plugs should be inspected and replaced according to your vehicle’s maintenance schedule, typically every 30,000-100,000 miles depending on the plug type. Replacing the spark plug in cylinder 4 resolves the issue in many cases.
Will P0304 go away on its own?
No, P0304 will not resolve without repair. The code will persist and likely worsen over time. While the check engine light might temporarily disappear if the misfire is intermittent, the underlying problem remains and will continue to damage your engine and catalytic converter. Professional diagnosis and repair are necessary.
Is it safe to drive with P0304?
It depends on the severity. A steady check engine light with P0304 allows limited driving to a repair shop, but a blinking light indicates a severe misfire that can cause catalytic converter damage within minutes. Do not drive at highway speeds or in heavy traffic with a blinking check engine light. Get the vehicle diagnosed and repaired as soon as possible to avoid expensive secondary damage.