What is the P0304 Code?
The P0304 diagnostic trouble code indicates that your vehicle’s engine control unit (ECU) has detected a misfire in cylinder 4. A misfire occurs when one or more combustion cycles in a cylinder fail to produce the proper power output. This is a high-severity code that requires prompt attention, as continued driving with a misfiring cylinder can damage your catalytic converter and engine components.
What Does P0304 Mean?
P0304 = Powertrain (P) | Generic OBD-II Code (0) | Fuel and Air Metering (3) | Cylinder 4 Misfire Detected (04)
When the ECU detects that cylinder 4 is not firing correctly or is producing insufficient combustion pressure, it logs this code and illuminates the check engine light. The engine’s crankshaft position sensor monitors the rotational speed of the crankshaft, and if it detects an unexpected variation in speed during the power stroke of cylinder 4, the misfire is recorded.
Misfires can be intermittent (occurring occasionally) or continuous (occurring every combustion cycle). The severity depends on how frequently the misfire occurs and how much damage it’s causing to the catalytic converter.
Common Symptoms of P0304
- Check Engine Light – The primary indicator that triggers when the code is set
- Rough Idle – Noticeable vibration or shaking when the vehicle is stopped or idling
- Loss of Power – Reduced acceleration and overall engine performance
- Engine Hesitation – Hesitation or stumbling when accelerating from a stop
- Fuel Smell – Unburned fuel in the exhaust creating a strong fuel odor
- Excessive Vibration – Noticeable shaking throughout the vehicle, especially at idle or low RPM
- Increased Fuel Consumption – Noticeably worse fuel economy due to incomplete combustion
- Black Smoke from Exhaust – Visible smoke indicating unburned fuel exiting the tailpipe
- Catalytic Converter Damage – Rotten egg smell from the exhaust (sulfur smell) indicating converter damage
Possible Causes of P0304
Listed from most to least common:
- Faulty Spark Plug (Most Common) – Worn, fouled, or incorrectly gapped spark plugs are the leading cause of misfires. Spark plugs should be replaced every 30,000-100,000 miles depending on the vehicle.
- Defective Ignition Coil – A failing coil pack or ignition coil for cylinder 4 cannot generate sufficient voltage to ignite the fuel mixture.
- Damaged Spark Plug Wire – On vehicles with traditional plug wires, a cracked or damaged wire prevents spark from reaching the plug.
- Fuel Injector Issues – A clogged, leaking, or malfunctioning fuel injector for cylinder 4 can cause improper fuel delivery.
- Low Fuel Pressure – A failing fuel pump or pressure regulator results in insufficient fuel pressure for proper atomization.
- Vacuum Leak – Unmetered air entering the engine disrupts the air-fuel ratio, causing misfires across multiple cylinders.
- Compression Loss – Worn piston rings, valve damage, or head gasket failure in cylinder 4 reduces compression needed for combustion.
- Carbon Buildup – Excessive carbon deposits on the intake valves or piston in cylinder 4 interfere with combustion.
- Engine Timing Issues – Incorrect ignition timing or a faulty timing chain/belt can cause misfires.
- Contaminated Fuel – Water or debris in the fuel tank can prevent proper combustion.
- Worn Engine Components – Valve damage, cylinder wall wear, or other internal engine damage reduces compression.
- ECU or Sensor Malfunction – Rarely, a faulty crankshaft position sensor or ECU can cause false misfire codes.
Diagnostic Steps for P0304
Follow these steps to diagnose the cause of your P0304 code:
Step 1: Retrieve and Document the Code
Use an OBD-II scanner to confirm the P0304 code and check for any additional codes. Multiple misfire codes (P0300-P0308) suggest a more general problem like vacuum leaks or fuel pressure issues. A single P0304 code points to a cylinder-specific problem.
Step 2: Inspect the Spark Plug
Remove the spark plug from cylinder 4 and inspect it for:
- Excessive wear or erosion
- Carbon fouling (black deposits)
- Oil fouling (wet, oily deposits)
- Improper gap spacing
- Damage or cracks
Compare it to a spark plug from another cylinder. If it’s fouled or worn, replace all spark plugs as a preventive measure.
Step 3: Check the Ignition Coil
Inspect the ignition coil or coil pack for cylinder 4. Look for:
- Cracks or damage to the coil body
- Corrosion on the connectors
- Loose connections
- Burn marks or discoloration
If the coil appears damaged, test it with a multimeter or swap it with a coil from another cylinder to see if the misfire follows the coil.
Step 4: Inspect Spark Plug Wires (If Applicable)
For vehicles with traditional plug wires, visually inspect the wire for cylinder 4:
- Cracks, splits, or damage to the insulation
- Loose connections at the plug or coil
- Corrosion or moisture inside the boot
Test the wire’s resistance with a multimeter (should be 3,000-10,000 ohms per foot of wire).
Step 5: Check Fuel Injector Operation
Listen for a clicking sound from the fuel injector for cylinder 4 while the engine is running. If you don’t hear a click, the injector may not be firing. You can also use a fuel injector tester or multimeter to check resistance and pulse signals.
Step 6: Test Fuel Pressure
Use a fuel pressure gauge to test the fuel system pressure at idle and under load. Compare the reading to your vehicle’s specification (typically 35-65 PSI depending on the vehicle). Low pressure indicates a failing fuel pump or pressure regulator.
Step 7: Perform a Compression Test
If the above steps don’t reveal the problem, perform a compression test on cylinder 4. Remove all spark plugs, connect a compression gauge to cylinder 4, and crank the engine. A healthy cylinder should have 100+ PSI (specifications vary by vehicle). Low compression in cylinder 4 suggests internal engine damage.
Step 8: Check for Vacuum Leaks
Inspect all vacuum hoses for cracks, splits, or loose connections. Listen for a hissing sound with the engine running to locate leaks. Spray carburetor cleaner around suspected leak areas—if the engine RPM changes, you’ve found a leak.
Step 9: Scan for Additional Codes
Clear the code and test drive the vehicle. If the code returns immediately, the problem is consistent. If it takes time to return, the misfire is intermittent, which may indicate a failing sensor or electrical connection issue.
Repair Cost Estimates
The cost to repair a P0304 misfire depends on the underlying cause:
- Spark Plug Replacement: $50-$200 (parts and labor for all plugs)
- Ignition Coil Replacement: $150-$400 per coil
- Spark Plug Wire Replacement: $100-$300 (set of wires)
- Fuel Injector Cleaning/Replacement: $200-$900 (cleaning is cheaper; replacement is more expensive)
- Fuel Pump Replacement: $400-$1,200
- Fuel Pressure Regulator: $200-$600
- Compression Repair (Valve Job): $1,500-$4,000+
- Head Gasket Replacement: $1,000-$3,000+
- Catalytic Converter Replacement: $800-$2,500+ (if damaged by prolonged misfiring)
Average Repair Cost: $150-$500 for most common causes (spark plugs, coils, or fuel injectors)
Can I Still Drive with P0304?
Severity: HIGH – Driving with a P0304 code is not recommended, though it may be possible for short distances.
Safety Concerns:
- Reduced engine power and acceleration, which can be dangerous in traffic
- Unburned fuel entering the exhaust can damage the catalytic converter, leading to expensive repairs ($800-$2,500+)
- Prolonged misfiring can cause internal engine damage
- Rough idle and vibration can indicate a serious problem that may worsen suddenly
Recommendation: Have the vehicle diagnosed and repaired as soon as possible. If you must drive it, do so cautiously and avoid highway driving. The longer you wait, the more damage can occur to the catalytic converter and engine.
FAQ About P0304
Q: Will P0304 go away on its own?
A: No, P0304 will not go away without repair. The code will persist until the underlying cause is fixed. You can clear the code with a scanner, but it will return when the misfire occurs again. Ignoring it will only lead to more expensive damage, particularly to the catalytic converter.
Q: Can a bad fuel filter cause P0304?
A: A severely clogged fuel filter can restrict fuel flow and contribute to misfires, but it’s not a common cause of a single-cylinder misfire. A bad fuel filter typically causes multiple cylinder misfires (P0300) and poor overall performance. However, if you haven’t changed your fuel filter recently, it’s worth checking as part of your diagnostic process.
Q: Is P0304 the same as P0303 or P0305?
A: No. P0304 specifically indicates a misfire in cylinder 4. P0303 indicates cylinder 3, P0305 indicates cylinder 5, and so on. If you have multiple misfire codes (P0300-P0308), the problem is likely a general issue affecting the entire engine, such as a vacuum leak, low fuel pressure, or bad ignition timing. A single P0304 code points to a problem specific to cylinder 4.
Q: How much will it cost to fix P0304?
A: Most P0304 repairs cost between $150-$500 and involve replacing a spark plug, ignition coil, or fuel injector. However, if the misfire has damaged the catalytic converter or if there’s internal engine damage, costs can exceed $2,000. The best way to minimize cost is to diagnose and repair the problem quickly before additional damage occurs.