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 the fuel-air mixture in that cylinder fails to ignite properly, or ignites inconsistently. This is a high-severity code that should be addressed promptly, as continued driving can lead to catalytic converter damage, reduced fuel economy, and potential engine damage.
What Does P0304 Mean?
P0304 is a standardized OBD-II code that specifically targets cylinder 4 misfire detection. Your engine’s ECU monitors each cylinder’s combustion events by analyzing crankshaft speed variations. When a cylinder fails to fire properly, it causes a brief hesitation in crankshaft rotation. If the ECU detects this pattern occurring on cylinder 4 over a certain threshold (typically within 1,000 revolutions), it triggers the P0304 code and illuminates the check engine light.
The “P” prefix indicates this is a powertrain code, “03” means it’s a fuel and air metering issue, and “04” specifies cylinder 4. Different vehicles have different cylinder configurations, so cylinder 4’s location varies—but the diagnostic approach remains consistent.
Common Symptoms
- Check engine light – Illuminates on the dashboard
- Engine hesitation or stuttering – Noticeable jerking during acceleration
- Rough idle – Engine vibrates or runs unevenly at stoplights
- Loss of power – Reduced acceleration and overall performance
- Increased fuel consumption – Unburned fuel exits the exhaust
- Engine knocking or pinging – Especially under load or acceleration
- Difficulty starting – Engine cranks but takes longer to turn over
- Rotten egg smell – Unburned fuel creates sulfur odor in exhaust
Possible Causes (Ranked by Frequency)
- Faulty Spark Plug – The most common cause. Worn, fouled, or gapped spark plugs fail to ignite the fuel-air mixture. Spark plugs typically need replacement every 30,000–100,000 miles depending on type.
- Bad Ignition Coil – The ignition coil generates the high voltage needed to fire the spark plug. A failing coil pack (especially common on cylinder-specific coils) prevents proper ignition.
- Faulty Fuel Injector – A clogged, leaking, or electrically failing fuel injector on cylinder 4 disrupts the fuel-air ratio, preventing proper combustion.
- Low Fuel Pressure – A failing fuel pump or clogged fuel filter reduces pressure, preventing adequate fuel delivery to cylinder 4.
- Vacuum Leak – Unmetered air entering the engine disrupts the fuel-air ratio, causing lean conditions that prevent ignition.
- Compression Loss – A blown head gasket, cracked piston, or worn piston rings on cylinder 4 reduce compression, making ignition difficult.
- Carbon Buildup – Excessive carbon deposits on the intake valve or piston crown can interfere with combustion.
- Timing Issues – Incorrect ignition timing (often from a faulty timing belt or sensor) prevents proper spark delivery.
- Engine Control Unit (ECU) Malfunction – Rare, but a faulty ECU or wiring issue can cause false misfire codes.
Diagnostic Steps
Follow this systematic approach to diagnose P0304:
Step 1: Scan for Additional Codes
Use an OBD-II scanner to check for other trouble codes. Related codes like P0300 (random misfire), P0301–P0308 (other cylinder misfires), or P0171/P0172 (fuel trim codes) provide valuable context. If multiple cylinders are misfiring, the issue is likely fuel pressure, timing, or vacuum-related rather than cylinder-specific.
Step 2: Visual Inspection
Locate cylinder 4 on your engine and inspect:
- Spark plug condition (remove and examine for wear, fouling, or gap issues)
- Ignition coil for cracks, corrosion, or loose connections
- Fuel injector connector for corrosion or loose wiring
- Vacuum hoses for cracks or disconnections
- Engine oil level and condition (low oil can affect compression)
Step 3: Replace Spark Plug and Coil
Start with the most common fixes. Replace the spark plug on cylinder 4 with the correct type and gap specification (consult your owner’s manual). If the misfire persists, swap the ignition coil from cylinder 4 with a neighboring cylinder and retest. If the code moves to the other cylinder, the coil is faulty.
Step 4: Check Fuel Injector
Use a fuel injector tester to verify the injector is firing. Listen for a clicking sound when the engine is cranked. A silent injector indicates an electrical or mechanical failure. Test fuel pressure with a fuel pressure gauge—it should match manufacturer specs (typically 40–60 PSI for port injection).
Step 5: Compression Test
Perform a wet and dry compression test on cylinder 4. Low compression indicates internal engine damage (blown gasket, cracked piston, worn rings). Dry compression should be at least 100 PSI; if adding oil temporarily improves compression, the rings are worn.
Step 6: Check Ignition Timing and Fuel Trim
Use a scope or timing light to verify ignition timing matches specifications. Check fuel trim values in the scanner’s live data—excessive positive or negative trim indicates fuel system issues.
Repair Cost Estimates
Repair costs vary widely depending on the underlying cause:
- Spark Plug Replacement: $15–$100 per plug (parts + labor)
- Ignition Coil Replacement: $75–$250 per coil
- Fuel Injector Cleaning: $50–$150
- Fuel Injector Replacement: $300–$900
- Fuel Pump Replacement: $400–$1,200
- Head Gasket Replacement: $1,000–$2,500+
- Compression Repair (piston/rings): $2,000–$5,000+
Most P0304 codes are resolved for under $300 with spark plug or coil replacement. Seek a professional diagnosis before authorizing major repairs.
Can I Still Drive?
Severity: High – You should avoid extended driving with a P0304 code.
While a single-cylinder misfire won’t immediately strand you, continued operation poses serious risks:
- Catalytic Converter Damage: Unburned fuel entering the exhaust ignites in the catalytic converter, causing overheating and permanent damage. Converter replacement costs $800–$2,500.
- Engine Damage: Prolonged misfiring can damage piston rings, valves, and cylinder walls.
- Reduced Fuel Economy: Expect 10–20% worse MPG.
- Failed Emissions Test: You won’t pass inspection with an active misfire code.
Recommendation: Drive directly to a mechanic for diagnosis. Avoid highway driving and heavy acceleration. If the misfire is severe (noticeable shaking), consider having the vehicle towed.
Frequently Asked Questions
Q: Can a bad battery cause a P0304 code?
A: Indirectly, yes. A weak battery reduces ignition coil voltage, making spark plugs less likely to fire. However, a bad battery typically triggers codes like P0133 or P0135. If you have a weak battery, charge or replace it and retest. A truly dead battery won’t allow the engine to start, so P0304 would be the least of your concerns.
Q: What’s the difference between P0304 and P0300?
A: P0300 is a random misfire code affecting multiple cylinders or the entire engine. P0304 specifically targets cylinder 4. If you have P0300 without cylinder-specific codes, the issue is usually fuel pressure, timing, or a vacuum leak affecting all cylinders. If you have P0304 (and maybe P0301), the problem is likely cylinder-specific, like a bad coil or spark plug.
Q: Will P0304 go away on its own?
A: No. The code will persist until the underlying cause is fixed. The check engine light may turn off temporarily if the misfire is intermittent, but the code will remain in the ECU’s memory and reappear under similar driving conditions. Clearing the code without fixing the problem is not a solution.
Q: Is it safe to drive with P0304 for a week until I can get to a mechanic?
A: Not recommended. A week of daily driving with a misfire significantly increases the risk of catalytic converter damage. If possible, limit driving to short trips to the mechanic, or have the vehicle towed. The cost of a tow ($100–$200) is far less than a catalytic converter replacement ($800–$2,500).