P0304: Cylinder 4 Misfire Detected

Quick answer

• What the code means: The Powertrain Control Module (PCM) has detected one or more misfires in cylinder 4 of the engine.
• Severity: Moderate to severe. Driving for extended periods can damage the catalytic converter or engine components. Get it checked out as soon as possible.
• Most common cause: Worn or faulty spark plug in cylinder 4.

What the P0304 code means

Official OBD-II Definition: "Cylinder 4 Misfire Detected"

In plain English, the P0304 code means that your car's computer has identified an issue with cylinder number 4 specifically, where the combustion process isn't happening correctly. During normal operation, each cylinder should ignite its air-fuel mixture reliably and powerfully. A "misfire" occurs when this ignition process is incomplete or fails entirely, leading to a loss of power and potentially harmful unburnt fuel entering the exhaust system.

The PCM monitors crankshaft speed and camshaft position to detect variances that indicate a misfire. When a significant change in rotation speed is detected during the power stroke of cylinder 4, the PCM flags a P0304 code and illuminates the Check Engine Light.

Common symptoms

• Check Engine Light (flashing during severe misfires)
• Rough idling or vibrations
• Reduced engine power or acceleration
• Poor fuel economy
• Engine hesitation or stumbling during acceleration
• A strong smell of raw fuel from the exhaust
• Difficulty starting the engine (less common)

Common causes

1. Faulty Spark Plug (Cylinder 4): Worn, fouled, or incorrectly gapped spark plugs are the most frequent culprits. A weak or absent spark directly leads to a misfire.
2. Ignition Coil Failure (Cylinder 4): Modern engines often use individual coil-on-plug (COP) ignition coils. If the coil for cylinder 4 fails, it won't deliver high voltage to the spark plug, preventing ignition.
3. Clogged or Faulty Fuel Injector (Cylinder 4): An injector that doesn't spray enough fuel, or sprays it incorrectly, will cause an imbalanced air-fuel mixture, leading to a misfire.
4. Vacuum Leaks: Leaks in the intake manifold or vacuum hoses can introduce unmetered air, leaning out the air-fuel mixture in certain cylinders, potentially causing a misfire in cylinder 4 if the leak is localized or significant enough to affect it.
5. Low Compression (Cylinder 4): This is a more severe mechanical issue involving worn piston rings, damaged valves, or a blown head gasket. Loss of compression means the cylinder cannot effectively ignite the air-fuel mixture.
6. Wiring Issues: Damaged wiring to the spark plug, ignition coil, or fuel injector for cylinder 4 can prevent proper operation.
7. PCM Issues: While rare, a faulty Powertrain Control Module could incorrectly interpret signals or fail to control ignition/fuel delivery to cylinder 4.

How a technician diagnoses it

1. Verify the Code and Freeze Frame Data: The technician will first connect an OBD-II scan tool to confirm the P0304 code and check the "freeze frame" data. This data provides a snapshot of engine conditions (engine speed, load, temperature, etc.) at the moment the code was set, which can offer valuable clues.
2. Inspect Visual Components: A visual inspection of the spark plug, ignition coil, and injector wiring for cylinder 4 will occur. Look for obvious signs of damage, corrosion, or leaks.
3. Spark Plug Inspection/Swap Test: The spark plug from cylinder 4 will be removed and inspected for wear, fouling, or damage. Often, a technician will swap the spark plug from cylinder 4 with one from a known good cylinder (e.g., cylinder 1) to see if the misfire code follows the plug. If P0301 now appears, the spark plug was the issue.
4. Ignition Coil Test/Swap Test: Using a spark tester or by swapping the ignition coil for cylinder 4 with another cylinder, the technician can determine if the coil is faulty. If the misfire moves to the new cylinder after swapping coils, the coil is bad.
5. Fuel Injector Test/Swap Test: The technician may use a noid light to check if the injector for cylinder 4 is receiving a pulse, or listen to it with a stethoscope. A fuel pressure gauge can check overall system pressure, and a resistance test can be performed on the injector itself. Similar to plugs and coils, swapping the injector with a known good cylinder can isolate the problem.
6. Compression Test: If ignition and fuel delivery components check out, a compression test will be performed on cylinder 4 and compared to other cylinders to check for mechanical issues. Low compression indicates a severe internal engine problem.
7. Vacuum Leak Test: Techniques like smoke testing or spraying carburetor cleaner around the intake manifold/vacuum lines can pinpoint active vacuum leaks.
8. Scan Tool Live Data Analysis: Monitoring live data such as misfire counts for individual cylinders, fuel trims, and O2 sensor readings can help identify other contributing factors.

Typical repair cost

• Spark Plug Replacement: $100 - $300 (per cylinder/set, includes diagnosis and minor parts)
• Ignition Coil Replacement: $150 - $400 (for one coil, including diagnosis)
• Fuel Injector Replacement: $250 - $600 (for one injector, including diagnosis and related parts)
• Addressing Vacuum Leaks: $150 - $500 (depending on the location and complexity of the leak)
• Engine Compression Repair: $1,000 - $4,000+ (for issues like head gasket, valve repair, or piston rings; highly variable)

These costs include labor and parts but can vary significantly based on vehicle make, model, and regional labor rates.

Can I keep driving with this code?

It is not recommended to keep driving with a P0304 code, especially if the Check Engine Light is flashing. A flashing light indicates a severe misfire that is actively causing damage, most notably to the catalytic converter due to unburnt fuel entering the exhaust. Driving with a consistent misfire will also lead to reduced fuel economy, poor performance, and potentially further engine damage if the underlying cause is severe (e.g., low compression).

While you might be able to drive a short distance to a repair shop, prolonged driving should be avoided.

Related codes

• P0300: Random/Multiple Cylinder Misfire Detected
• P0301 - P0308: Misfire Detected in specific cylinders (e.g., P0301 for cylinder 1, P0302 for cylinder 2, etc.)
• P0171 / P0174: System Too Lean (Bank 1 or Bank 2) - sometimes accompanies misfires due to fuel mixture issues.
• P0420 / P0430: Catalyst System Efficiency Below Threshold (Bank 1 or Bank 2) - often a consequence of driving with a misfire.

Get this scanned at a trusted shop on Repairius.