P0304 Code: Causes, Symptoms, Diagnosis & How to Fix It
The P0304 code means Cylinder 4 Misfire Detected.
Your vehicle's powertrain control module (PCM) has detected that cylinder 4 is not firing correctly. A misfire occurs when combustion does not happen properly or consistently inside the cylinder.
P0304 identifies the affected cylinder, but it does not automatically identify the failed component. The problem could involve the spark plug, ignition coil, fuel injector, intake system, engine compression, wiring, timing, or another engine-management component.
The physical location of cylinder 4 varies by engine design, so use the vehicle manufacturer's service information to identify the correct cylinder before testing components.
P0304 Code Symptoms
Common symptoms include:
Check Engine Light
Rough or shaking engine
Rough idle
Hesitation during acceleration
Engine stumbling
Reduced engine power
Poor fuel economy
Hard starting
Engine stalling
Unusual exhaust odor
Flashing Check Engine Light
A severe misfire can make the engine run noticeably rough and may become more obvious during acceleration or under load.
Common Causes of P0304
1. Faulty Spark Plug
A worn, fouled, damaged, or incorrectly gapped spark plug can prevent cylinder 4 from igniting the air-fuel mixture correctly.
2. Bad Ignition Coil
A weak or failed ignition coil can cause cylinder 4 to lose the spark required for combustion.
3. Fuel Injector Problem
A clogged, restricted, leaking, or electrically faulty injector can prevent cylinder 4 from receiving the correct amount of fuel.
4. Vacuum or Intake Leak
An intake or vacuum leak can create an incorrect air-fuel mixture and contribute to a cylinder-specific misfire.
5. Low Fuel Pressure
A weak fuel pump or another fuel-delivery problem can cause insufficient fuel delivery and contribute to misfires.
6. Low Engine Compression
A burned valve, worn piston rings, head-gasket problem, or another mechanical fault can reduce compression and cause a persistent misfire.
7. Wiring or Connector Problems
Damaged wiring, loose connectors, corrosion, or electrical faults affecting the ignition or injector can trigger P0304.
8. Engine Timing Problems
Incorrect camshaft or crankshaft timing can interfere with combustion and cause misfire conditions.
How to Diagnose the P0304 Code
A systematic diagnosis can help prevent unnecessary parts replacement.
1. Scan for Additional Codes
Use an OBD2 scanner to check for stored and pending DTCs. Look for other misfire, fuel-system, lean-condition, or sensor codes.
Record the freeze-frame data before clearing the codes.
2. Inspect the Spark Plug
Remove the cylinder 4 spark plug and inspect it for:
Carbon buildup
Oil or fuel fouling
Excessive wear
Damaged electrodes
Incorrect gap
Spark-plug condition can provide clues about what is happening inside the cylinder.
3. Test the Ignition Coil
Inspect the cylinder 4 ignition coil, boot, connector, and wiring.
If the vehicle uses interchangeable ignition coils, swap the cylinder 4 coil with another cylinder. If the misfire moves to the other cylinder, the coil becomes a strong suspect.
4. Check the Fuel Injector
Inspect the cylinder 4 injector and its electrical connection. Depending on the vehicle, testing may include injector resistance, electrical operation, fuel delivery, or injector balance.
5. Check for Intake or Vacuum Leaks
Inspect vacuum hoses, intake components, and manifold areas for leaks. A smoke test can help locate leaks that are difficult to identify visually.
6. Perform a Compression Test
If the ignition and fuel systems appear normal, test engine compression. Low compression can indicate an internal mechanical problem such as a valve, piston, ring, or head-gasket issue.
7. Check Timing and Engine Sensors
If necessary, inspect camshaft and crankshaft synchronization and related sensor data. Follow the vehicle manufacturer's diagnostic procedure when timing or sensor problems are suspected.
How to Fix P0304
The correct repair depends on the confirmed cause. Possible repairs include:
Replace a worn or damaged spark plug
Replace a defective ignition coil
Repair ignition wiring or connectors
Repair or replace a faulty fuel injector
Repair an intake or vacuum leak
Correct low fuel pressure
Repair damaged sensor wiring
Correct engine timing problems
Repair an internal engine problem causing low compression
After the repair, clear the code and perform a test drive while monitoring misfire data to confirm that P0304 does not return.
Avoid replacing expensive components based only on the P0304 code. The code identifies the misfiring cylinder, not necessarily the failed component.
Is P0304 Serious?
P0304 should not be ignored. A persistent misfire can reduce engine performance and may eventually damage the catalytic converter or other components.
If the Check Engine Light is flashing, the engine is shaking severely, the vehicle has major power loss, or it is stalling, avoid unnecessary driving and have the vehicle diagnosed promptly.
If the Check Engine Light is steady and the vehicle is running normally, arrange diagnosis as soon as practical.
P0304 vs. P0303: What's the Difference?
Code | Meaning |
P0303 | Cylinder 3 Misfire Detected |
P0304 | Cylinder 4 Misfire Detected |
The diagnostic process is similar, but the technician must identify and test the correct cylinder and its corresponding ignition, fuel, intake, and mechanical components.
P0304 vs. P0300
P0300 indicates a random or multiple-cylinder misfire without identifying one specific cylinder.
P0304 specifically identifies cylinder 4 as the cylinder where the PCM detected the misfire.
Frequently Asked Questions
Can a bad spark plug cause P0304?
Yes. A worn, fouled, damaged, or incorrectly gapped spark plug can cause a cylinder 4 misfire.
Can a bad ignition coil cause P0304?
Yes. A defective ignition coil can prevent cylinder 4 from receiving the spark needed for proper combustion.
Can I drive with P0304?
It depends on the severity. If the vehicle has severe shaking, major power loss, stalling, or a flashing Check Engine Light, avoid continued driving.
Does P0304 mean cylinder 4 needs to be repaired?
Not necessarily. P0304 identifies a misfire in cylinder 4, but the actual cause could be the spark plug, ignition coil, injector, vacuum leak, fuel system, compression, wiring, or another problem.
Can low compression cause P0304?
Yes. Mechanical problems that reduce compression can cause a persistent cylinder 4 misfire.
