P0304 Cylinder #4 Misfire Detected

P0304 – Cylinder #4 Misfire Detected means the engine control module (ECM/PCM) has detected abnormal combustion in cylinder number 4.

The code tells you which cylinder is misfiring, but not why it is misfiring. The cause could be something relatively simple, such as a worn spark plug or weak ignition coil, or it could involve the fuel injector, wiring, intake system, compression, valve-train, or engine timing.

The ECM generally identifies a misfire by monitoring changes in crankshaft rotational speed. When cylinder #4 fails to produce its expected combustion force, the crankshaft does not accelerate in the normal pattern. If the variation is large enough, the ECM records P0304.

What Does P0304 Mean in Practice?

A normal combustion event produces torque that contributes to smooth crankshaft rotation. When cylinder #4 does not burn the mixture correctly, that contribution is reduced or missing.

The severity can range from an occasional stumble to a continuous misfire.

Possible symptoms include:

• Rough idle

• Engine vibration

• Hesitation during acceleration

• Reduced engine power

• Poor throttle response

• Increased fuel consumption

• Engine shaking under load

• Difficult starting

• Intermittent stalling

• Check Engine Light

• Flashing Check Engine Light during severe misfire

A vehicle can sometimes continue running with P0304, particularly when the misfire is intermittent. However, the underlying problem should not be ignored.

The Main Causes of P0304

P0304 is best approached by checking the complete combustion process in cylinder #4.

The main areas are ignition, fuel, air, compression and timing.

Spark plug

A worn or damaged spark plug is one of the first things to inspect.

Possible problems include excessive electrode wear, incorrect gap, carbon deposits, fuel fouling, oil contamination or cracked ceramic.

The condition of the #4 spark plug can also provide clues about the underlying problem.

Ignition coil

On engines with individual ignition coils, a weak coil can cause P0304.

A coil does not necessarily have to fail completely. Some coils work normally at idle but break down when engine load increases.

During acceleration, cylinder pressure rises and the ignition system has to produce a stronger spark. A marginal coil may therefore cause a misfire only under load.

Fuel injector

Cylinder #4 may not be receiving the correct amount of fuel.

The injector could be restricted, internally damaged or electrically defective. Its connector and wiring can also cause problems.

Air or vacuum leak

An intake leak near cylinder #4 can introduce additional air and create a lean mixture. This can produce a cylinder-specific misfire.

Mechanical problem

If ignition, fuel and air supply are all functioning correctly, cylinder compression and valve-train operation should be investigated.

Can a Bad Ignition Coil Cause P0304?

Yes.

If the engine uses interchangeable individual ignition coils, a coil swap test can help determine whether the coil is responsible.

Move the suspected cylinder #4 coil to another cylinder and monitor the misfire codes.

For example, if P0304 disappears and a misfire code appears for the cylinder that received the coil, the misfire has followed the coil. This is strong evidence that the coil is defective.

If P0304 remains on cylinder #4, the coil becomes less likely to be the cause.

This test is more useful than replacing the coil without verification because it gives you evidence about whether the fault follows the component.

Inspect the Cylinder #4 Spark Plug

The spark plug should be removed and compared with the plugs from the other cylinders.

A fuel-wet plug may indicate that fuel is entering the cylinder but is not being ignited correctly.

A heavily carbonized plug can indicate an ignition or mixture problem.

An oil-fouled plug can suggest that oil is entering the combustion chamber.

A damaged electrode or incorrect gap can directly cause a misfire.

If replacing the spark plug eliminates P0304 temporarily but the code returns, the plug may have been affected by another underlying problem.

Check the Fuel Injector

If the ignition system checks out, the fuel injector serving cylinder #4 should be investigated.

Depending on the engine, useful tests include:

• Injector control signal testing

• Injector current waveform analysis

• Injector balance testing

• Injector flow testing

• Fuel-pressure testing

• Fuel-pressure testing under load

A clogged injector may deliver too little fuel and create a lean misfire.

An injector that leaks excessively can create an overly rich mixture and also cause poor combustion.

A fuel-pressure problem affecting the entire engine can cause multiple-cylinder misfires, while a consistently isolated P0304 makes a cylinder-specific injector or wiring problem more suspicious.

Wiring and Connector Problems

A P0304 code does not necessarily mean that the ignition coil or injector itself is defective.

The wiring supplying or controlling these components can also cause the misfire.

Inspect the area around cylinder #4 for:

• Broken wires

• Chafed insulation

• Melted wiring

• Corrosion

• Loose terminals

• Poor terminal tension

• Oil contamination

• Water intrusion

• Damaged connectors

Intermittent wiring faults are particularly important when P0304 appears only under certain driving conditions.

Engine movement during acceleration can move the wiring harness enough to interrupt a damaged conductor or weak terminal connection.

A wiggle test while monitoring live misfire data can sometimes expose this type of fault.

Could an Intake Leak Cause P0304?

Yes.

An intake manifold leak close to cylinder #4 can allow additional air into the engine. This unmetered air can create a lean mixture and cause the cylinder to misfire.

Possible sources include:

• Intake manifold gasket

• Intake runner seal

• Vacuum hose

• Intake-system sealing surfaces

• Other components connected to the intake system

If the ignition system has already been verified, a smoke test can help locate an intake leak.

What If Cylinder #4 Has Low Compression?

If spark, fuel and air supply are normal, mechanical condition becomes the next major area of investigation.

Low compression in cylinder #4 can result from:

• Burned valve

• Damaged valve seat

• Worn piston rings

• Damaged piston

• Head-gasket failure

• Valve-train malfunction

• Camshaft problem

• Excessive deposits affecting valve operation

A compression test should be interpreted by comparing cylinder #4 with the other cylinders.

If cylinder #4 has significantly lower compression, a leak-down test can help determine where the pressure is escaping.

Air escaping through the intake suggests an intake-valve problem. Air escaping through the exhaust suggests an exhaust-valve problem. Air entering the crankcase can indicate piston or piston-ring problems.

Can Engine Timing Cause P0304?

Yes, but a general timing problem should not be the first assumption when only cylinder #4 is misfiring.

Problems involving the timing chain, timing belt, camshaft position, variable valve timing (VVT), or cam/crank synchronization can affect combustion.

Timing problems may also produce other symptoms or DTCs, such as:

• Multiple-cylinder misfires

• Camshaft position codes

• Crankshaft position codes

• Cam/crank correlation codes

• Difficult starting

• Reduced engine performance

• Abnormal VVT data

If the misfire consistently remains on cylinder #4, cylinder-specific ignition, fuel, air and mechanical causes should generally be investigated first.

Pay Attention to When the Misfire Occurs

The conditions under which P0304 appears can be extremely useful.

Use the scan tool's freeze-frame information and misfire counters to determine whether the problem occurs:

• At idle

• During acceleration

• Under heavy load

• At high RPM

• During cold starts

• After the engine reaches operating temperature

A misfire that appears only during hard acceleration can point toward an ignition coil that breaks down under load or a fuel-delivery problem.

A misfire concentrated at idle may make an intake leak or mixture problem more interesting.

A temperature-dependent misfire can point toward a component that changes behavior as it heats up.

P0304 With Other DTCs

Always check the complete list of stored and pending trouble codes.

P0304 combined with an ignition-coil circuit code can direct attention toward the corresponding ignition circuit.

P0304 combined with fuel-pressure-related codes makes fuel delivery more important.

P0304 accompanied by camshaft, crankshaft or correlation codes raises suspicion of a timing or synchronization problem.

If several cylinders are misfiring simultaneously, the cause may be something affecting the engine as a whole rather than a fault limited to cylinder #4.

This is why clearing the code and waiting for it to return is not a complete diagnostic procedure.

Cylinder Numbering Is Not Universal

The physical location of cylinder #4 should be confirmed using the manufacturer's service information.

Cylinder numbering varies between engine manufacturers and configurations. Do not assume that cylinder #4 is always in a particular position.

This is especially important when identifying the correct ignition coil, spark plug, injector or wiring harness for testing.

Can P0304 Damage the Catalytic Converter?

A severe misfire can damage the catalytic converter.

When cylinder #4 fails to burn the air-fuel mixture correctly, unburned fuel can enter the exhaust system. Excessive fuel reaching the catalytic converter can cause it to overheat.

A flashing Check Engine Light is particularly important because it generally indicates a misfire severe enough to potentially damage the emissions system.

If the engine is shaking heavily and the Check Engine Light is flashing, continued driving should be minimized until the fault is diagnosed.

A Practical Diagnostic Order for P0304

A logical diagnostic process is:

1. Scan all stored and pending DTCs.

2. Check freeze-frame information and misfire counters.

3. Inspect the cylinder #4 spark plug.

4. Test or swap the ignition coil if applicable.

5. Inspect ignition and injector wiring.

6. Check injector operation.

7. Verify fuel pressure and delivery when indicated.

8. Check for intake or vacuum leaks.

9. Compare cylinder #4 compression with the other cylinders.

10. Perform a leak-down test if compression is abnormal.

11. Investigate valve timing, VVT and cam/crank synchronization if other evidence supports it.

12. Consider an ECM/PCM fault only after the external causes have been eliminated.

P0304 Does Not Automatically Mean a Bad Engine

A P0304 code can come from something as simple as a worn spark plug or poor electrical connection. It can also result from a defective ignition coil, injector or intake leak.

In more serious cases, the cause may be low compression, valve damage or another internal engine problem.

The DTC itself cannot tell you which category is responsible.

A good diagnostic strategy is therefore to compare cylinder #4 with a cylinder that is operating normally.

If moving the ignition coil moves the misfire, the coil becomes the primary suspect. If the misfire remains on #4, continue with injector, wiring and intake checks. If the ignition and fuel systems are proven good, compression and leak-down testing become much more important.

Final Diagnostic Summary

P0304 – Cylinder #4 Misfire Detected means the ECM/PCM has detected abnormal combustion associated with cylinder #4.

It does not automatically mean the spark plug, ignition coil or injector is defective, and it does not prove that the engine has internal damage.

The correct approach is to investigate the complete combustion process: spark, fuel, air, compression and timing.

Start with the easiest cylinder-specific tests and use component swaps, live misfire data and electrical testing to determine where the fault actually follows. If cylinder #4 continues to misfire after the ignition and fuel systems have been verified, move toward intake, compression, valve-train and timing diagnosis.

The objective is not simply to clear P0304. It is to determine why cylinder #4 is producing abnormal combustion compared with the other cylinders and correct that underlying problem.