BMW P0304 Code Meaning: Cylinder 4 Misfire Causes, Symptoms, and Fixes

BMW P0304 Code Meaning Explained

The BMW P0304 code meaning is straightforward: the engine control unit has detected a misfire in cylinder 4.

In practical terms, the combustion event in that cylinder is incomplete or absent often enough to trigger a diagnostic trouble code and, in many cases, a check engine light.

On BMW engines, a misfire can stem from ignition, fuel delivery, air leaks, compression loss, or even sensor and software issues.

Understanding the exact cause matters because a cylinder 4 misfire can range from a simple spark plug problem to a deeper engine fault that should not be ignored.

What the P0304 Code Means on a BMW

OBD-II code P0304 is a generic misfire code that applies across manufacturers, including BMW, but the brand’s engine management systems can store additional fault data that helps narrow the diagnosis.

The code points specifically to cylinder 4, not to the entire engine.

BMW engines such as the N52, N54, N55, B48, and B58 families may set P0304 for different underlying reasons, but the code interpretation remains the same: cylinder 4 is not firing as expected.

Because the engine computer monitors crankshaft speed variations, it can often detect a weak or intermittent misfire before the driver notices major drivability issues.

Common Symptoms of a BMW Cylinder 4 Misfire

A P0304 code often comes with noticeable drivability symptoms.

Some are subtle at idle, while others become more pronounced under acceleration or load.

• Rough idle or shaking at stoplights
• Reduced engine power
• Hesitation during acceleration
• Fuel smell from the exhaust
• Check engine light flashing or staying on
• Increased fuel consumption
• Poor cold-start behavior

If the check engine light is flashing, the misfire may be severe enough to damage the catalytic converter.

In that case, continued driving can turn a relatively simple repair into a much more expensive one.

Most Common Causes of BMW P0304

Cylinder misfires on BMW engines usually fall into a few familiar categories.

The challenge is identifying whether the problem is isolated to cylinder 4 or part of a broader issue affecting multiple cylinders or the entire engine.

1. Faulty spark plug

Worn, fouled, or incorrectly gapped spark plugs are one of the most common reasons for a BMW misfire code.

If cylinder 4’s spark plug is degraded, the air-fuel mixture may not ignite consistently, especially under load.

2. Bad ignition coil

BMW ignition coils are frequent failure points.

A weak coil on cylinder 4 can cause intermittent misfires, rough idle, and poor acceleration.

In many cases, moving the coil to another cylinder is a useful diagnostic step because the code may follow the coil.

3. Fuel injector problems

A clogged, leaking, or electrically faulty injector can prevent cylinder 4 from receiving the right amount of fuel.

Direct-injection BMW engines are especially sensitive to injector performance, and injector faults can sometimes mimic ignition problems.

4. Vacuum leak or intake air leak

Unmetered air entering the intake can lean out the mixture and create a misfire.

Common leak points include intake boots, manifold gaskets, PCV components, valve cover systems, and vacuum hoses.

Even a small leak near cylinder 4 can affect combustion stability.

5. Low compression or mechanical engine fault

If spark and fuel are both present, the issue may be mechanical.

Burned valves, worn piston rings, damaged valve train components, or head gasket problems can reduce compression in cylinder 4 and trigger P0304.

This type of fault usually requires compression or leak-down testing.

6. Carbon buildup

On BMW direct-injection engines, intake valve carbon buildup can restrict airflow and disrupt combustion.

This is especially relevant on higher-mileage vehicles and can contribute to rough idle and random or cylinder-specific misfires.

7. Wiring or connector issues

Damaged wiring, loose connectors, oil intrusion, or corrosion in the cylinder 4 ignition or injector circuit can create an intermittent misfire.

Electrical problems are often overlooked because they may appear only when the engine is hot or vibrating.

How to Diagnose BMW P0304 Correctly

A proper diagnosis should follow a logical order rather than replacing parts at random.

BMW-specific scan tools can be especially helpful because they often show misfire counters, freeze-frame data, and related fault codes.

Start with the scan data

Read all stored and pending codes, not just P0304.

Related codes for fuel trim, mixture control, boost leaks, VANOS issues, or injector faults can point toward a bigger root cause.

Freeze-frame data can also show whether the misfire happened at idle, cruise, or under load.

Inspect and swap ignition components

Check the spark plug and ignition coil for cylinder 4.

If the coil or plug appears worn, swap it with another cylinder and clear the code.

If the misfire moves to the new cylinder, the swapped part is likely the cause.

Check for air leaks

Inspect the intake system for cracks, loose clamps, torn boots, and degraded vacuum hoses.

A smoke test is one of the fastest ways to find hidden leaks on BMW engines, especially around the intake manifold and crankcase ventilation system.

Test the injector

If ignition parts and leaks check out, evaluate the fuel injector.

This may involve listening for injector operation, checking electrical signal, looking at fuel trims, or swapping injectors if the engine design allows it.

Direct-injection systems often require more advanced testing.

Measure compression and leak-down

When the misfire persists after basic checks, compression testing and leak-down testing help determine whether cylinder 4 has a mechanical issue.

Consistent low compression on one cylinder usually means the problem is internal and not solved by ignition or fuel parts alone.

Can You Drive a BMW with P0304?

Short trips may be possible if the misfire is mild, but driving with a persistent cylinder 4 misfire is not recommended.

Misfires can overheat and damage the catalytic converter, reduce fuel economy, and place extra stress on the engine and ignition system.

If the engine is shaking severely, the check engine light is flashing, or the car has lost significant power, the safest choice is to stop driving and diagnose the issue immediately.

The longer a misfire continues, the more likely it is to cause additional repair costs.

Typical Repair Options for a BMW P0304 Code

The correct fix depends on the cause, but the most common repairs are relatively straightforward when the problem is caught early.

• Replace worn spark plugs
• Replace a failed ignition coil
• Clean or replace a faulty fuel injector
• Repair vacuum leaks or cracked intake components
• Fix wiring or connector damage
• Perform carbon cleaning on intake valves if needed
• Repair mechanical engine damage in severe cases

After repairs, clear the code and road test the vehicle under similar conditions that triggered the misfire.

Confirm that the misfire counters stay at zero and no related codes return.

BMW Models Where P0304 Commonly Appears

P0304 can appear on many BMW models, including the 3 Series, 5 Series, X3, X5, and 7 Series, as well as performance variants like M Performance and turbocharged six-cylinder models.

The underlying cause often depends more on the specific engine family and maintenance history than on the vehicle body style.

For example, coil and plug issues are common across many BMW gasoline engines, while injector concerns may be more prominent on direct-injection turbo models.

Higher-mileage vehicles with neglected maintenance are also more likely to develop vacuum leaks, carbon buildup, and worn ignition parts.

What to Check Before Replacing Parts

Before buying parts, confirm the basic condition of cylinder 4 and the surrounding systems.

A few simple checks can prevent unnecessary expense and repeated repairs.

• Verify the correct cylinder numbering for your BMW engine
• Inspect the spark plug for oil, carbon, or wear
• Check coil boot condition and connector security
• Look for intake leaks or cracked hoses
• Review other trouble codes and fuel trim data
• Test battery voltage and charging system health

Low system voltage, poor grounds, and weak batteries can also contribute to drivability issues that resemble a misfire, particularly on modern BMWs with sensitive engine management systems.

When to Seek Professional BMW Diagnosis

If the misfire returns after a simple coil or plug swap, or if compression and injector testing are needed, professional diagnosis is the best path.

BMW-specific scan tools, smoke machines, and leak-down equipment can significantly reduce guesswork.

A qualified technician can also interpret adaptation values, misfire counters, and fuel trim data to identify whether P0304 is the result of ignition failure, fuel delivery problems, air leakage, or internal engine wear.