What Does P0304 Mean? Cylinder 4 Misfire Explained

If your scanner just displayed P0304, don’t assume cylinder 4 needs an expensive engine repair. The code identifies where the misfire was detected, not which component caused it.

I’ve found that this distinction prevents a lot of unnecessary parts swapping. This guide explains what P0304 means, what symptoms matter, and how to narrow the fault down logically.

What Does P0304 Mean?

P0304 is an OBD-II diagnostic trouble code for a misfire detected on cylinder 4. Your engine control module noticed that cylinder contributing less consistently than expected.

A misfire means combustion didn’t occur normally during one or more firing events. The cylinder may have weak spark, incorrect fuel delivery, poor compression, or another condition that interrupts combustion.

Definition: P0304 = Cylinder 4 Misfire Detected. The code tells you which cylinder is misfiring, but it does not identify the failed component.

How does the computer detect a cylinder misfire?

Modern engine computers monitor tiny changes in crankshaft speed while the engine runs. A properly firing cylinder briefly adds power and helps accelerate the crankshaft.

If cylinder 4 repeatedly fails to make its expected contribution, the computer can recognize the uneven crankshaft acceleration. It then records a diagnostic trouble code when its programmed conditions are met.

• P identifies a powertrain-related code.
• 03 places the fault in the ignition or misfire family.
• 04 identifies cylinder 4 as the cylinder where the misfire was detected.

Where is cylinder 4 located?

Cylinder 4’s physical location depends on the engine design and manufacturer. Don’t assume it is always the fourth cylinder from the front of the vehicle.

Inline engines and V-shaped engines may use different numbering arrangements. Check the vehicle’s service information before disconnecting coils, injectors, or spark plugs.

Diagnostic note: Verify the cylinder numbering for your exact make, model, year, and engine before moving or replacing parts.

Is P0304 Serious, and Can You Keep Driving?

P0304 should be diagnosed soon because continued misfiring can affect drivability, emissions, fuel use, and the catalytic converter. The urgency increases sharply if the check-engine light flashes.

A steady check-engine light may appear with a mild or intermittent misfire. You should still investigate it rather than simply clearing the code.

What does a flashing check-engine light mean?

A flashing malfunction indicator light can indicate a severe active misfire. Unburned fuel may enter the exhaust and create damaging heat in the catalytic converter.

AAA advises drivers to treat a flashing check-engine light as a more serious condition than a steady light. If the engine also shakes or loses power, driving should be minimized.

• Steady light: Diagnose the code promptly and watch for worsening symptoms.
• Flashing light: Reduce load and speed, then stop somewhere safe if the engine runs poorly.
• Severe shaking: Avoid continuing a normal trip.
• Major power loss: Consider roadside assistance or towing.

Common symptoms of P0304

Some vehicles store P0304 before the driver notices an obvious problem. Others develop unmistakable drivability symptoms.

• Check-engine light
• Rough or uneven idle
• Engine vibration or shaking
• Hesitation during acceleration
• Reduced engine power
• Hard starting in some cases
• Poorer fuel economy
• Fuel or exhaust odor when the misfire is severe

Section Takeaway: A steady P0304 deserves prompt diagnosis. A flashing check-engine light combined with an active misfire deserves much faster attention.

What Causes a P0304 Cylinder 4 Misfire?

The most common P0304 causes fall into three groups: ignition, fuel delivery, and mechanical compression. Wiring, air leaks, or engine-management problems can also contribute.

One mistake I see in diagnostic work is treating the code description as a parts recommendation. P0304 does not automatically mean “replace the spark plug” or “replace the coil.”

Ignition problems

A damaged spark plug, coil, coil boot, or related connection may prevent the air-fuel mixture from igniting properly. Ignition faults are often among the first things technicians check.

Still, replacing all plugs and coils without testing can waste money. Compare cylinder 4 with known-good cylinders whenever the service procedure allows it.

Fuel injector or fuel delivery problems

Cylinder 4 also needs the correct amount of fuel at the right time. A restricted injector, electrically failed injector, or damaged injector circuit can create the same P0304 code.

Fuel pressure problems usually affect more than one cylinder, but vehicle-specific conditions vary. This is why companion codes and live data matter.

Low compression or mechanical problems

A cylinder can have fuel and spark yet still misfire if it cannot compress the mixture properly. Possible mechanical causes include valve sealing problems, piston or ring damage, or head-gasket concerns.

Mechanical testing usually comes after easier ignition and fuel checks unless other symptoms point there first.

Section Takeaway: P0304 identifies the affected cylinder, not the failed part. Test ignition, fuel delivery, wiring, air leaks, and compression before choosing a repair.

How Do You Diagnose P0304?

Start by confirming P0304 and recording the conditions under which it appeared. Then test the simplest likely causes before moving toward fuel-system or mechanical diagnosis.

A basic code reader can reveal P0304, while a more capable scan tool may show freeze-frame data, fuel trims, Mode $06 information, or cylinder misfire counters where the vehicle supports them.

1. Scan the vehicle before clearing anything. Record P0304 plus pending, stored, and related trouble codes.
2. Review freeze-frame data. Note engine speed, load, coolant temperature, and other available information from when the fault set.
3. Confirm cylinder numbering. Use correct service information for the engine.
4. Inspect cylinder 4’s ignition components. Look for plug wear, damaged boots, loose connectors, oil contamination, or corrosion.
5. Test rather than guess. Where the manufacturer permits it, compare or swap a suspected component with a known-good cylinder and see whether the misfire follows.
6. Check the injector and wiring. Follow vehicle-specific electrical and fuel-system test procedures.
7. Check compression if needed. A compression or leak-down test can reveal an internal mechanical problem.
8. Verify the repair. Clear codes only after the fault is addressed, then confirm that the misfire does not return.

Why freeze-frame data matters

Freeze-frame data is a snapshot of operating conditions when the computer stored a fault. It can show whether the misfire occurred at idle, during acceleration, when cold, or under another repeatable condition.

That information helps separate an intermittent problem from a constant one. It can also stop you from testing only at idle when the fault happens under load.

Don’t ignore other codes

P0304 often makes more sense when viewed with every stored and pending code. A lean condition, injector circuit code, sensor fault, or random misfire code may change your diagnostic direction.

AAA’s OBD information also emphasizes that diagnostic trouble codes provide clues rather than automatically identifying the part that must be replaced.

Pro tip: If testing requires fuel-system work, high-voltage ignition checks, or internal engine diagnosis, use the manufacturer’s service procedure or a qualified repair professional.

Section Takeaway: Save the scan data first, verify cylinder 4, and work from simple tests toward deeper mechanical diagnosis. Don’t clear the evidence before you’ve looked at it.

How Do You Fix Code P0304?

You fix P0304 by repairing the condition that is making cylinder 4 misfire. There is no single universal P0304 repair.

The correct fix depends entirely on what testing finds.

• Replace a worn or damaged spark plug when confirmed faulty.
• Replace or repair a failed ignition coil or coil connection.
• Repair damaged wiring or connectors.
• Service or replace a faulty fuel injector when properly diagnosed.
• Repair an intake leak affecting mixture control.
• Correct fuel-delivery problems when testing confirms them.
• Repair internal engine problems when compression testing identifies them.

Can you fix P0304 yourself?

Some owners can handle basic inspection or spark-plug work if they have the correct tools and service information. Diagnosis becomes more difficult when the problem involves injectors, wiring, fuel pressure, or compression.

A scan tool helps, but it doesn’t replace testing. The most expensive repair is often the part you replace before proving it is bad.

How much does P0304 cost to repair?

There is no reliable single P0304 repair price because the code can have several unrelated causes. A plug or connector problem and an internal compression problem involve completely different parts and labor.

Vehicle make, engine layout, parts availability, local labor rates, and diagnostic time all affect the final bill. Get the cause confirmed before comparing repair estimates.

Locksmith’s diagnostic note: Ask what test confirmed the failed component before authorizing a costly repair. A trouble code alone isn’t proof that a specific part has failed.

Should you clear P0304 after the repair?

You can clear the code after completing the repair if the service procedure permits it. Then operate the vehicle under appropriate conditions and rescan it.

Some emissions-related diagnostic information may behave differently from an ordinary stored code. Let the vehicle complete its required self-checks instead of repeatedly erasing codes to hide a recurring fault.

How Is P0304 Different From Other Misfire Codes?

P0304 is cylinder-specific, while P0300 indicates a random or multiple-cylinder misfire. P0301, P0302, and P0303 identify cylinders 1, 2, and 3 respectively.

If P0304 appears alone, start with conditions affecting cylinder 4. If several cylinder codes appear together, widen the diagnosis to problems capable of affecting multiple cylinders.

For unusual repeat failures, you can also check your VIN and vehicle information through the NHTSA recall and manufacturer communications search. Vehicle-specific service information remains important because diagnostic procedures can differ by engine and model year.

P0304 FAQ

What does code P0304 mean on an OBD2 scanner?

P0304 means the vehicle’s computer detected a misfire on cylinder 4. It tells you where the abnormal combustion was detected, but additional testing is required to identify whether spark, fuel, wiring, air delivery, or engine compression caused it.

Is P0304 usually a spark plug?

A worn or fouled spark plug can cause P0304, but the code does not prove the plug is faulty. Ignition coils, injectors, electrical connections, air leaks, and low compression can produce the same cylinder-specific misfire code.

Can a bad ignition coil cause P0304?

Yes. A weak or failed ignition coil can prevent cylinder 4 from producing a strong spark. When appropriate for the vehicle, technicians may compare ignition operation with another cylinder to determine whether the fault follows the suspected coil.

Can a fuel injector cause P0304?

Yes. A restricted, electrically faulty, or poorly controlled cylinder 4 injector can cause a misfire. Injector diagnosis may require electrical checks, scan data, fuel-system testing, or manufacturer-specific procedures rather than simply replacing the injector.

Can low compression cause code P0304?

Yes. If cylinder 4 has poor compression, combustion may remain weak even when fuel and ignition components work correctly. A compression test or cylinder leak-down test can help identify valve, piston, ring, or other internal mechanical concerns.

Will P0304 clear itself?

The warning light may eventually turn off if the fault stops occurring under the vehicle’s required monitoring conditions. However, that does not prove the underlying cause is repaired, and diagnostic information may remain stored for some time.

Can I drive with P0304?

Short-distance driving may be possible when the light is steady and the engine runs normally, but the code should be diagnosed promptly. If the check-engine light flashes, the engine shakes heavily, or power drops significantly, limit driving and seek service.

P0304 looks simple on a scanner screen, but it is only the beginning of the diagnosis. Confirm the code, save the scan data, identify cylinder 4 correctly, and test before buying parts.

That method takes a little more time upfront. It usually saves far more time than replacing components until the misfire disappears.