P0304 Code Explained (Cylinder 4 Misfire)

Why You Got a P0304 Code (Simple Explanation)

The engine computer monitors how each cylinder contributes to engine speed and smoothness.

When cylinder 4 starts missing combustion events or slowing down unexpectedly, the computer can trigger a P0304 code.

In simple terms, cylinder 4 may be fine sometimes, then start misfiring when you accelerate, climb a hill, merge onto the highway, or put the engine under more load.

Common Symptoms of a P0304 Code

P0304 is often more noticeable when the engine has to work harder. Instead of shaking badly all the time, the car may hesitate when accelerating, feel weak uphill, or stumble while merging onto the highway.

• Hesitation during acceleration
• Loss of power uphill
• Engine struggling under load
• Check engine light stays on or flashes
• Jerking or stumbling when pressing the gas
• Rough idle in some cases
• Reduced fuel economy

What the Pattern of a Cylinder 4 Misfire Can Tell You

P0304 tells you where the misfire was detected, but the pattern can help decide what to check next. Treat it as a clue, not proof of one failed part.

What changed before the code appeared matters. A misfire that began right after service should send you back to the parts and connectors that were touched before you assume a new fault appeared somewhere else.

What Makes P0304 Different From P0300, P0301, P0302, and P0303?

P0304 means the computer is seeing the misfire on cylinder 4. That is different from P0300, where the misfire is random or spread across more than one cylinder.

For a beginner, the main point is simple: do not treat P0304 like a general engine problem first. Start with cylinder 4 checks — spark plug, coil, injector, connector, and wiring near that cylinder.

Best first move: if your engine uses coil-on-plug ignition, compare cylinder 4 with another cylinder before buying parts.

What “Cylinder 4” Means

Cylinder 4 means the engine computer has identified the misfire on the cylinder the manufacturer labels as number 4.

The physical location of cylinder 4 depends on the engine design, so do not assume its exact position without checking your vehicle information.

The code gives you a starting point, not a final diagnosis. Cylinder 4 could have a spark, fuel, wiring, or compression problem.

If your scanner shows misfires across different cylinders instead of only one, see P0300 (Random/Multiple Cylinder Misfire Detected).

Common Causes of a P0304 Misfire

1. Worn spark plug on cylinder 4

This is one of the first things many owners end up checking. If the spark plug is worn, fouled, or damaged, cylinder 4 may not fire correctly.

2. Weak or failing ignition coil

A weak ignition coil is one of the most common real-world causes of a P0304 code. The misfire often becomes more noticeable during acceleration, uphill driving, or whenever the engine is under load.

3. Fuel injector problem

Sometimes the injector for cylinder 4 becomes partially clogged or starts spraying fuel unevenly, especially on higher-mileage engines.

In some cases, the engine may run rougher during idle or after cold starts before improving slightly.

4. Wiring or connector issue

A damaged connector, loose wiring, or poor electrical connection can affect the spark plug, coil, or injector for that cylinder.

5. Vacuum leak or mixture problem

Air/fuel imbalance can make combustion unstable. Broader mixture-related issues can also trigger codes like P0174.

6. Low compression or mechanical issue

Burned valves, head gasket issues, or internal engine wear can also cause a cylinder-specific misfire.

What a Scanner Can Help You See

Even a beginner-friendly scanner can help you avoid guessing.

• Stored and pending codes
• Whether the misfire is limited to one cylinder or not
• Freeze-frame data from when the fault was detected
• Whether fuel-trim or catalyst-related codes are present too

If you are still choosing your first scanner, see our Best OBD2 Scanners Under $50

Not sure whether a phone-based tool or standalone tool is easier? Read: Bluetooth vs Wired OBD2 Scanners

Can I Drive With a P0304 Code?

Sometimes yes — but be careful.

If the check engine light is flashing, the engine is shaking badly, or the car has clear power loss, you should avoid driving if possible.

If the light is solid and the car still runs mostly normal, short-term driving may be possible while you diagnose it, but you should not ignore it.

If you are unsure whether it is safe to keep driving, read: Can I drive with the check engine light on?

Beginner Mistakes to Avoid

Replacing random parts without checking cylinder 4 first

P0304 already tells you which cylinder has the problem. Start there instead of guessing across the whole engine.

Ignoring a flashing check engine light

A flashing light is a strong clue that the misfire is active and more urgent.

Clearing the code before writing it down

Once you clear it, you lose useful clues like freeze-frame data and related codes.

Assuming it is always “just a spark plug”

Spark plugs are common, but ignition, injector, wiring, air/fuel, and mechanical issues can all cause P0304.

When P0304 Stays on Cylinder 4

If you swap the cylinder 4 ignition coil with another cylinder and P0304 still returns on cylinder 4, that result matters. The moved coil becomes less likely to be the whole explanation, so the next checks should stay focused on cylinder 4.

1. Verify the spark plug. Check that the cylinder 4 plug is the correct type, seated properly, and not obviously worn, fouled, cracked, or damaged.
2. Use a simple coil swap where practical. If the misfire moves with the coil, the coil becomes much more suspicious. If it does not, do not keep buying coils.
3. Check the wiring and connector. Look for a loose connection or visible damage around the cylinder 4 coil and injector wiring.
4. Consider the injector. If the ignition-side checks do not explain the problem, fuel delivery to cylinder 4 becomes a more reasonable next area to investigate.
5. Move to compression or mechanical checks if needed. A P0304 that keeps returning after the simpler checks may need deeper testing rather than more parts swapping.

Beginner rule: if P0304 stays on cylinder 4, follow the evidence in order instead of replacing the most common part again.

Frequently Asked Questions

Does P0304 always mean a bad ignition coil?

No. Ignition coils are common causes, but spark plugs, injectors, wiring problems, vacuum leaks, and low compression can also trigger a cylinder 4 misfire.

Can a bad spark plug cause only cylinder 4 to misfire?

Yes. A worn or fouled spark plug on cylinder 4 can cause a single-cylinder misfire while the rest of the engine still runs relatively normal.

Why does P0304 sometimes feel worse during cold starts?

Some ignition and fuel-delivery problems become easier to notice when the engine is cold, especially during the first few minutes after startup.

Can P0304 come and go?

Yes. Some misfires only happen during acceleration, cold starts, wet weather, or heavier engine load.

Final Beginner Takeaway

P0304 means cylinder 4 is misfiring. The important part is this: the code tells you where the problem is happening, but not exactly which part failed.

A simple starting point: scan the car, write down all codes, and check the cylinder 4 spark plug and ignition coil before buying random parts.

If the engine is shaking hard or the light is flashing, treat it as more urgent than a simple emissions code.