P0304 Code Explained (Cylinder 4 Misfire)
A P0304 code means the engine computer detected a misfire on cylinder 4. Many drivers notice it most during acceleration, uphill driving, or when the engine is under load.
The car may feel mostly normal at idle, then stumble, hesitate, or lose power when you press the gas.
If the engine feels rough at idle, see Car Shakes When Idling . If the car jerks, bucks, or hesitates during acceleration, see Car Jerks When Accelerating .
- If the engine is shaking or the light is flashing, avoid driving
- Check the spark plug and ignition coil on cylinder 4
- Swap the coil with another cylinder to see if the misfire moves
- Clear the code and see if it returns
Here’s what that means in real driving: cylinder 4 is not firing cleanly every time. It may feel mild at idle, but show up more clearly when the engine is working harder.
If you are brand new to scanning codes, learn how to use an OBD2 scanner step-by-step . If your check engine light just came on and you have not read the basics yet, start here: What Does the Check Engine Light Mean?
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 P0304 Often Feels Like in Real Life
A cylinder 4 misfire may feel worse when the engine is under load. Many drivers notice it most while accelerating, climbing a hill, or merging onto the highway.
- Hesitation during acceleration: The car may stumble or feel uneven when you press the gas.
- Loss of power uphill: The misfire may become more noticeable when climbing hills or driving with extra load.
- Flashing check engine light under load: The light may flash during harder acceleration, uphill driving, or highway merging.
- The problem feels worse during cold starts: In some cases, the engine runs rough for the first few minutes before improving slightly.
- The misfire follows the ignition coil: A common beginner test is swapping the cylinder 4 ignition coil with another cylinder to see if the code moves.
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).
Why Cylinder 4 Matters
P0304 is different from a random misfire because the computer has narrowed the problem to cylinder 4.
For beginners, this helps reduce guesswork because the first inspection can stay focused on cylinder 4 ignition, fuel, and wiring components.
Why Misfires Matter
A misfire means the air-fuel mixture in a cylinder is not burning correctly. That can make the engine run rough and send unburned fuel into the exhaust.
If the misfire keeps returning, that extra fuel can overheat and damage the catalytic converter. That is one reason a P0304 code should not be ignored for long.
Over time, repeated misfires can contribute to catalyst-efficiency problems like P0420. In some cases, broader air/fuel issues can also overlap with codes like P0171 or P0175.
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.
Quick real-world test: swap the cylinder 4 coil with another cylinder and see if the misfire code follows it.
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?
How Serious Is P0304?
Fairly serious.
P0304 is more serious than many simple emissions codes because it involves combustion problems, not just a sensor reading or a small leak.
If the misfire continues, it can:
- Cause rough running and poor acceleration
- Waste fuel
- Damage the catalytic converter
- Point to a larger ignition, fuel, or mechanical problem
Beginner rule: if the light is flashing or the engine is shaking hard, treat it as urgent.
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.
Where Most People Start
If you want to keep this simple, follow this order:
- Read and write down all stored codes
- Pay attention to whether the check engine light is solid or flashing
- Check the spark plug and ignition coil for cylinder 4 first
- Look for related fuel-trim or misfire codes
- Think about injector, wiring, or compression issues only after the basics
Beginner move: start with the spark plug and ignition coil on cylinder 4 before replacing more expensive parts.
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.