P0304 Symptoms: Signs of a Cylinder 4 Misfire

P0304 is one of those codes you should take seriously without immediately assuming the engine needs a major repair. It simply tells you the computer detected repeated misfires on cylinder 4.

In diagnostic work, I treat the code as a starting point rather than a parts list. This guide explains the symptoms, severity, likely causes, and practical checks that narrow the problem down.

What Does the P0304 Code Mean?

P0304 means the engine control module or powertrain control module detected a misfire associated with cylinder 4. It is a generic OBD-II powertrain trouble code.

A cylinder misfire happens when combustion doesn’t occur correctly during one or more firing events. The affected cylinder may produce weak power or no useful power during that event.

What does each part of P0304 indicate?

• P: The fault belongs to the powertrain system.
• 0: It is a generic OBD-II code rather than a manufacturer-specific code.
• 3: The code family relates to ignition or misfire detection.
• 04: The detected misfire is associated with cylinder 4.

P0304 identifies the cylinder with the detected problem. It does not identify which component failed.

A worn spark plug can cause it, but so can an ignition coil, fuel injector, vacuum leak, wiring problem, or loss of compression. Testing matters.

How does the computer know cylinder 4 is misfiring?

The engine computer monitors changes in crankshaft speed as the cylinders fire. A weak combustion event can create a small, measurable change in crankshaft acceleration.

The exact monitoring strategy varies by vehicle. Modern systems may combine crankshaft, camshaft, load, temperature, and other operating information.

Definition: A cylinder misfire is a combustion failure that prevents one cylinder from producing its expected power during a firing event.

Section Takeaway: P0304 tells you where the misfire was detected, not which part needs replacement.

What Are the Most Common P0304 Symptoms?

The most common P0304 symptoms are rough idle, shaking, hesitation, reduced power, and a check-engine light. The symptoms can range from barely noticeable to severe.

Rough or shaky idle

A rough idle is one of the easiest symptoms to notice. The steering wheel, seat, or entire vehicle may vibrate while you sit at a stop.

The engine may also sound uneven. You might hear a repeating stumble instead of a smooth exhaust rhythm.

Hesitation during acceleration

The engine may hesitate when you pull away from a stop or accelerate onto a highway. Cylinder 4 isn’t contributing consistently, so the engine cannot produce power smoothly.

• Light throttle: You may notice a small stumble or surge.
• Heavy throttle: The misfire may become much more obvious.
• Uphill driving: Added engine load can expose a weak ignition component.
• Cold starts: Some misfires become noticeable only when the engine is cold.

Loss of engine power

A constant cylinder 4 misfire can make the engine feel noticeably weaker. The effect is often more obvious on smaller engines.

You may need more throttle than normal to maintain speed. Passing or climbing hills can also feel harder.

Check-engine light

P0304 normally appears with the malfunction indicator lamp, commonly called the check-engine light. The light may be steady or flashing depending on the severity and vehicle strategy.

A flashing light deserves immediate attention. It can indicate a misfire severe enough to threaten the catalytic converter.

Poor fuel economy and fuel smell

Fuel economy may decline because the engine isn’t converting the air-fuel mixture into power efficiently. Other control adjustments may further affect consumption.

With a severe misfire, you may sometimes notice an unusual exhaust or fuel smell. Don’t assume the smell proves the injector itself is defective.

Hard starting or stalling

A single-cylinder misfire doesn’t always cause a no-start condition. However, severe ignition, fuel, or mechanical faults can make the engine harder to start.

Stalling becomes more likely when the engine is already running poorly at low speed. Other stored fault codes can provide important clues.

Section Takeaway: The strongest P0304 pattern is an illuminated check-engine light combined with rough running, shaking, hesitation, or reduced power.

Can You Drive With a P0304 Code?

You shouldn’t ignore P0304. Whether you can move the vehicle a short distance depends on how badly the engine is misfiring.

A mild intermittent misfire is different from violent shaking with a flashing check-engine light. Always judge the symptoms, not just the code number.

Stop driving if these symptoms appear

• Flashing check-engine light: Treat an active severe misfire as urgent.
• Heavy engine shaking: Severe vibration suggests the cylinder isn’t contributing normally.
• Major power loss: Reduced acceleration can create a driving hazard.
• Stalling: Don’t continue if the engine cannot remain running reliably.
• Abnormal heat or odor: Stop if you smell overheating, burning, or strong raw fuel.

Unburned fuel entering the exhaust can overheat a catalytic converter. That can turn a relatively manageable misfire into a much more expensive repair.

For broader road and vehicle safety information from NHTSA, use official government guidance rather than relying on dashboard-code advice alone.

Locksmith’s note: If the vehicle is shaking badly or losing power in traffic, prioritize getting it somewhere safe. Diagnosis can wait until the car is no longer creating a road hazard.

Section Takeaway: A steady light with mild symptoms still needs diagnosis, while a flashing light or severe shaking is a strong reason to stop driving.

What Causes a P0304 Cylinder 4 Misfire?

P0304 can come from an ignition, fuel, air, electrical, or mechanical problem affecting cylinder 4. Spark plugs and ignition coils are common suspects, but replacing them without testing can waste money.

Common ignition-related causes

• Worn spark plug: Excessive wear or fouling can weaken or prevent ignition.
• Faulty ignition coil: A weak coil may fail more often under load.
• Damaged coil boot: Cracks, oil contamination, or carbon tracking can allow spark leakage.
• Wiring problem: Damaged connectors or wiring can interrupt coil control.

Fuel-related causes

• Restricted injector: Cylinder 4 may receive too little fuel.
• Electrical injector fault: Wiring or connector problems can prevent normal operation.
• Fuel pressure problem: Low system pressure can contribute to misfires, often on more than one cylinder.

Air and mechanical causes

• Vacuum leak: An intake leak near cylinder 4 can create a lean mixture.
• Low compression: Valve, piston, ring, or head-gasket problems can reduce cylinder pressure.
• Valve-train problem: Incorrect valve operation can interfere with cylinder filling or sealing.
• Timing problem: Mechanical or variable-valve-timing faults can create wider misfire patterns.

If a known vehicle issue seems possible, you can also use the official NHTSA recall and vehicle-information search with the correct VIN, year, make, and model.

Section Takeaway: Don’t assume P0304 automatically means “replace the spark plug.” The fault can originate in spark, fuel, air, wiring, or engine mechanics.

How Do You Diagnose P0304 Correctly?

Start with the scan data, then test the simplest likely causes before moving into mechanical diagnosis. Don’t clear the code before recording useful information.

1. Scan every stored and pending code

Confirm P0304 and look for other trouble codes. A companion fuel-trim, injector, airflow, or random-misfire code can change the diagnostic direction.

Save freeze-frame data when your scanner provides it. It can show engine speed, load, temperature, and other conditions present when the fault was recorded.

2. Confirm which cylinder is number 4

Never guess cylinder numbering from physical position alone. Cylinder layouts and numbering schemes vary between manufacturers and engine families.

Use service information for the exact year, make, model, and engine. This prevents testing or replacing a component on the wrong cylinder.

3. Inspect the cylinder 4 ignition components

• Look for a worn, fouled, damaged, or contaminated spark plug.
• Inspect the ignition coil and boot for cracking or carbon tracking.
• Check connectors for loose terminals, corrosion, or damaged wiring.
• Look for oil or water inside the spark-plug well.

On engines with identical coil-on-plug components, a technician may compare or swap suitable components between cylinders as a diagnostic test. Only do this when the parts are interchangeable and the procedure is appropriate for that vehicle.

4. Check the fuel side

If the ignition system checks out, verify injector operation and electrical control. A clogged, electrically failed, or improperly controlled injector can create the same P0304 code.

Pro tip: don’t apply power directly to an injector or probe control circuits without the correct wiring information. Modern engine electronics can be damaged by improper testing.

5. Test engine mechanical condition

A misfire that stays on cylinder 4 after spark and fuel checks may require compression testing. A leak-down test can provide more information about where cylinder pressure is escaping.

• Intake valve leakage
• Exhaust valve leakage
• Piston or piston-ring damage
• Head-gasket leakage
• Other internal mechanical faults

One mistake I avoid is treating an OBD2 code as proof that a named component has failed. The scanner tells you where the computer detected abnormal behavior; testing finds the cause.

Section Takeaway: Preserve the scan data, verify cylinder numbering, check ignition and fuel, then move to compression or deeper testing when needed.

How Is a P0304 Misfire Repaired?

The correct P0304 repair depends entirely on the confirmed cause. There isn’t one universal part that fixes every cylinder 4 misfire.

Possible repairs after diagnosis

• Replace a worn or damaged spark plug.
• Replace a confirmed faulty ignition coil or boot.
• Repair damaged coil or injector wiring.
• Repair or replace a faulty fuel injector.
• Repair an intake or vacuum leak.
• Correct a fuel-delivery problem.
• Repair an internal compression or valve-train problem.

How much does P0304 cost to fix?

Repair cost varies widely because P0304 describes a symptom detected by the computer, not a specific failed component. A basic ignition repair can cost far less than injector, cylinder-head, or internal-engine work.

Labor rates also vary by region, engine layout, and vehicle design. Diagnosis before parts replacement is usually the best way to control unnecessary expense.

Pro tip: ask the shop what testing confirmed the failed component before approving a larger repair. A trouble code alone isn’t proof that a coil, injector, or engine part needs replacement.

Should you clear P0304 after the repair?

The code may be cleared after the underlying fault has been repaired and the necessary information has been recorded. Then operate the vehicle according to the manufacturer’s recommended verification procedure.

Recheck for pending codes and misfire data when your scanner supports them. Returning P0304 means the underlying problem is still present or another fault remains.

Section Takeaway: Fix the verified cause rather than replacing parts based only on the code description, then rescan the vehicle to confirm the misfire stays gone.

P0304 Symptoms FAQ

What is the first symptom of a P0304 code?

A check-engine light or slightly rough idle may be the first sign. Intermittent misfires can be subtle, while a constant cylinder 4 misfire usually causes noticeable shaking or hesitation.

Can a bad spark plug cause P0304?

Yes. A worn, fouled, cracked, or incorrectly operating spark plug can cause cylinder 4 to misfire, but the code doesn’t prove the plug is bad. The coil, injector, wiring, air supply, and compression should also be considered.

Can a bad ignition coil cause P0304?

Yes. A weak or failed ignition coil can prevent reliable spark on cylinder 4. Testing should confirm the coil before replacement because injector and mechanical problems can produce similar symptoms.

Can P0304 cause the car to shake?

Yes. Cylinder 4 contributes less torque when it misfires, which can make the engine run unevenly. The vibration may be strongest at idle and can become more pronounced during acceleration.

Can P0304 cause poor acceleration?

Yes. A cylinder that isn’t burning its mixture correctly cannot deliver normal power. You may notice hesitation, stumbling, sluggish acceleration, or greater power loss when climbing hills or placing the engine under load.

Can low compression cause P0304?

Yes. Burned valves, piston or ring problems, head-gasket leakage, and other mechanical faults can reduce compression in cylinder 4. Compression or leak-down testing becomes important when ignition and fuel checks don’t reveal the cause.

Will P0304 go away by itself?

An intermittent misfire may temporarily disappear, and the warning light can sometimes change after several drive cycles. That doesn’t prove the fault repaired itself, so stored and pending codes should still be checked.

Final Takeaway: P0304 means cylinder 4 is misfiring, but it doesn’t tell you which part failed. Start with scan data, take a flashing check-engine light seriously, and diagnose spark, fuel, air, wiring, and compression before buying parts.

If you’re troubleshooting at home, record every code and freeze-frame detail before clearing the computer. A few minutes of careful diagnosis can prevent a long trail of unnecessary replacement parts.