P0304 Repair Cost: How Much Does It Cost to Fix a Cylinder 4 Misfire?

P0304 can look like a simple spark-plug problem, but the final repair bill depends entirely on why cylinder 4 is misfiring. That is why I treat the trouble code as the start of the diagnosis rather than a shopping list.

This guide explains realistic 2026 cost ranges, common causes, driving risks, and the tests that can help prevent unnecessary repairs.

What Does P0304 Mean?

P0304 means “Cylinder 4 Misfire Detected.” The powertrain control module has detected that cylinder 4 is not contributing to combustion as expected.

A misfire happens when the air-fuel mixture inside a cylinder does not burn correctly. The problem may involve spark, fuel, airflow, compression, wiring, or another engine condition.

Definition: P0304 is a cylinder-specific OBD-II diagnostic trouble code. It tells you where the computer detected the misfire, but it does not tell you which component caused it.

What symptoms can appear with P0304?

Some vehicles only display a check-engine light. Others develop obvious drivability problems.

• Rough idle: The engine may shake or feel uneven while stopped.
• Hesitation: Acceleration may feel weak or inconsistent.
• Power loss: Cylinder 4 may contribute less power than expected.
• Hard starting: The engine may crank longer or start roughly.
• Poor fuel economy: Incomplete combustion can reduce efficiency.
• Flashing warning light: A severe active misfire can make the check-engine light flash.

Do not assume cylinder 4 is located in the same place on every engine. Inline, V-type, and horizontally opposed engines use different cylinder numbering arrangements.

Check the correct service information for your exact make, model, engine, and model year before touching parts.

Technician’s note: Confirm cylinder numbering before removing a plug, coil, or injector. Testing the wrong cylinder wastes time and can create new problems.

How Much Does P0304 Repair Cost?

P0304 repair cost varies because there is no single “P0304 part.” You pay for the diagnosis and for whatever component or mechanical problem caused cylinder 4 to misfire.

As of 2026, the ranges below are broad U.S. repair estimates rather than quotes. Labor rates, engine design, parts quality, and regional pricing can change the total substantially.

The cheapest outcome is often a straightforward ignition repair. A worn plug, damaged boot, loose connector, or failed coil can sometimes be corrected without major engine work.

The expensive outcomes begin when ignition parts test normally. An injector problem, intake leak, wiring fault, or loss of compression can require more labor and testing.

Why shouldn’t you replace the coil immediately?

P0304 does not mean “replace ignition coil number 4.” A code reader identifies the affected cylinder, not the failed component.

A coil may be responsible, but an injector or compression problem can produce nearly identical symptoms. Testing first protects your repair budget.

• Vehicle design: Some plugs and coils are accessible in minutes, while others require substantial disassembly.
• Parts choice: OEM, original-equipment-supplier, and aftermarket components can have different prices.
• Labor rate: Independent shops and dealership service departments may charge different hourly rates.
• Related repairs: Oil leaks into plug wells or damaged connectors may need attention at the same time.
• Diagnostic depth: Intermittent misfires can require more testing than constant misfires.

Cost note: Ask for a diagnosis and written estimate before authorizing expensive parts. Prices can change significantly by vehicle and location.

Section Takeaway: P0304 can be inexpensive when the cause is a plug or simple ignition fault, but mechanical problems can push the repair into the thousands. Diagnose the cause before choosing a repair.

What Usually Causes a P0304 Cylinder 4 Misfire?

P0304 normally comes from a problem affecting spark, fuel, air, or mechanical compression in cylinder 4. Several failures can create the same code.

Worn or fouled spark plug

A damaged, worn, contaminated, or incorrectly gapped spark plug may not ignite the mixture consistently. Misfires can become more noticeable during acceleration when ignition demand rises.

Weak or failed ignition coil

Many modern gasoline engines use one ignition coil for each cylinder. A weak cylinder 4 coil can cause rough idle, hesitation, and P0304.

Damage to a coil boot, connector, or wiring can create similar symptoms. Inspect the complete ignition circuit rather than focusing only on the coil body.

Fuel injector problem

A restricted injector may deliver too little fuel. A leaking or electrically faulty injector can also interfere with normal combustion.

Injector diagnosis may involve electrical checks, scan data, balance testing, or other manufacturer-approved procedures.

Vacuum or intake leak

Unmetered air entering near the affected intake runner can create a mixture that is too lean. Intake-gasket leaks sometimes become more noticeable during cold starts.

Low compression

A cylinder needs adequate compression to burn the mixture effectively. Valve leakage, piston-ring wear, head-gasket trouble, or valvetrain damage may reduce cylinder pressure.

Compression problems become more important when the plug, coil, injector, and wiring test normally.

Wiring or connector damage

Heat, corrosion, vibration, rodents, or previous repair work can damage coil and injector circuits. A loose connector may create an intermittent P0304 without a failed component.

• Inspect the cylinder 4 coil connector.
• Inspect the injector connector.
• Look for stretched or damaged wiring.
• Check for oil or moisture contamination.
• Review any work performed shortly before the code appeared.

Repair note: Do not order an injector, coil, sensor, or other expensive component solely because it appears on a generic causes list.

Is It Safe to Drive With P0304?

A mild P0304 with a steady check-engine light may allow limited driving, but an active severe misfire should not be ignored. Strong shaking, stalling, major power loss, or a flashing light makes the situation more urgent.

A continuing misfire can send unburned fuel into the exhaust. That can raise exhaust temperature and put the catalytic converter at risk.

Stop driving when you notice these warning signs

• Flashing check-engine light: Treat it as an urgent active engine fault.
• Strong shaking: The cylinder may be misfiring heavily.
• Major power loss: Merging or crossing traffic may become unsafe.
• Repeated stalling: The engine may shut off unexpectedly.
• Fuel or burning smell: Stop safely and investigate the cause.
• Overheating: Shut the engine off in a safe location.

AAA advises drivers to react quickly when a flashing check-engine light appears, especially when the vehicle loses power, shakes, or behaves abnormally.

When the engine runs badly, towing can cost less than turning a manageable misfire into a damaged catalytic converter or larger engine repair.

Safety note: An OBD2 scanner tells you which fault was recorded. It cannot decide whether the vehicle is safe to drive.

How Do You Diagnose P0304 Before Paying for Repairs?

Start by confirming P0304 and recording all available diagnostic information. Do not clear the code before saving useful evidence.

AAA’s OBD diagnostic information also emphasizes that trouble codes provide clues rather than automatically identifying the required repair.

1. Scan every stored and pending code

P0304 may appear alone or alongside P0300, fuel-trim codes, injector circuit faults, camshaft codes, or other misfire codes. Related codes can change the diagnostic path.

2. Save freeze-frame information

Freeze-frame data records operating conditions when the fault was detected. Useful information can include engine speed, coolant temperature, load, vehicle speed, and fuel trim.

A cold-start-only misfire needs a different investigation than one occurring during hard acceleration. Save the evidence first.

3. Inspect accessible components

• Check visible ignition wiring and connectors.
• Look for oil or water inside spark-plug wells.
• Inspect intake hoses and obvious vacuum connections.
• Check whether recent work left anything disconnected.
• Review the vehicle’s spark-plug maintenance history.

4. Test ignition components

The spark plug and coil are logical early checks because they directly affect one cylinder. Follow the manufacturer’s testing procedure for your engine.

Component swapping is sometimes useful on coil-on-plug engines, but it should only be done when the components are interchangeable and the service information permits it.

5. Check fuel delivery

If ignition tests normally, cylinder 4’s injector and electrical circuit deserve attention. Professional testing can separate an injector fault from a wiring or control problem.

6. Check compression when necessary

A persistent cylinder-specific misfire with good spark and fuel delivery can indicate a mechanical issue. Compression and cylinder leak-down testing help determine whether cylinder 4 can seal properly.

7. Check recalls before paying for major repairs

Before authorizing an expensive repair, enter your VIN into the NHTSA recall lookup. A recall or manufacturer campaign can affect how a specific vehicle problem should be handled.

Do not assume a recall applies just because another owner has the same code. Coverage depends on the exact vehicle and VIN.

How can you reduce the final repair bill?

• Pay for diagnosis first: Testing may cost less than replacing several good parts.
• Bring scanner information: Saved codes and freeze-frame data can provide useful starting evidence.
• Review maintenance records: Old spark plugs or recent repairs may provide clues.
• Ask for test results: Request the evidence supporting an expensive injector or mechanical repair.
• Compare written estimates: This becomes more valuable when internal engine work is recommended.

The biggest money-saving principle is simple: do not confuse a diagnostic code with a parts recommendation.

Pro tip: Record codes and freeze-frame information before disconnecting the battery or clearing the check-engine light.

Section Takeaway: Good P0304 diagnosis moves from scanner evidence to ignition, fuel, air, wiring, and mechanical testing. Spending a little on accurate diagnosis can prevent much larger parts-swapping costs.

Final Takeaway

P0304 means cylinder 4 is misfiring, but it doesn’t name the defective part. Spark plugs and ignition coils are common suspects, yet injectors, intake leaks, wiring faults, and low compression can create exactly the same code.

Start with your OBD2 scanner, save the evidence, inspect cylinder 4, and test before replacing anything. If the check-engine light flashes or the engine runs badly, stop unnecessary driving and arrange proper diagnosis.

The fastest repair isn’t the one that changes parts first. It’s the one that proves the cause first.

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.

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.

How Do You Diagnose P0304 Without Replacing the Wrong Parts?

Start with scan data, then move from simple visual checks toward more involved testing. Save evidence before clearing the code.

Step 1: Scan Every Stored and Pending Code

Connect your OBD2 scanner and record all codes. P0304 appearing beside P0300, fuel-trim, injector-circuit, sensor, or timing codes can change your diagnostic direction.

Also save freeze-frame data when your scanner supports it. Freeze-frame data records operating conditions around the time the computer stored the fault.

• Engine RPM
• Vehicle speed
• Engine load
• Coolant temperature
• Short-term fuel trim
• Long-term fuel trim

As of 2026, many affordable scanners can display more than a code description. Live data and manufacturer-supported misfire information can make troubleshooting more useful.

Step 2: Inspect Cylinder 4 Carefully

Check the visible ignition and fuel-delivery components around cylinder 4. Look for anything loose, contaminated, cracked, overheated, or corroded.

• Spark plug condition
• Ignition coil and boot
• Injector connector
• Coil connector
• Nearby wiring harness
• Oil or water inside the spark-plug well
• Damaged vacuum or intake connections

Don’t disconnect or probe circuits randomly. Some modern ignition systems use sensitive electronics and manufacturer-specific procedures.

Step 3: Determine Whether the Fault Follows a Component

On engines where components are interchangeable, technicians sometimes compare cylinder components to see whether the misfire follows one. This method must only be used when the parts are truly compatible and the manufacturer permits it.

For example, if a verified interchangeable coil is moved and the misfire moves with it, that gives stronger evidence than P0304 alone. Never assume coil or injector compatibility from appearance.

Technician’s note: Check your manufacturer’s service information before swapping ignition or fuel components. Connector design, calibration, and testing procedures can differ by engine.

Step 4: Check Fuel and Air

If ignition checks don’t reveal the problem, inspect fuel delivery and intake sealing. A cylinder-specific injector problem or intake leak can keep P0304 active.

Fuel-trim information may help identify a lean condition, but scanner numbers need context. Compare operating conditions rather than relying on one reading.

Step 5: Test Compression When Necessary

A persistent cylinder-specific misfire with good ignition and fuel operation raises concern about mechanical condition. Compression testing can show whether cylinder 4 is significantly different from the others.

A leak-down test goes deeper by showing where cylinder pressure may be escaping. Possible paths include intake valves, exhaust valves, piston rings, and the cooling system.

Manufacturer procedures should guide specifications and test conditions. If you own a Ford, for example, start with official Ford support information rather than a generic firing-order diagram.

Section Takeaway: The best P0304 diagnosis moves from scan evidence to ignition, fuel, air, wiring, and finally mechanical testing. The code itself never proves which part failed.

Frequently Asked Questions About P0304 Repair Cost

Can P0304 be fixed with new spark plugs?

Yes, if a worn, fouled, damaged, or incorrectly gapped spark plug is causing cylinder 4 to misfire. However, P0304 alone does not prove the plug is bad. A coil, injector, intake leak, wiring fault, or compression problem can produce the same trouble code.

How much does it cost to replace an ignition coil for P0304?

A single-coil repair may broadly fall around $150 to $450, including parts and labor, but some vehicles cost less or considerably more. Engine access, OEM versus aftermarket parts, shop rates, and whether related plugs or boots need replacement all influence the final estimate.

Does P0304 mean the fuel injector is bad?

No. A faulty cylinder 4 injector is one possible cause, but the code only confirms that the computer detected a misfire on that cylinder. Ignition problems, vacuum leaks, wiring faults, and low compression should also remain in the diagnostic plan until testing eliminates them.

Can I clear P0304 and keep driving?

Clearing the code does not repair the cause. The warning light may return as soon as the computer detects another misfire. Save the diagnostic information before clearing anything, and avoid driving if the light flashes or the engine shakes, stalls, or loses substantial power.

Is P0304 expensive to fix?

Not always. Spark-plug or coil-related repairs can be relatively affordable, while injector, intake, wiring, cylinder-head, valve, or compression repairs cost more. The diagnostic result determines the bill, which is why replacing parts from the code description alone is a poor strategy.

Can P0304 damage the catalytic converter?

A severe continuing misfire can allow unburned fuel into the exhaust and expose the catalytic converter to excessive heat. A flashing check-engine light deserves immediate attention. Reduce engine load, move to a safe location, and arrange professional diagnosis when severe symptoms continue.

What Should You Do Next?

If your scanner shows P0304, first look at how the vehicle is behaving. A flashing warning light, strong shaking, stalling, unusual odor, overheating, or major power loss means you should stop driving as soon as it is safe.

If the light is steady and the engine runs normally, arrange diagnosis soon. Record P0304, related codes, and freeze-frame data before anything is cleared.

• Confirm cylinder 4: Use service information for the exact engine.
• Inspect before buying: Look for obvious connector, wiring, plug-well, or intake problems.
• Test common causes: Check ignition and fuel delivery before assuming internal engine damage.
• Verify mechanical health: Use compression testing when ignition and fuel checks do not explain the misfire.
• Get a written estimate: Compare diagnosis, parts, labor, and warranty coverage before approving an expensive repair.

P0304 repair cost is inexpensive in some cases and substantial in others. The code tells you which cylinder needs attention, not what you need to buy.

Diagnose first, repair the confirmed cause, and avoid replacing good parts simply to make the warning light disappear.