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P0304 Code Explained: Cylinder 4 Misfire Causes, Symptoms & Fixes

P0304 Code

If your Check Engine Light is on and your OBD-II scanner displays P0304, the Engine Control Module has detected a misfire specifically affecting Cylinder 4.

A misfire happens when the air/fuel mixture inside a cylinder fails to burn correctly. Cylinder 4 may be receiving insufficient spark, too little fuel, too much air, or inadequate compression.

Unlike P0300, which indicates random or multiple-cylinder misfires, P0304 identifies the affected cylinder. That makes diagnosis easier—at least in theory. The vehicle still expects you to test things instead of replacing every ignition component within arm’s reach.

P0304 commonly appears on vehicles from Ford, Chevrolet, GMC, Honda, Toyota, Nissan, Hyundai, Kia, Subaru, Volkswagen, Audi, BMW, Dodge, Jeep, Ram, Mazda, and many other manufacturers.

Quick Answer

ItemInformation
CodeP0304
DescriptionCylinder 4 Misfire Detected
Severity⭐⭐⭐⭐☆ High
Safe to Drive?Only briefly if the misfire is mild
Most Common CausesSpark plug, ignition coil, injector, vacuum leak, low compression
Estimated Repair Cost$30–$5,000+

What Does the P0304 Code Mean?

Diagnostic Trouble Code P0304 means the ECM has detected an abnormal combustion event in Cylinder 4.

The ECM monitors crankshaft speed during each cylinder’s power stroke. When a cylinder fires correctly, it slightly accelerates the crankshaft.

If Cylinder 4 produces less power than expected, the crankshaft speed changes unevenly. After the ECM detects enough irregular events, it stores P0304 and illuminates the Check Engine Light.

A Cylinder 4 misfire may be:

What Is Cylinder 4?

Cylinder numbering varies by manufacturer and engine design.

On many inline four-cylinder engines, Cylinder 4 is located at the opposite end from Cylinder 1. However, the physical orientation depends on how the engine is installed.

On V6 and V8 engines, Cylinder 4 may be located on either cylinder bank depending on the manufacturer’s numbering system.

Never assume Cylinder 4’s location based solely on where you are standing. Consult:

Replacing the wrong coil because “it looked like number four” is a proud automotive tradition best left to someone else.

What Causes a Cylinder to Misfire?

For proper combustion, Cylinder 4 needs:

A failure in any of these areas can trigger P0304.

The problem may be isolated to Cylinder 4 or caused by a larger engine issue that affects Cylinder 4 more severely than the others.

Symptoms of a P0304 Code

Common symptoms include:

A mild intermittent misfire may produce only a warning light. A severe misfire can make the engine feel like it is trying to escape its own engine mounts.

How Serious Is P0304?

Severity: ⭐⭐⭐⭐☆ High

P0304 should be diagnosed promptly.

A continuing misfire can allow unburned fuel to enter the exhaust system. This may overheat and damage the catalytic converter.

Additional risks include:

A flashing Check Engine Light indicates an active misfire severe enough to threaten the catalytic converter.

Can You Drive With P0304?

Short-distance driving may be possible if:

Stop driving if:

Continuing to drive with a severe misfire can turn a $100 ignition repair into a $2,000 catalytic converter replacement. Cars enjoy financial escalation almost as much as contractors.

Most Common Causes of P0304

CauseLikelihood
Worn or fouled spark plug⭐⭐⭐⭐⭐
Failed ignition coil⭐⭐⭐⭐⭐
Damaged spark plug wire⭐⭐⭐⭐
Faulty fuel injector⭐⭐⭐⭐
Injector wiring problem⭐⭐⭐
Vacuum leak near Cylinder 4⭐⭐⭐
Intake manifold gasket leak⭐⭐⭐
Low compression⭐⭐⭐
Burned valve⭐⭐
Worn piston rings⭐⭐
Valve-train problem⭐⭐
Timing issue⭐⭐
Failed ECM driver⭐

Ignition-related faults are the best place to begin, but compression and fuel delivery should not be ignored.

Most Common Repairs

RepairTypical Cost
Replace spark plug$30–$250
Replace spark plug set$100–$500
Replace ignition coil$80–$400
Replace spark plug wire set$100–$350
Clean fuel injector$100–$300
Replace fuel injector$200–$800
Repair injector wiring$100–$500
Repair vacuum leak$75–$500
Replace intake manifold gasket$300–$1,200
Repair burned valve$1,500–$4,000
Internal engine repair$3,000–$8,000+

Repair cost depends on engine design, accessibility, labor rate, and the actual cause of the misfire.

Tools You May Need

Diagnosing P0304 may require:

A capable scan tool can help identify when the misfire occurs and whether other cylinders are also affected.

Step-by-Step P0304 Diagnosis

Step 1: Check for Additional Trouble Codes

Scan the vehicle for all stored, pending, and permanent codes.

Related codes may include:

These codes can narrow the diagnosis.

Examples:

Record freeze-frame data before clearing anything.

Step 2: Review Freeze-Frame Data

Record:

Freeze-frame data can reveal whether the misfire occurred:

A misfire that appears only under load often points toward ignition weakness. A cold-start misfire may suggest injector leakage, valve problems, or coolant intrusion.

Step 3: Identify Cylinder 4 Correctly

Confirm Cylinder 4’s physical location using manufacturer information.

Do not rely on:

Correct cylinder identification is essential before swapping or testing components.

Step 4: Inspect the Cylinder 4 Spark Plug

Remove the Cylinder 4 spark plug and inspect it.

Look for:

Compare the plug with those from nearby cylinders.

Spark plug appearance may provide important clues:

Spark Plug ConditionPossible Cause
Wet with fuelNo spark or weak spark
Black carbon depositsRich mixture or weak ignition
Oil foulingWorn rings, valve seals, or PCV problem
White depositsLean condition or overheating
Steam-cleaned appearancePossible coolant intrusion
Cracked insulatorSpark leakage or improper installation

Replace worn or damaged plugs using the correct type and gap.

Step 5: Swap the Ignition Coil

If the engine uses coil-on-plug ignition, move the Cylinder 4 ignition coil to another cylinder.

For example:

If P0304 changes to P0302, the ignition coil is likely faulty.

If P0304 remains, continue testing.

This swap test is one of the fastest ways to isolate a bad coil without donating money to the parts store unnecessarily.

Step 6: Inspect Spark Plug Wires

If the engine uses spark plug wires, inspect the Cylinder 4 wire for:

A weak wire may misfire only under load or during damp weather.

Step 7: Test Spark Output

Use an approved spark tester to verify ignition output.

A healthy ignition system should produce a strong, consistent spark.

Weak or intermittent spark may indicate:

Do not test spark by holding the plug wire with your hand unless your diagnostic plan includes unexpected personal enlightenment.

Step 8: Test the Cylinder 4 Fuel Injector

Listen to the injector using a mechanic’s stethoscope.

A functioning injector usually produces a steady clicking sound.

If the injector does not click, test:

A noid light can confirm whether the ECM is commanding the injector.

The injector may still click but be clogged, restricted, leaking, or mechanically damaged.

Step 9: Perform an Injector Swap Test

Where practical, move the Cylinder 4 injector to another cylinder.

After reassembly:

If the misfire follows the injector, the injector is likely defective.

Injector swapping can involve new seals, fuel-system depressurization, and careful installation. Fuel leaks near a running engine are generally discouraged by manufacturers, technicians, firefighters, and people with functioning eyebrows.

Step 10: Check for Vacuum Leaks

A vacuum leak near the Cylinder 4 intake runner can create a cylinder-specific lean condition.

Inspect:

A smoke test is the safest and most accurate way to locate intake leaks.

Fuel trims that improve as RPM increases often suggest a vacuum leak.

Step 11: Perform a Compression Test

If ignition and fuel delivery appear normal, test Cylinder 4 compression.

Compare all cylinders.

Compression should generally remain within approximately 10% to 15% across the engine, although manufacturer specifications always take priority.

Low Cylinder 4 compression may indicate:

Step 12: Perform a Leak-Down Test

A cylinder leak-down test helps identify where compression is escaping.

Listen for air at:

Leak-down testing is especially useful when compression is low or the misfire continues despite good spark and fuel delivery.

Step 13: Inspect Valve-Train Operation

If Cylinder 4 has low compression, inspect:

A collapsed lifter, worn cam lobe, tight valve clearance, or broken valve spring can cause a persistent cylinder-specific misfire.

Step 14: Check Mechanical Timing

Incorrect timing normally affects multiple cylinders, but some timing or camshaft problems can affect one cylinder more severely.

Inspect:

Mechanical timing should be checked when P0304 appears with correlation or camshaft codes.

Step 15: Verify the Repair

After completing repairs:

  1. Reinstall all components correctly.
  2. Torque spark plugs to specification.
  3. Clear stored codes.
  4. Start the engine.
  5. Monitor live misfire counters.
  6. Check idle quality.
  7. Road-test under the conditions shown in freeze-frame data.
  8. Rescan for pending codes.
  9. Inspect for fuel, vacuum, or coolant leaks.
  10. Confirm the Check Engine Light remains off.

A successful repair should eliminate the Cylinder 4 misfire count under idle, cruise, and acceleration.

How to Diagnose P0304 by Symptom

P0304 at Idle Only

Possible causes include:

P0304 Under Acceleration

Possible causes include:

P0304 During Cold Start

Possible causes include:

P0304 After Engine Warms Up

Possible causes include:

P0304 With a Flashing Check Engine Light

This indicates a severe active misfire.

Reduce engine load and stop driving as soon as safely possible.

Common P0304 Diagnostic Mistakes

Replacing All Ignition Coils

A coil swap test can identify whether the Cylinder 4 coil is actually defective.

Replacing the Spark Plug Without Inspecting It

The plug’s condition may reveal oil burning, coolant intrusion, excessive fuel, or a lean mixture.

Ignoring Compression

Spark and fuel cannot compensate for a burned valve or damaged piston.

Replacing the Injector Without Testing It

Test injector command, resistance, operation, and fuel delivery first.

Clearing Codes Before Reading Freeze-Frame Data

Freeze-frame data reveals when the misfire occurred.

Assuming Cylinder 4 Is in the Same Location on Every Engine

Cylinder numbering varies. Verify before testing.

Ignoring Related Codes

P0204, P0354, lean codes, timing codes, and fuel-pressure codes can point directly toward the cause.

Vehicle-Specific P0304 Problems

Ford and Lincoln

Common causes include:

Common models include:

Chevrolet and GMC

Common causes include:

Common models include:

On applicable V8 engines, lifter or camshaft problems should be considered when ignition and injector tests are normal.

Honda and Acura

Common causes include:

Common models include:

Valve-clearance inspection is especially important when a persistent misfire remains after ignition repairs.

Toyota and Lexus

Common causes include:

Common models include:

Nissan and Infiniti

Common causes include:

Common models include:

Hyundai and Kia

Common causes include:

Subaru

Common causes include:

Volkswagen and Audi

Common causes include:

Direct-injection engines may develop intake-valve carbon buildup that contributes to cold-start misfires.

BMW and MINI

Common causes include:

Dodge, Jeep, and Ram

Common causes include:

P0304 Repair Costs

RepairEstimated Cost
Spark plug replacement$30–$250
Full spark plug service$100–$500
Ignition coil replacement$80–$400
Plug-wire replacement$100–$350
Injector cleaning$100–$300
Fuel injector replacement$200–$800
Wiring repair$100–$500
Vacuum leak repair$75–$500
Intake manifold gasket$300–$1,200
Compression diagnosis$150–$400
Valve repair$1,500–$4,000
Engine replacement$4,000–$10,000+

Related Trouble Codes

P0304 may appear with:

Frequently Asked Questions

What does P0304 mean?

P0304 means the ECM has detected a combustion misfire in Cylinder 4.

Can I drive with P0304?

Only briefly if the misfire is mild and the Check Engine Light is not flashing.

A flashing warning light or severe engine shaking means you should stop driving.

Can a bad spark plug cause P0304?

Yes.

A worn, fouled, damaged, or improperly gapped spark plug is one of the most common causes.

Can a bad ignition coil cause P0304?

Yes.

A coil swap test can often confirm the diagnosis. If the misfire moves to another cylinder with the coil, replace the coil.

Can a bad fuel injector cause P0304?

Yes.

A clogged, leaking, electrically open, or mechanically failed Cylinder 4 injector can cause the code.

Can low compression cause P0304?

Yes.

Burned valves, worn rings, damaged pistons, valve-train problems, and head-gasket failures can all reduce Cylinder 4 compression.

Will replacing the spark plugs fix P0304?

It may if the Cylinder 4 spark plug is worn or fouled.

It will not fix a failed coil, injector, vacuum leak, wiring fault, or compression problem.

Why does P0304 return after replacing the coil?

Possible reasons include:

Can a vacuum leak cause only Cylinder 4 to misfire?

Yes.

An intake manifold gasket leak near the Cylinder 4 runner can create a localized lean condition.

Can P0304 damage the catalytic converter?

Yes.

A severe misfire can send unburned fuel into the converter, causing it to overheat and fail.

What is the difference between P0300 and P0304?

Final Thoughts

The P0304 Cylinder 4 Misfire Detected code means Cylinder 4 is not producing consistent combustion.

The most common causes are a worn spark plug, failed ignition coil, damaged plug wire, faulty fuel injector, vacuum leak, or low compression.

Start with the easiest tests:

  1. Confirm the Cylinder 4 location.
  2. Inspect the spark plug.
  3. Swap the ignition coil.
  4. Test the fuel injector.
  5. Check for intake leaks.
  6. Perform compression and leak-down testing if necessary.

Avoid replacing every ignition component simply because the engine is shaking. Methodical testing is faster, cheaper, and considerably less entertaining to the parts-store cashier.

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