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
| Item | Information |
|---|---|
| Code | P0304 |
| Description | Cylinder 4 Misfire Detected |
| Severity | ⭐⭐⭐⭐☆ High |
| Safe to Drive? | Only briefly if the misfire is mild |
| Most Common Causes | Spark 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:
- Intermittent
- Constant
- Present only at idle
- Present under acceleration
- Present when cold
- Present after the engine warms up
- Severe enough to make the Check Engine Light flash
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:
- Factory service manual
- Firing-order diagram
- Underhood emissions label
- Manufacturer repair information
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:
- Correct spark
- Proper fuel delivery
- Adequate compression
- Correct air/fuel mixture
- Proper valve timing
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:
- Check Engine Light
- Flashing Check Engine Light
- Rough idle
- Engine shaking
- Poor acceleration
- Hesitation
- Loss of power
- Hard starting
- Increased fuel consumption
- Fuel odor from the exhaust
- Engine stalling
- Reduced throttle response
- Failed emissions inspection
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:
- Catalytic converter failure
- Spark plug fouling
- Cylinder-wall fuel wash
- Engine-oil contamination
- Burned valves
- Piston damage
- Reduced fuel economy
- Internal engine damage
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:
- The Check Engine Light is steady
- The misfire is occasional
- The engine is not shaking severely
- There is no major power loss
- The engine is not overheating
Stop driving if:
- The Check Engine Light flashes
- The engine shakes violently
- Power drops significantly
- A strong raw-fuel smell develops
- The vehicle stalls
- The catalytic converter glows red
- Multiple misfire codes appear
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
| Cause | Likelihood |
| 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
| Repair | Typical 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:
- OBD-II scanner
- Scan tool with live misfire data
- Spark plug socket
- Ratchet and extensions
- Torque wrench
- Digital multimeter
- Spark tester
- Noid light
- Fuel-pressure gauge
- Compression tester
- Leak-down tester
- Smoke machine
- Mechanic’s stethoscope
- Service manual
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:
- P0300
- P0301
- P0302
- P0303
- P0305
- P0171
- P0172
- P0204
- P0354
- P0420
- P0430
These codes can narrow the diagnosis.
Examples:
- P0204 may indicate a Cylinder 4 injector circuit problem.
- P0354 may indicate a Cylinder 4 ignition coil circuit fault.
- P0171 may suggest a vacuum leak or lean condition.
- P0300 may indicate the problem is not isolated to Cylinder 4.
Record freeze-frame data before clearing anything.
Step 2: Review Freeze-Frame Data
Record:
- Engine RPM
- Engine load
- Coolant temperature
- Vehicle speed
- Short-term fuel trim
- Long-term fuel trim
- Throttle position
- Intake-air temperature
Freeze-frame data can reveal whether the misfire occurred:
- At idle
- Under acceleration
- During cold start
- At highway speed
- When the engine was hot
- During heavy load
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:
- Guessing
- Online photos of a different engine
- The nearest visible ignition coil
- A forum post written by someone named “BoostedDad47”
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:
- Worn electrodes
- Excessive gap
- Carbon fouling
- Oil contamination
- Fuel saturation
- Cracked porcelain
- White overheating deposits
- Coolant contamination
- Damaged threads
Compare the plug with those from nearby cylinders.
Spark plug appearance may provide important clues:
| Spark Plug Condition | Possible Cause |
| Wet with fuel | No spark or weak spark |
| Black carbon deposits | Rich mixture or weak ignition |
| Oil fouling | Worn rings, valve seals, or PCV problem |
| White deposits | Lean condition or overheating |
| Steam-cleaned appearance | Possible coolant intrusion |
| Cracked insulator | Spark 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:
- Move Cylinder 4 coil to Cylinder 2.
- Clear the codes.
- Run or road-test the vehicle.
- Rescan for misfire codes.
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:
- Cracks
- Burn marks
- Loose terminals
- Carbon tracking
- Excessive resistance
- Contact with hot exhaust components
- Moisture intrusion
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:
- Failed ignition coil
- Damaged plug wire
- Poor electrical connection
- Coil driver problem
- Low system voltage
- Ignition module fault
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:
- Electrical power
- ECM control pulse
- Connector condition
- Injector resistance
- Harness continuity
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:
- Clear the codes.
- Run the engine.
- Rescan.
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:
- Intake manifold gasket
- Vacuum hoses
- PCV system
- Brake-booster hose
- Injector seals
- Intake runner
- EVAP connections
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:
- Burned valve
- Bent valve
- Worn piston rings
- Damaged piston
- Head-gasket failure
- Broken valve spring
- Camshaft lobe wear
- Timing problem
Step 12: Perform a Leak-Down Test
A cylinder leak-down test helps identify where compression is escaping.
Listen for air at:
- Intake manifold: intake valve leak
- Exhaust pipe: exhaust valve leak
- Oil-filler opening: piston rings or cylinder wear
- Cooling system: head-gasket or cylinder-head problem
- Adjacent cylinder: possible head-gasket failure
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:
- Rocker arms
- Camshaft lobes
- Valve springs
- Lifters
- Pushrods
- Valve clearance
- Variable valve timing components
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:
- Timing belt or chain alignment
- Camshaft timing
- Variable valve timing operation
- Camshaft phasers
- Timing-chain tension
- Reluctor wheels
Mechanical timing should be checked when P0304 appears with correlation or camshaft codes.
Step 15: Verify the Repair
After completing repairs:
- Reinstall all components correctly.
- Torque spark plugs to specification.
- Clear stored codes.
- Start the engine.
- Monitor live misfire counters.
- Check idle quality.
- Road-test under the conditions shown in freeze-frame data.
- Rescan for pending codes.
- Inspect for fuel, vacuum, or coolant leaks.
- 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:
- Vacuum leak near Cylinder 4
- Intake manifold gasket leak
- Dirty injector
- Low compression
- Valve problem
P0304 Under Acceleration
Possible causes include:
- Weak ignition coil
- Excessive spark plug gap
- Damaged plug wire
- Low fuel pressure
- Restricted injector
P0304 During Cold Start
Possible causes include:
- Leaking injector
- Coolant intrusion
- Tight valve clearance
- Intake gasket leak
- Weak ignition coil
P0304 After Engine Warms Up
Possible causes include:
- Heat-sensitive ignition coil
- Injector winding failure
- Valve-clearance issue
- Wiring expansion or connector fault
- Low compression when hot
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:
- Ignition coil failure
- Spark plug wear
- Injector problems
- Coolant intrusion on certain engines
- Camshaft or valve-train wear
Common models include:
- Ford F-150
- Ford Escape
- Ford Explorer
- Ford Fusion
- Ford Mustang
- Lincoln MKX
Chevrolet and GMC
Common causes include:
- Ignition coil or plug-wire problems
- Fuel injector failure
- AFM or DFM lifter issues
- Camshaft wear
- Intake manifold leaks
Common models include:
- Chevrolet Silverado
- Chevrolet Tahoe
- Chevrolet Equinox
- GMC Sierra
- GMC Yukon
- GMC Terrain
On applicable V8 engines, lifter or camshaft problems should be considered when ignition and injector tests are normal.
Honda and Acura
Common causes include:
- Worn spark plug
- Ignition coil failure
- Tight valve clearance
- Fuel injector problem
- Intake leak
Common models include:
- Honda Civic
- Honda Accord
- Honda CR-V
- Honda Pilot
- Acura TL
- Acura MDX
Valve-clearance inspection is especially important when a persistent misfire remains after ignition repairs.
Toyota and Lexus
Common causes include:
- Ignition coil failure
- Worn spark plugs
- Fuel injector restriction
- Intake gasket leak
- Low compression
Common models include:
- Toyota Camry
- Toyota Corolla
- Toyota RAV4
- Toyota Tacoma
- Lexus ES
- Lexus RX
Nissan and Infiniti
Common causes include:
- Ignition coil failure
- Injector problems
- Vacuum leaks
- Camshaft timing issues
- Low compression
Common models include:
- Nissan Altima
- Nissan Maxima
- Nissan Rogue
- Nissan Pathfinder
- Infiniti G35
- Infiniti Q50
Hyundai and Kia
Common causes include:
- Ignition coil
- Spark plug wear
- Fuel injector problem
- Carbon buildup
- Engine mechanical issues
Subaru
Common causes include:
- Ignition coil or plug-wire failure
- Oil-fouled spark plug
- Injector problem
- Compression loss
- Vacuum or intake leak
Volkswagen and Audi
Common causes include:
- Ignition coil failure
- Spark plug wear
- Fuel injector problem
- Carbon buildup on intake valves
- PCV or intake leak
Direct-injection engines may develop intake-valve carbon buildup that contributes to cold-start misfires.
BMW and MINI
Common causes include:
- Ignition coil failure
- Fuel injector problems
- Spark plug wear
- Vacuum leaks
- Intake-valve carbon buildup
- Low compression
Dodge, Jeep, and Ram
Common causes include:
- Ignition coil or spark plug failure
- Fuel injector problems
- HEMI lifter or camshaft wear
- Intake manifold leak
- Low compression
P0304 Repair Costs
| Repair | Estimated 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:
- P0300 – Random/Multiple Cylinder Misfire Detected
- P0301 – Cylinder 1 Misfire Detected
- P0302 – Cylinder 2 Misfire Detected
- P0303 – Cylinder 3 Misfire Detected
- P0305 – Cylinder 5 Misfire Detected
- P0171 – System Too Lean, Bank 1
- P0172 – System Too Rich, Bank 1
- P0204 – Cylinder 4 Injector Circuit Malfunction
- P0354 – Ignition Coil D Primary/Secondary Circuit
- P0420 – Catalyst System Efficiency Below Threshold, Bank 1
- P0430 – Catalyst System Efficiency Below Threshold, Bank 2
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:
- Bad spark plug
- Faulty injector
- Intake leak
- Damaged wiring
- Low compression
- Valve problem
- Incorrect cylinder identification
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?
- P0300 indicates random or multiple-cylinder misfires.
- P0304 specifically identifies Cylinder 4.
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:
- Confirm the Cylinder 4 location.
- Inspect the spark plug.
- Swap the ignition coil.
- Test the fuel injector.
- Check for intake leaks.
- 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.
