How to Fix P0304: Cylinder 4 Misfire Diagnosis and Repair

P0304 can make an engine shake, hesitate, lose power, or idle roughly. The code points you toward one cylinder, but it does not tell you which component has failed.

I often see drivers start replacing ignition parts as soon as a scanner shows a misfire code. A faster approach is to save the evidence first and let the fault pattern decide what you test.

What Does P0304 Mean?

P0304 means the engine computer has detected a misfire associated with cylinder 4. It identifies the affected cylinder, not the defective component.

A misfire occurs when a cylinder fails to produce its expected combustion power. The air-fuel mixture may fail to ignite, ignite weakly, or burn incorrectly.

Definition: P0304 is an OBD2 powertrain diagnostic trouble code indicating a detected misfire on cylinder 4.

What does the code tell you?

• P: The fault belongs to the powertrain system.
• 0: It belongs to the standardized generic-code family.
• 3: This code family includes ignition and misfire faults.
• 04: The detected misfire is associated with cylinder 4.

The engine computer may detect misfires by monitoring changes in crankshaft speed and other operating data. Exact monitoring strategies and thresholds vary by manufacturer.

P0304 can appear alone or beside codes such as P0300, injector circuit faults, lean-condition codes, or ignition-coil circuit codes. Those companion codes can change your diagnostic direction.

Is P0304 related to the key fob or immobilizer?

Normally, no. A transponder chip, smart key, or immobilizer problem usually affects authorization to start the vehicle rather than combustion in one numbered cylinder.

In locksmith callouts, I occasionally meet drivers who suspect their key because the engine starts poorly and shakes. Once P0304 appears after the engine has started, I move the diagnosis toward cylinder 4 combustion rather than the remote or keyless-entry system.

Section Takeaway: P0304 tells you where the misfire was detected, but testing is still required to determine why it happened.

Is It Safe to Drive With P0304?

Driving with P0304 depends on how badly the engine is misfiring. A flashing check-engine light, strong shaking, major power loss, or repeated stalling deserves immediate attention.

An active misfire can send unburned fuel into the exhaust system. That can increase exhaust temperatures and may damage the catalytic converter.

Stop driving and arrange professional help if you notice:

• A flashing check-engine light
• Severe engine shaking or bucking
• Major loss of acceleration
• Repeated stalling
• Abnormal overheating
• A strong raw-fuel smell
• Loud mechanical knocking

If the warning light remains steady and the engine feels normal, you may have an intermittent misfire. You should still diagnose it promptly rather than assuming the problem disappeared.

Your owner’s manual should guide the final decision because manufacturers can provide model-specific warnings. When in doubt, towing avoids turning a small fault into a larger repair.

Section Takeaway: A severe or flashing-light P0304 condition should be treated as a stop-driving problem, not a code to ignore.

What Causes a P0304 Cylinder 4 Misfire?

P0304 can come from ignition, fuel, airflow, electrical, or mechanical problems affecting cylinder 4. Spark plugs and ignition coils are common suspects, but they are not the only possibilities.

Ignition problems

• Worn, fouled, cracked, or incorrectly specified spark plug
• Weak or failed ignition coil
• Damaged coil boot
• Carbon tracking around the plug or boot
• Loose or corroded ignition connector
• Damaged ignition wiring
• Faulty plug wire on engines that use conventional wires

Fuel-delivery problems

• Restricted or malfunctioning fuel injector
• Injector connector problem
• Damaged injector wiring
• Fuel-pressure problem affecting combustion
• Contaminated or poor-quality fuel

Air and intake problems

• Vacuum leak near the affected intake runner
• Intake manifold gasket leak
• Cracked hose or disconnected vacuum line
• Abnormal air entering the engine after the airflow measurement point

Mechanical causes

• Low cylinder compression
• Burned or leaking valve
• Valve-train problem
• Worn piston rings or cylinder damage
• Head-gasket leakage
• Camshaft or valve-timing problem

The operating condition matters. A misfire that appears only at idle suggests a different path from one that occurs during hard acceleration.

Cold-start-only P0304 also deserves special attention. Moisture, deposits, intake leakage, or mechanical sealing problems can become more noticeable before the engine warms.

Section Takeaway: Don’t call P0304 a “bad coil code.” Treat ignition as one branch of a larger cylinder-specific diagnostic process.

How Do You Diagnose P0304 Correctly?

The best P0304 diagnosis starts with scan data, then moves from simple checks to targeted component tests. Avoid clearing the code before recording the evidence.

1. Scan every available trouble code

Read stored, pending, and permanent codes when your scanner supports them. Write down anything related to fuel trim, injectors, ignition coils, airflow, camshaft timing, or crankshaft position.

As of 2026, basic OBD2 readers can retrieve P0304 on compatible vehicles. More capable scanners may provide freeze-frame information, enhanced misfire counters, or Mode $06 test data.

2. Save the freeze-frame information

Freeze-frame data captures operating conditions near the time the fault was recorded. Useful fields can include engine speed, coolant temperature, calculated load, vehicle speed, and fuel trim.

• Cold engine: Look harder at cold-start fuel, intake, moisture, and mechanical patterns.
• Warm idle: Intake leaks or cylinder sealing deserve attention.
• Heavy load: Weak ignition and fuel-delivery problems become stronger suspects.
• Intermittent event: Wiring, connectors, heat, and vibration may matter.

3. Confirm where cylinder 4 is located

Do not assume cylinder 4 is simply the fourth ignition coil you can see. Cylinder numbering differs across inline, V-type, and horizontally opposed engines.

Use the manufacturer’s service information or owner’s technical documentation to confirm the cylinder layout. Testing the wrong cylinder wastes time and can create new confusion.

4. Inspect the spark plug and ignition system

With the engine off and safely cooled, inspect accessible wiring and connectors before disassembly. Look for loose plugs, damaged insulation, moisture, oil in the plug well, or a coil boot that is cracked or carbon tracked.

When removing a spark plug is within your skill level, compare cylinder 4 with a neighboring cylinder. Deposits can offer clues about oil, fuel, coolant, or an ignition problem.

Technician’s note: Follow the vehicle manufacturer’s service information for coil removal, plug torque, plug specification, and high-voltage precautions.

5. Determine whether the misfire follows the ignition component

On some coil-on-plug engines, a technician may compare identical coils between cylinders. If the misfire moves with the suspect coil, that strongly points toward the coil rather than the original cylinder.

Do this only when the components are identical and manufacturer procedures allow it. Record the original positions first.

6. Test the fuel injector

If cylinder 4 has good spark, the injector becomes a stronger suspect. Start with its connector and visible wiring before assuming the injector itself has failed.

A professional may use injector balance testing, circuit testing, current analysis, or other manufacturer-approved methods. These tests can separate an electrical fault from a restricted injector.

7. Check for an intake leak

A leak near one intake runner can lean out a single cylinder enough to create a misfire. Fuel trims and a smoke test are often more reliable than spraying chemicals around a running engine.

Cracked hoses and recently disturbed intake connections are worth inspecting first. Recent repair history is often an important clue.

8. Check compression if spark and fuel look normal

Persistent P0304 with good ignition and fuel should push mechanical condition higher on the list. A compression test compares cylinder 4 with the other cylinders.

A cylinder leak-down test can provide more detail by showing where pressure escapes. That can help identify valve, piston, ring, or head-gasket problems.

9. Check recalls and manufacturer information

Before paying for an expensive diagnosis, enter the VIN in the NHTSA recall lookup. A recall or manufacturer communication may affect the correct repair path for a specific vehicle.

Never assume a repair bulletin applies solely because another owner has the same code. Vehicle year, engine, production date, and VIN can change applicability.

Section Takeaway: Diagnose P0304 in layers: scan data, cylinder identification, ignition, injector, air leak, then mechanical condition.

How Do You Fix P0304?

The correct P0304 fix is the repair that addresses the confirmed cause of the cylinder 4 misfire. Clearing the code without repairing the fault does not fix anything.

If the spark plug is faulty

Replace the plug using the correct manufacturer-specified type and installation procedure. Inspect the old plug first because its condition may reveal a deeper problem.

A plug coated in oil or coolant may foul again after replacement. In that case, repairing the underlying leak or mechanical problem matters more than installing another plug.

If the ignition coil is faulty

Replace the confirmed faulty coil or other failed ignition component. Inspect the connector, boot, and plug well at the same time so you do not overlook the condition that damaged it.

Do not automatically replace every coil because one cylinder misfires. Some manufacturers or repair situations may justify replacing multiple aged components, but that decision should follow service information and vehicle condition.

If the injector is faulty

Repair a damaged connector or circuit when testing proves the electrical side caused the problem. Replace or professionally service the injector when testing confirms the injector itself is defective or restricted.

Do not replace an injector based only on P0304. Ignition and mechanical faults can create almost identical symptoms.

If an intake or vacuum leak is responsible

Repair the leaking hose, gasket, connection, or intake component. Then verify fuel trims and misfire behavior after the repair.

A smoke-tested leak location gives you more confidence than replacing intake parts based on suspicion. Evidence saves money.

If compression is low

Low compression may require cylinder-head, valve-train, piston, ring, timing, or head-gasket repair. The next step depends on compression and leak-down results.

This is where professional diagnosis usually becomes worthwhile. Mechanical engine work can become expensive quickly if the failure point has not been confirmed.

When should you call a repair shop?

• You have a flashing check-engine light.
• The engine shakes badly or stalls.
• The spark plug and coil appear normal.
• The misfire remains after a confirmed ignition repair.
• You need injector balance testing.
• You need a smoke, compression, or leak-down test.
• You suspect timing or internal engine damage.

If you need professional help, the AAA Approved Auto Repair locator is one way to find repair facilities offering engine diagnostic services. Independent specialists and dealership service departments are other options.

Pro tip: Ask the shop what test confirmed the failed component before approving a major repair. P0304 alone is not proof that a specific part needs replacement.

What should you do after the repair?

1. Confirm all disconnected components are correctly reinstalled.
2. Clear codes only after saving the original scan information.
3. Start the engine and check for an immediate returning misfire.
4. Review available live misfire counters.
5. Road-test only when the vehicle is operating safely.
6. Re-scan for stored, pending, or returning codes.

Some permanent emissions-related codes may remain until the vehicle completes its required self-tests. That does not automatically mean the repair failed.

Section Takeaway: Fix the verified fault, then confirm the repair with operating data rather than relying only on a cleared warning light.

Frequently Asked Questions About P0304

Can bad spark plugs cause P0304?

Yes. A worn, fouled, damaged, or incorrectly specified spark plug can cause cylinder 4 to misfire, but P0304 does not prove the plug is bad. Inspect and test the ignition system before buying parts.

Can a bad ignition coil cause P0304?

Yes. A weak ignition coil can produce P0304, especially when the misfire becomes worse under load. Confirm the coil with manufacturer-approved testing or a controlled comparison before replacing it.

Can a fuel injector cause a P0304 code?

Yes. A restricted injector, damaged injector circuit, or poor electrical connection can prevent cylinder 4 from receiving the correct fuel delivery. Injector testing becomes especially important when the spark plug and ignition coil pass inspection.

Can low compression cause P0304?

Yes. Burned valves, ring problems, valve-train faults, head-gasket leakage, and other mechanical problems can reduce cylinder 4 compression. Compression and leak-down testing help separate mechanical faults from ignition or fuel problems.

Why did P0304 return after replacing the ignition coil?

The coil may not have been the original cause, or another problem may exist alongside it. Inspect the spark plug, injector, intake system, wiring, compression, and related codes before replacing additional parts.

Can P0304 clear itself?

An intermittent misfire may stop long enough for the warning light to change after successful drive cycles. However, the original cause may still exist, so scan pending and stored data instead of assuming the vehicle repaired itself.

Does P0304 mean cylinder 4 needs to be replaced?

No. P0304 identifies a detected misfire in cylinder 4, not a destroyed cylinder. Many cases involve serviceable ignition, fuel, wiring, air-leak, or mechanical issues that require targeted diagnosis.

What Should You Do Next?

Start by recording every code and freeze-frame value before touching the engine. Then verify the actual location of cylinder 4 and inspect the simplest ignition and wiring possibilities first.

If the warning light flashes or the vehicle runs badly, stop driving and arrange professional diagnosis. NHTSA’s broader vehicle safety resources can also help you check safety information connected with your vehicle.

• Step 1: Save P0304 and all related scan data.
• Step 2: Verify the manufacturer’s cylinder numbering.
• Step 3: Inspect the plug, coil, boot, wiring, and connector.
• Step 4: Test the injector and look for localized intake leakage.
• Step 5: Check compression when ignition and fuel tests pass.
• Step 6: Repair only the fault that your testing confirms.
• Step 7: Re-scan and verify that the misfire stays gone.

P0304 gives you a useful head start because the computer has already narrowed the problem to cylinder 4. Use that advantage to test systematically instead of replacing parts until the warning light disappears.