P0304 Code Explained: How to Diagnose and Fix a Cylinder 4 Misfire

What is P0304 – Cylinder 4 Misfire Detected?

When your OBD-II scanner shows P0304, your engine control module has detected that cylinder 4 is misfiring. Unlike random misfire codes (P0300), this is specific to one cylinder, making it easier to isolate and fix if you follow a structured diagnostic path.

Ignoring it can cause catalytic converter damage, poor fuel economy, and engine performance issues.

Common symptoms of P0304 include:

• Rough idle, especially in drive or park

• Engine shaking or vibrations

• Hesitation during acceleration

• Poor fuel economy

• Check engine light flashing or steady

⚠️ Flashing light? Stop driving immediately. High-rate misfires can cause serious damage in minutes.

Why Cylinder 4 Misfires Occur

All cylinder misfires fall into four main categories:

1. Spark / Ignition

2. Fuel Delivery

3. Air / Vacuum

4. Mechanical / Compression

1. Spark / Ignition Issues

Spark Plugs – Cylinder 4 may misfire if its spark plug is worn, fouled, or improperly gapped. Carbon buildup, oil fouling, or heat cracking are common.

Ignition Coils – Coil-on-plug systems can fail over time due to heat cycles, moisture intrusion, or oil leaks.

How to test: Swap cylinder 4 coil with another cylinder; if misfire moves, the coil is faulty.

Vehicles most prone:

• Ford F-150 3.5L EcoBoost

• Chevy Silverado 5.3L

• Honda Accord 2.4L

• Toyota Camry 2.5L

• Dodge Charger 3.6L Pentastar

Repair cost: $150–$400 (spark plug + coil)

2. Fuel Injector Problems

Cylinder 4 misfire can occur if the injector:

• Is clogged

• Has electrical failure

• Low fuel pressure

Direct injection engines are particularly prone to injector issues. Symptoms often show under acceleration, not just at idle.

Repair cost: $350–$800

3. Air / Vacuum Leaks

Unmetered air entering near cylinder 4 can create a lean mixture, causing misfire. Common culprits:

• Intake manifold gasket leak

• Cracked PCV hose

• Stuck EVAP purge valve

• Brake booster vacuum leak

How to test: Smoke test or vacuum gauge check.

Repair cost: $200–$600

4. Mechanical / Compression Issues

If compression is low in cylinder 4, no spark or fuel adjustment will fix it:

• Burned valve

• Worn piston rings

• Blown head gasket

• Camshaft lobe wear

Compression test range: 150–210 PSI (varies by engine). Anything 15–20% lower than other cylinders indicates mechanical failure.

Repair cost: $1,000+

Professional Diagnostic Steps for P0304

1. Scan and record freeze-frame data

2. Monitor live misfire counts

3. Inspect ignition coil and spark plug for cylinder 4

4. Test fuel injector pulse and continuity

5. Perform smoke test for vacuum leaks

6. Conduct compression test if ignition and fuel check out

Proper diagnostics avoid unnecessary parts replacement and recurring misfires.

Vehicles Most Likely to Show P0304

Certain engines are more prone to cylinder 4 misfires due to their ignition, fuel, or design characteristics. For example, Ford F-150 3.5L EcoBoost trucks often experience coil-on-plug failures because of high heat in the engine bay. Chevy Silverado 5.3L and Traverse 3.6L models with V8 engines are also known to develop coil and injector issues as they age.

In the Honda lineup, the Accord 2.4L and CR-V engines commonly face spark plug wear over time, especially in high-mileage vehicles. Toyota Camry 2.5L and Corolla 1.8L engines can experience misfires due to coil pack deterioration and carbon buildup. Similarly, Dodge Charger 3.6L Pentastar and Grand Caravan engines may develop misfires from aging coils or fuel injectors.

Vehicles with 150,000 miles or more, or those that haven’t had regular maintenance, are generally more likely to encounter a P0304 cylinder 4 misfire. If your vehicle matches any of these models and you notice a rough idle, engine shaking, or the check engine light flashing, it’s wise to have a professional diagnose it promptly.

Real Case Studies

Case 1 – 2016 Honda Accord 2.4L
The customer complained of rough idle and engine shaking at stoplights. Diagnostics confirmed a P0304 misfire. Swapping the coil verified the coil was defective. Repair: Coil replacement, total cost $350.

Case 2 – 2015 Ford F-150 3.5L EcoBoost
The truck misfired under acceleration. The cylinder 4 fuel injector was clogged. Repair: Injector replacement, total cost $600.

Case 3 – 2013 Chevy Silverado 5.3L
The owner ignored the misfire for weeks, which resulted in catalytic converter damage. Repair involved coil replacement plus catalytic converter replacement, costing $1,700.

Early diagnosis is key to avoiding secondary damage, such as catalytic converter failure.

Repair Cost Estimates

• Spark plug replacement: $150–$300

• Ignition coil replacement: $200–$400

• Fuel injector replacement: $350–$800

• Vacuum leak repair: $250–$600

• Mechanical engine repair: $1,000+

Safety Considerations

• Steady check engine light: Short trips only.

• Flashing check engine light: Stop immediately.

• Misfires can damage catalytic converters, oxygen sensors, and engine mounts if left unresolved.

How Instant Car Fix Can Help

Instant Car Fix provides mobile diagnostics and repair services right at your home or workplace, including:

• On-site OBD-II scanning

• Spark plug, ignition coil, and fuel injector testing

• Cylinder-specific misfire repair

• Transparent pricing with no hidden fees

Skip the shop and get your vehicle safely running again. Schedule your P0304 cylinder 4 misfire repair today!

Frequently Asked Questions

Q1: Can bad fuel cause a P0304 misfire?
A: Only temporarily. Persistent misfires indicate hardware or mechanical failure.

Q2: Should I replace all spark plugs to fix P0304?
A: Only if testing shows all plugs are worn or faulty. Swap testing helps isolate the issue.

Q3: Can low compression trigger a P0304 misfire?
A: Yes. If ignition and fuel systems check out, compression problems must be evaluated.

Q4: Can I ignore a cylinder 4 misfire?
A: No. Ignoring it risks catalytic converter damage and reduced engine life.