Toyota Error Code P0304 – Cylinder 4 Misfire Detected

Error Code: P0304
Affected Component: Cylinder 4 Misfire Detected
System: Engine / Ignition / Emissions
Severity: Moderate – Direct impact on combustion efficiency and emission levels

🧠 What Does Toyota Error Code P0304 Mean?

P0304 is a powertrain diagnostic trouble code (DTC) indicating a misfire specifically occurring in cylinder 4. This means that the ECM has identified inconsistent combustion events, either from poor spark, incorrect fuel-air mixture, or mechanical inefficiencies. The misfire compromises engine stability and emissions integrity, often triggering VSC or TRAC interventions due to drivetrain irregularities.

📍 When and Why It Appears

This code typically appears under these conditions:

• High load acceleration or sustained high RPMs
• Cold starts with poor combustion initiation
• Fuel trim compensations exceeding thresholds
• Unstable ignition timing detected via knock sensor feedback

⚠️ Key Symptoms of a Cylinder 4 Misfire

• Illuminated Check Engine Light (CEL)
• Rough idle, shaking engine at stop or during drive
• Audible sputtering from the exhaust tip
• Sudden loss of power or hesitation on throttle input
• Increased emissions and possible catalyst damage if ignored

🔎 Root Causes of P0304

• Ignition Failure: Worn spark plug, failing coil pack, or poor plug wire insulation
• Fuel Delivery: Dirty or sticking fuel injector in cylinder 4
• Air Intake Anomalies: Vacuum leak or intake gasket leak skewing AFR
• Mechanical: Low compression in cylinder 4 from valve, piston ring, or head gasket failure
• ECU / Wiring: Faulty injector driver signal or coil ground path

🧪 Advanced Diagnostic Procedure

1. Conduct a full OBD-II scan and retrieve freeze frame data
2. Swap ignition coils between cylinders (e.g., 4 to 1) and observe if misfire follows
3. Use a noid light to check injector pulse signal at cylinder 4
4. Run a cylinder balance test using a scan tool or oscilloscope
5. Perform a compression and leak-down test on cylinder 4

🔧 How to Fix Toyota Code P0304

1. Replace or clean spark plug in cylinder 4 (DENSO/NGK recommended)
2. Replace ignition coil or verify ground continuity
3. Clean or replace fuel injector – verify resistance and spray pattern
4. Reseat ECU connectors, inspect for pin corrosion or harness breaks
5. Reset fault codes and recheck live misfire data after a cold and hot start

💰 Estimated Repair Cost

• Ignition coil or spark plug: ₱1,200 – ₱3,500
• Fuel injector service or replacement: ₱2,000 – ₱6,000
• Compression/leak-down test and diagnostics: ₱1,500 – ₱3,000
• Full scan with live data and freeze frame analysis: ₱1,000 – ₱2,000

🧠 Expert Insight

• P0304 paired with P0420 or P0171 often indicates unburnt fuel reaching the catalyst – address promptly
• Swapping parts like coils and injectors between cylinders helps isolate faults quickly without new parts
• If misfire disappears when the engine warms up, suspect spark plug gap or weak coil under cold load
• Always inspect coil boot for signs of arc tracking or carbon fouling

❓ FAQ – Toyota P0304

Can P0304 damage my catalytic converter?

Yes. Continuous misfiring dumps unburnt fuel into the exhaust, which can overheat and destroy the catalytic converter.

Will replacing the spark plug fix this?

Possibly—but only if the root cause is the plug. Proper diagnosis should always be done before replacing parts.

Can it be wiring or ECM-related?

Absolutely. Coil and injector circuits should be tested for continuity, ground integrity, and driver pulse from the ECM.

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