P0304 Code: Honda Accord (2018-2023) - Causes, Symptoms & Fixes

Comprehensive diagnostic guide: P0304 for 2018-2023 Honda Accord

Note on data availability

• According to the data you provided, there are no NHTSA owner complaints for P0304 on 2018–2023 Honda Accord, and no recalls found in the NHTSA database for this make/model/year. No recalls found in NHTSA database. This means there is limited official complaint/recall data specific to this exact scenario in the dataset. Real-world results may vary and misfire symptoms can occur even when formal data is sparse. Always use a structured diagnostic approach and consider other data sources if needed.

1. CODE MEANING AND SEVERITY

• What the code means: P0304 is a generic OBD-II cylinder misfire trouble code indicating the engine’s control module detected a misfire in cylinder 4.
• Severity and potential impacts:
  • Short-term: Rough engine operation, rough idle, reduced power, and increased emissions.
  • Medium-term risks: Prolonged misfire can overheat and damage the catalytic converter, increase fuel consumption, and accelerate wear on ignition/fuel-system components.
  • Long-term: If left unresolved, could lead to engine damage (e.g., piston/valve issues) in severe or persistent misfire conditions.
• Note: Misfires can be intermittent and caused by many things that aren’t permanent failures. A positive P0304 should be confirmed with follow-up testing and component checks.

2. COMMON CAUSES ON HONDA ACCORD (2018–2023)

• Ignition system
  • Worn or fouled spark plug for cylinder 4 (gap, electrode wear, oil/fuel fouling).
  • Faulty ignition coil on cylinder 4 (coil packed with the spark plug boot or internal coil failure).
  • Spark plug boot connector or wiring fault to cylinder 4.
• Fuel system
  • Faulty or dirty fuel injector for cylinder 4 (sticking or not injecting properly).
  • Low or unstable fuel pressure affecting one cylinder (inconsistent spray pattern).
• Air and vacuum
  • Vacuum leaks or cracked hoses around the intake manifold, PCV, or throttle body.
  • Mass Air Flow (MAF) sensor or intake air leaks altering air/fuel mixture to cylinder 4.
• Engine mechanical
  • Low compression in cylinder 4 due to worn rings, a burnt valve, blown gasket, or valve train issues.
• Electrical/PCM/wiring
  • Damaged harness or connector to coil and/or injector for cylinder 4.
  • Grounding issues or intermittent PCM fault (less common).
• Model-specific notes
  • The Accord uses coil-on-plug ignition and, in many trims, direct-injection engines. Direct injection can be prone to carbon buildup on injectors over time, potentially contributing to misfires if injector performance degrades.
  • Given there are multiple engine options in the 2018–2023 Accord (1.5L turbo with direct injection and 2.0L turbo with direct injection), the root cause often follows the same categories but may present differently depending on engine type and mileage.

3. SYMPTOMS

• Common symptoms you might notice with P0304:
  • Illumination of the check engine light (CEL) or MIL.
  • Visible rough idle or vibration, especially at idle or under load.
  • Reduced engine power or sluggish acceleration.
  • Increased or rough engine noise on cylinder 4’s firing.
  • Slightly higher or uneven fuel consumption.
  • In some cases, no obvious symptom beyond the MIL; a scan is required to confirm DTC P0304.
• If P0304 is accompanied by P0300 (random/multiple misfire) or P0301, P0302, P0303, P0305, or P0306, you may have broader misfire/contributing factors to investigate.

4. DIAGNOSTIC STEPS
   Prepare and verify

• Confirm the fault: Use an OBD-II scanner to confirm P0304 and note any related codes. Check the freeze-frame data for engine RPM, load, coolant temp, fuel trim, and other sensor readings at the time of the misfire.
• Gather symptoms and mileage history: Rough idle duration, when misfire occurs (idle, acceleration, hot/cold engine), and any oil/fuel history.

Step-by-step diagnostic flow

• Step 1: Visual and quick inspection
  • Inspect spark plug on cylinder 4 for wear, fouling (oil/fuel), or damage. Check the spark plug boot for cracks and proper seating.
  • Inspect ignition coil and coil boot for damage or corrosion at the connector.
  • Inspect electrical connectors to coil and injector for corrosion or looseness.
  • Check intake hoses and PCV system for cracks or leaks.
• Step 2: Swap test (parallel to corrosion-free approach)
  • Swap the coil from cylinder 4 with the coil from another cylinder (e.g., cylinder 2). Clear codes and drive to reproduce the misfire. If the misfire follows the coil (P0302 or P0304 moves to the swapped cylinder), the coil is likely at fault. If the misfire remains on cylinder 4, the coil is likely OK and you should investigate spark plug or injector or mechanical issues.
  • If a coil swap isn’t conclusive, swap the spark plug from cylinder 4 with a plug from another cylinder (with appropriate care and safety). If the misfire follows the spark plug, the plug is likely faulty.
• Step 3: Spark plug check and replacement (cylinder 4)
  • If the plug is worn, fouled, or damaged, replace with a quality OEM-grade spark plug suitable for the engine (note: many Honda engines use specific heat range plugs; confirm OEM spec).
  • After replacement, re-scan and clear codes; test drive to verify misfire resolution.
• Step 4: Fuel injector check (cylinder 4)
  • Listen for injector click with a mechanic’s stethoscope, or use a noid light to ensure injector is receiving control signal.
  • If you suspect injector issues, consider injector cleaning or replacement. Cleaning may be done digitally or with a service; replacement injector is more definitive if cleaning doesn’t help.
• Step 5: Fuel system and compression checks
  • Check fuel pressure to ensure adequate delivery to all cylinders; a regulator or pump issue could cause lean/fuel-starved conditions.
  • Perform a compression test on cylinder 4 and compare with adjacent cylinders. Low compression indicates mechanical issues (valve, piston rings, head gasket).
• Step 6: Vacuum and air handling checks
  • Perform a vacuum leak test (spray on suspected areas such as intake manifold gaskets, PCV hoses, and vacuum lines) and observe engine behavior.
  • Check for dirty or restricted MAF sensor readings or dirty throttle body that could affect air-fuel ratio.
• Step 7: Check other related codes and sensor data
  • If P0304 is accompanied by fuel trim anomalies (short-term and long-term trims), address underlying air/fuel balance issues.
  • Scan for O2 sensor behavior once the immediate misfire condition is resolved to ensure downstream sensors stabilize.
• Step 8: Reassess
  • If all of the above checks fail to fix the misfire, consider mechanical issues or a PCM-related fault. In rare cases, a software update or recalibration from a dealer may be needed.

Notes on testing and tools

• A good OBD-II scanner with live data and the ability to view misfire counters is helpful.
• Tools: spark plug socket and torque wrench, torque spec for spark plugs (see OEM spec), noid light or injector tester, stethoscope for injectors, compression tester, careful use of spark plug boot protection, and logic to avoid cross-contamination or oiling of plugs.
• Torque and service details: If you replace spark plugs, torque them to the OEM specification (typically within a low to mid range in the teens of ft-lbs, e.g., around 13–22 N·m or 9–16 ft-lbs depending on model and plug type; always verify with the specific Honda manual for your engine).

5. RELATED CODES

• Primary related misfire codes:
  • P0300: Random/Multiple Cylinder Misfire Detected
  • P0301: Cylinder 1 Misfire Detected
  • P0302: Cylinder 2 Misfire Detected
  • P0303: Cylinder 3 Misfire Detected
  • P0304: Cylinder 4 Misfire Detected (the current code)
  • P0305: Cylinder 5 Misfire Detected (if applicable to a V6 or region-specific engine)
  • P0306: Cylinder 6 Misfire Detected (same caveat as above)
• Other related codes you might see during misfire investigations:
  • P0171 / P0174: System too Lean (possible misfire-related condition)
  • P0130–P0150 family: Oxygen sensor-related diagnostics that may appear if the misfire affects combustion and sensor readings
  • P0420 / P0430: Catalyst efficiency codes that can appear if a misfire persists and affects exhaust aftertreatment

6. REPAIR OPTIONS AND COSTS (2025 PRICES)
   Note: Prices vary by region, shop, and vehicle specifics. All ranges are approximate and intended as a guide.

• Spark plug replacement (cylinder 4) only
  • Parts: OEM spark plug(s) or equivalent: $10–$35 each (many engines require one plug for cylinder 4)
  • Labor: 0.4–0.8 hours
  • Total estimate: $60–$180 (assuming one plug and no other issues)
• Spark plug replacement + ignition coil (cylinder 4)
  • Parts: Spark plug ($10–$35) + ignition coil ($60–$150)
  • Labor: 0.6–1.2 hours
  • Total estimate: $140–$330
• Ignition coil replacement (cylinder 4 only)
  • Parts: $60–$150
  • Labor: 0.5–1.0 hours
  • Total estimate: $110–$320
• Fuel injector service (cleaning) or replacement (cylinder 4)
  • Cleaning
    • Parts: minimal
    • Labor: 0.5–1.0 hours
    • Total estimate: $60–$150
  • Replacement injector
    • Parts: $120–$300
    • Labor: 1.0–2.0 hours
    • Total estimate: $200–$500
• Fuel pressure check/repair
  • Labor: 0.5–1.0 hours
  • Cost: $60–$150 (plus any pump/regulator costs if replacements are needed)
• Vacuum leak repair (e.g., PCV hose, intake gaskets)
  • Parts: $20–$100
  • Labor: 0.5–1.5 hours
  • Total estimate: $60–$250
• Compression test/repair for cylinder 4
  • Compression test: $100–$200 (diagnostic test)
  • If mechanical issue is found (valve, piston rings, head gasket), repair cost increases significantly
  • Estimated repair (minor to moderate mechanical work): $1,000–$4,000 depending on extent
• Head gasket/valve train repair
  • Parts: several hundred to thousands
  • Labor: many hours
  • Total estimate: $1,500–$4,000+ depending on damage and labor rates
• PCM/ECU-related issue
  • Rare; diagnosis and potential reprogramming may be required
  • Cost: typically $100–$300 diagnostic, plus any software/update costs

Tips for choosing a repair path

• Start with the simplest, least expensive fixes (spark plug, coil, injector) if misfire is isolated to cylinder 4 and there are no conflicting codes.
• If a mechanical issue is suspected (low compression, abnormal readings, persistent misfire after component swaps), escalate to more thorough testing or a specialist.
• Get a quoted diagnostic fee upfront and confirm whether the diagnosis is applied toward a repair if you proceed.
• If you are near the higher mileage range, consider injector cleaning and spark plug replacement as routine maintenance to prevent future misfires.

7. DIY VS PROFESSIONAL

• DIY-friendly scenarios
  • If you have a functioning OBD-II scanner, basic hand tools, and experience with spark plug/coils, replacing a single spark plug and possibly the corresponding coil is within DIY reach.
  • Pros: lower cost, faster resolution for simple issues; you gain familiarity with the vehicle.
  • Cons: if the problem is deeper (injector, compression, or mechanical), misdiagnosis can waste time and money and could cause engine damage.
• Professional route
  • Recommended when:
    • The misfire persists after replacing spark plug and coil.
    • You are unsure about compression testing or injector diagnostics.
    • Direct injection systems are involved and you lack injector cleaning/testing equipment.
  • Pros: proper diagnosis, access to equipment (fuel pressure gauge, compression tester, leak-down tester, oscilloscope, professional injector tests), and proper torque specs.
• Tools and tasks commonly handled by pros
  • Fuel pressure testing with a gauge
  • Fuel injector flow testing or ultrasonic cleaning
  • Compression and leak-down testing
  • Cylinder balance testing
  • Coil/injector harness diagnostics
• Honda-specific considerations
  • Both 1.5L and 2.0L Accord engines use coil-on-plug ignition; replacing only the spark plug is often a good first step, but the presence of direct injection means carbon buildup and injector issues can be more nuanced than in port-fuel-injection engines.
• Safety and warranty
  • Follow safety procedures (disconnect battery when removing ignition components, relieve fuel pressure if required, avoid spills) and check warranty implications if under warranty or service plan.

8. PREVENTION

• Regular maintenance
  • Replace spark plugs at the OEM-recommended interval for your engine (often around 60k–100k miles for many modern engines, but verify for your specific Accord engine).
  • Replace ignition coils if you notice misfire symptoms or upon OEM recommendations.
  • Use high-quality fuel and keep your fuel system clean with periodic injector cleaning if recommended by Honda service literature.
• Air and vacuum care
  • Replace air filters regularly to prevent unmetered air from causing misfires.
  • Inspect vacuum lines, PCV system, and intake manifold gaskets for cracks or leaks.
• Direct injection considerations
  • If your Accord uses direct injection (common in turbo variants), carbon buildup on injectors is a potential risk over time. Periodic injector cleaning or professional inspection can help.
  • Use the manufacturer-recommended oil and maintain oil changes to protect valve train and engine seals.
• Drive habits
  • Regularly drive the vehicle under varied loads (not always short trips) to prevent injector and spark plug fouling and to keep the exhaust system in good shape.
  • Address any running roughness or hesitation early; timely diagnosis reduces risk of downstream damage.
• Monitoring and records
  • Maintain a log of maintenance intervals, fuel quality, and any driving symptoms. This can help with quicker diagnostics if a misfire recurs.

Bottom line

• P0304 on a 2018–2023 Honda Accord points to a cylinder 4 misfire. Begin with the simplest potential causes (spark plug and ignition coil), then move to the injector, fuel system, and air/vacuum checks. If problems persist after component swaps and basic tests, evaluate compression and mechanical integrity. Given data limitations (no confirmed complaints or recalls in the provided dataset), rely on a thorough diagnostic approach and confirm findings with live data and cylinder testing. If you’re not comfortable performing these checks yourself, seek a qualified technician to perform a methodical inspection to avoid unnecessary repairs and ensure proper repair quality.