P0304 Code: Honda Civic (2022-2024) - Causes, Symptoms & Fixes

Recall/status note:

• Recall status: No recalls found in NHTSA database
• Data note: In the provided data for 2022-2024 Honda Civic, there are no NHTSA owner complaints listed for this issue. This guide uses general OBD-II knowledge and Honda-specific considerations to diagnose P0304, but it is not based on reported NHTSA complaints for this exact model/year.

CODE MEANING AND SEVERITY

• Code meaning: P0304 = Cylinder 4 Misfire Detected. The engine control module (ECM/PCM) has detected that cylinder 4 is not combusting as it should.
• Severity and impact: A cylinder misfire reduces engine efficiency, can cause rough idling, hesitation, and loss of power. Prolonged or frequent misfires can overheat the catalytic converter and raise emissions. If the misfire is persistent, you may see the MIL (Check Engine Light) on and fuel economy drop. In modern Civics, a misfire on cylinder 4 can be caused by ignition, fuel delivery, air/fuel mixture, or mechanical issues; further diagnosis is needed to identify the root cause.

COMMON CAUSES ON HONDA CIVIC

Note: This section reflects typical failure points for P0304 in a modern Honda Civic (2022-2024), including both 1.5L turbo and 2.0L options. The exact cause depends on engine version (direct injection vs port injection, coil design, etc.).

Most common:

• Faulty spark plug on cylinder 4 (worn, fouled, oil-fouled, or damaged)
• Faulty ignition coil on cylinder 4 or coil-on-plug wiring/connector (coil pack failure or damaged boot)
• Faulty or sticking fuel injector on cylinder 4 (clogged, leaking, or not delivering proper spray)
• Intake or vacuum leaks near cylinder 4 (manifold gasket, PCV hoses, vacuum lines)
• Dirty or faulty mass air flow (MAF) sensor or throttle body leading to improper air/fuel mixture
• Fuel delivery/pressure issue (weak fuel pump, failing pressure regulator) affecting cylinder 4
• Low compression on cylinder 4 due to mechanical wear (valve, piston rings, head gasket, camshaft problems)
• Carbon buildup on intake valves or deposits around the injector (more common with direct-injection engines)
• Exhaust/oxygen sensor or other sensor related to fueling (less common but possible if truly misfiring condition is caused by a fueling error)
• Wiring or connector problems (corroded or loose connectors to spark, coil, or injector)

Notes:

• In the 2022-2024 Civic family, the 1.5L turbo engines rely on direct injection and coil-on-plug ignition; carbon buildup and injector issues can be a recurring factor. The 2.0L variant may have different injector/valve behavior but can still present P0304 for the same root causes (spark, fuel, air, or compression issues).

SYMPTOMS

• MIL/Check Engine Light on (P0304 stored or current)
• Rough idle or engine shake at idle
• Noticeable loss of power and reduced acceleration
• Hesitation or stuttering during acceleration; engine may stumble under load
• Increased fuel consumption
• Occasional misfire symptoms that may be intermittent, or persist under steady drive
• Potential misfire at specific operating conditions (idle, highway boost, or high engine load)

DIAGNOSTIC STEPS

A systematic approach to confirm P0304 and identify root cause:

Verify the code and data

• Use a reputable OBD-II scanner to confirm P0304 is present and note freeze-frame data (RPM, vehicle speed, load, spark advance, fuel trim, etc.).
• Check for additional codes (P0300 for random/multiple misfires; other P03xx codes related to cylinder 4’s fuel/ignition path).

Inspect and test the spark system on cylinder 4

• Visual inspection: Remove spark plug boot and inspect spark plug on cylinder 4 for wear, fouling, oil fouling, or damage.
• Spark plug condition: If the plug is fouled (oil/fuel) or worn, replace. If you have high mileage, consider replacing all four plugs for balanced performance.
• Swap test: Swap the ignition coil (and/or spark plug) from cylinder 4 with another cylinder (e.g., cylinder 2) and clear codes. If the P0304 moves to P0302 after the swap, the issue is likely coil or plug on cylinder 4; if it stays on 4, the problem is likely in the circuit around cylinder 4 (injector, wiring, or compression).
• Coil/boot check: Inspect coil boot for cracks or arcing; test coil primary/secondary resistance if you have the specs and a multimeter. Look for corrosion at the connector.

Inspect and test the fuel delivery to cylinder 4

• Fuel injector: Listen for a click with the engine cranking (with a mechanic’s stethoscope) or use injector noid light to confirm signal. Swap the injector with another cylinder and observe whether the misfire follows the injector.
• Injector resistance: Check resistance of the injector on cylinder 4 and compare to spec or a known-good injector. A stuck or open injector can cause a misfire.
• Fuel pressure: Verify general fuel pressure to ensure it meets spec for the engine. A weak fuel pump or faulty regulator can cause misfires, especially under load.

Check for air leaks and intake path integrity

• Vacuum leaks: Carefully inspect hoses around cylinder 4 and intake manifold gaskets; listen for hissing sounds with the engine running.
• PCV system: Ensure PCV valve and lines are clean and not cracked.
• MAF and throttle body: If dirty, clean MAF sensor and throttle body per service manual; recheck after cleaning.

Check engine compression and mechanical condition

• Compression test: Perform a dry compression test on cylinder 4 and adjacent cylinders for reference.
• Wet compression test: If compression is low on cylinder 4, perform a wet test by adding a small amount of lubricant to the cylinder and retesting; a significant improvement indicates worn rings or piston/cylinder wall issues.
• If low compression is confirmed, diagnose for intake/exhaust valve seal, piston ring wear, or head gasket issues.

Fuel trims and sensor diagnostics

• Use scan tool data to observe short-term and long-term fuel trims at warm idle and steady cruising.
• If long-term trim is consistently high (lean condition) on bank containing cylinder 4, suspect vacuum leak, lean fueling, or MAF fault.
• Check oxygen sensors if data suggests an ongoing fueling fault, but don’t jump to sensor replacement as the first step.

Clean, test, and verify

• After inspecting/repairing the suspected item (plug, coil, injector, vacuum leak, etc.), clear codes and perform a road test to verify that cylinder 4 no longer misfires.
• If misfire persists after dotting common causes (plug, coil, injector), broaden the check to compression and mechanical issues, or consult a professional.

Safety and re-check

• Ensure battery is disconnected when wiring tasks require it.
• Reconnect all electrical connectors securely after testing.
• Re-scan for codes after road test to confirm resolution or capture new codes.

RELATED CODES

• P0300: Random/Multiple Cylinder Misfire Detected
• P0301: Cylinder 1 Misfire
• P0302: Cylinder 2 Misfire
• P0303: Cylinder 3 Misfire
• P0305: Cylinder 5 Misfire (if applicable in your engine family)
• Other codes that may accompany misfire: P0171/P0174 (fuel trims), P0101-P0103 (MAF), P0130-P0134 (O2 sensors), P0172 (rich condition) or P0175 (rich condition) depending on fueling diagnosis. These can help point to air/fuel path issues but are not the direct cause of the misfire.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Notes:

• Prices are rough nationwide ranges in USD and vary by region, shop, and vehicle specifics (engine variant, whether you do DIY or shop work, and whether OEM or aftermarket parts are used).
• Labor rates commonly range from about $90 to $160 per hour depending on region and shop type.

A. Spark plug replacement ( cylinder 4 or all four )

• Parts: $8–$18 per plug (OEM or aftermarket; many Civics use iridium/platinum-style plugs)
• Labor: 0.3–0.8 hours for a single cylinder; 0.8–1.2 hours to replace all four
• Estimated total: $60–$150 (single-cylinder approach) to $150–$350 (all four plugs with labor)

B. Ignition coil replacement (cylinder 4)

• Parts: $60–$120 for an OEM or reputable aftermarket coil
• Labor: 0.3–1.0 hours
• Estimated total: $120–$250

C. Fuel injector replacement (cylinder 4)

• Parts: $100–$250
• Labor: 0.8–2.0 hours (depends on accessibility and whether injectors are individual or bank-mounted)
• Estimated total: $200–$650

D. Fuel system service (injector cleaning or fuel treatment)

• Parts/service: $40–$120 for injector cleaning service; some shops charge more for professional cleaning
• Estimated total: $60–$200

E. Vacuum leak repair (PCV hoses, gaskets, vacuum lines)

• Parts: $20–$60
• Labor: 0.2–0.5 hours
• Estimated total: $60–$180

F. MAF sensor or O2 sensor (diagnostic steps or replacement as needed)

• MAF sensor:
  • Parts: $60–$250
  • Labor: 0.3–1.0 hours
  • Total: $100–$350
• O2 sensor:
  • Parts: $80–$300
  • Labor: 0.3–1.0 hours
  • Total: $120–$400

G. Compression/valve repair (if mechanical issue is found)

• Parts: varies widely; head/gasket work or valve job
• Estimated total: $1,000–$4,000+ (highly engine/repair dependent)

H. Exhaust catalytic converter-related harm prevention

• If misfire caused catalyst overheating, you may need to address the catalyst later; costs vary widely

I. ECU/Programming

• If ECU learning or calibration is required after a hard misfire scenario, some shops may charge for re-learn; cost varies

Important note:

• If cylinder 4 misfire is caused by a mechanical problem (low compression, valve issue), replacing plugs/coils/injectors alone will not fix the issue and costs can escalate with a mechanical repair.

DIY VS PROFESSIONAL

• DIY viability: The tasks listed below are generally within reach for an experienced DIYer with basic hand tools, a torque wrench, a spark plug socket, and a multimeter.
  • Most common quick fixes: inspecting/replacing spark plug(s), swapping test coils/plugs to identify a faulty component, testing or swapping a faulty injector, and basic vacuum leak checks.
    Known caveats: Modern Civics (including 2022-2024 models) may include direct-injection systems and tight engine bays; some tasks (injector removal, precise fuel pressure checks, compression testing, and certain diagnostics) require proper tools and safety precautions.
• When to call a professional:
  • If misfire persists after plugging and coil tests or you cannot isolate the problem with basic tests.
  • If you suspect low compression or mechanical valve/piston issues.
  • If you need precise fuel pressure testing, advanced injector testing, or ECU re-learn procedures.
  • If you are uncomfortable working around high-voltage ignition coils or modern electronic systems.

PREVENTION

• Regular maintenance:
  • Replace spark plugs at the manufacturer’s recommended interval (typically every 30k–60k miles for many Civics; consult owner’s manual for exact interval and gap specs).
  • Inspect and replace ignition coils if signs of wear or cracking are observed.
  • Use high-quality gasoline with proper detergents; avoid ethanol-heavy fuel if possible, and add fuel system cleaners only as recommended by Honda and the fuel additive manufacturer.
• Direct-injection considerations (common on the 1.5L turbo Civic):
  • Carbon buildup on intake valves can contribute to misfires; consider periodic intake valve cleaning or carbon buildup mitigation as recommended by a shop familiar with direct injection engines.
• Fuel system hygiene:
  • Keep the fuel system clean; replace the fuel filter if applicable and maintain proper fuel pressure.
  • Clean the MAF sensor and throttle body if dirty (per service interval) to help maintain accurate air measurement.
• Vacuum integrity:
  • Regularly inspect hoses for cracks and leaks; replace worn hoses promptly.
• Driving habits:
  • Regular highway driving can help keep the engine and fuel system in good condition; avoid short trips that leave the engine cold for long periods.
• Diagnostics readiness:
  • If the MIL reappears, revisit the diagnostic steps and consider a more thorough test (swap tests, compression tests, or professional injector testing) to catch less obvious causes.

Conclusion

• P0304 on a 2022–2024 Honda Civic indicates cylinder 4 is not firing properly. In most cases, the problem is spark, fuel delivery, or air path related (plug, coil, injector, or vacuum leak). Mechanical problems (low compression) are possible but less common as a first fix.
• Start with the simplest, most common causes (plug and coil on cylinder 4, then injector, then vacuum/path issues, and only then move to compression/valve concerns).
• Because the data provided shows no recalls and no listed complaints for this exact issue, there is no recall-based remediation to reference. If you need a precise diagnostic path tailored to your exact Civic variant (1.5L turbo vs 2.0L, MPFi vs direct injection specifics), consult a qualified technician with access to your vehicle’s service history and live data.

If you’d like, I can tailor the diagnostic steps further to match your exact Civic variant (engine option and production month) and walk you through a swap-test or compression test checklist with tool-by-tool instructions.