P0304 Code: Subaru Forester (2019-2024) - Causes, Symptoms & Fixes

Diagnostic Guide: P0304 (Cylinder 4 Misfire) for 2019-2024 Subaru Forester

Data note

• OWNER COMPLAINTS: No NHTSA complaints found for this make/model/year/issue combination.
• OFFICIAL RECALLS: No recalls found in NHTSA database.
• Recalls/TSBs: No recalls found in NHTSA database.
• This guide uses general automotive knowledge for technical explanations and the limited data provided. Data limitations mean there are no confirmed complaint or recall cues to tailor this guide to actual reported cases beyond the general code meaning.

CODE MEANING AND SEVERITY

• What the code means: P0304 indicates the engine control module (ECM/PCM) detected a misfire in Cylinder 4. A misfire means the air/fuel mixture in that cylinder did not ignite properly or at the expected time.
• Severity and potential consequences:
  • Mild, intermittent misfire may cause rough idle or reduced performance but may not immediately harm the engine.
  • Persistent misfire can lead to increased exhaust emissions, poor fuel economy, engine roughness, and potential damage to the catalytic converter or downstream sensors if the misfire is sustained.
  • In some cases, misfires can be a symptom of a bigger problem (fuel delivery, ignition, or mechanical issues) that requires diagnosis beyond the single cylinder.

COMMON CAUSES ON SUBARU FORESTER (2019–2024)

• Ignition system
  • Worn or fouled spark plug in cylinder 4.
  • Faulty ignition coil for cylinder 4 or upgrade to a bad coil/harness connection.
  • Damaged or corroded coil boot or ignition wiring connector.
• Fuel system
  • Dirty, clogged, or failing fuel injector on cylinder 4.
  • Low or unstable fuel pressure or volume affecting cylinder 4.
• Air and vacuum systems
  • Vacuum leak near cylinder 4 (PCV hoses, intake manifold gaskets, or vacuum lines).
  • Mass airflow sensor (MAF) or intake air leaks affecting air/fuel mixture.
• Engine mechanical
  • Low compression in cylinder 4 due to worn piston rings, valve seal, or a head gasket issue (less common but possible).
• Miscellaneous
  • Electrical harness damage or poor connector contact to the coil or injector.
  • ECU/calibration issues are unlikely but possible if other codes accompany P0304.

SYMPTOMS TYPICALLY ASSOCIATED WITH P0304

• Check Engine Light (CEL) or MIL illuminated.
• Rough idle or vibration, especially at cold start or idle.
• Noticeable loss of power or hesitation during acceleration.
• Poor fuel economy and/or engine surging or stumble under load.
• Misfire may be intermittent; in some cases you may not notice until under load or at specific RPM.
• Possible accompanying codes: P0300 (random/multiple misfires) or P035x (ignition coil circuit faults) if the coil for cylinder 4 is implicated.

DIAGNOSTIC STEPS (Structured approach)

Before performing any repair, document codes and gather live data with a capable OBD-II scanner.

A. Verify and scope the issue

• Confirm P0304 is current and note freeze-frame data (engine RPM, load, fuel trims, temperature, etc.).
• Check for related codes (P0300, P0354, P0301–P0303, etc.).
• Clear codes after recording data, then drive to see if P0304 returns and how consistently.

B. Visual and quick checks

• Inspect spark plug on cylinder 4: gap, electrode wear, oil fouling, or heavy carbon buildup.
• Inspect ignition coil and coil boot for signs of cracking, oil intrusion, or heat damage.
• Check wiring harness and connectors to coil and injector for corrosion or loose pins.
• Look for obvious vacuum leaks around the intake manifold and PCV system near cylinder 4.

C. Component swap and/or testing (to isolate the faulty part)

• Spark plug test:
  • If plug looks worn/fouled, replace with the correct Subaru plug type and correct gap.
• Coil test (coil-on-plug system):
  • Swap the cylinder 4 coil with a known good coil from another cylinder (e.g., cylinder 2). Clear codes and drive; if P0304 moves to the new cylinder, the coil is likely faulty.
• Fuel injector test:
  • Increase suspicion if spark plug and coil test are OK. Listen for injector operation with a mechanic’s stethoscope or use an injector pulse test. Swap in a working injector if feasible to see if the misfire follows.
• Fuel pressure test:
  • Measure rail pressure with a suitable gauge to ensure it meets the Forester’s spec. Low pressure can cause misfires under load.
• Compression test / leak-down test:
  • With engine warm and properly cranked, check cylinder 4’s compression and compare to other cylinders. Low compression suggests mechanical issues (valves, rings, head gasket) requiring further diagnosis.
• Vacuum/air path test:
  • Use a spray (e.g., brake cleaner) around the intake manifold and PCV hoses while the engine is running to detect vacuum leaks (a change in idle indicates a leak).
• Electrical diagnosis:
  • Check for damaged or loose wiring on the coil and injector circuits (open/short to ground or battery). If P0354-type symptoms appear (coil circuit fault for cylinder 4), inspect the coil driver in the PCM as well.

D. Final steps

• If a replacement part fixed the issue, reassemble, clear codes, and perform a road test to ensure the misfire does not return.
• If the problem persists after testing common ignition and fuel delivery items, consult a professional technician for deeper diagnostics (possible mechanical issue or ECU problem).

RELATED CODES

• Misfire family: P0300 (random/multiple misfires), P0301 (cylinder 1), P0302 (cylinder 2), P0303 (cylinder 3), P0305 (cylinder 5), P0306 (cylinder 6), P0307 (cylinder 7), P0308 (cylinder 8) for multi-cylinder engines.
• Ignition coil circuit related: P0351–P0358 (cylinder-specific ignition coil primary/secondary circuit faults). For cylinder 4 specifically, P0354 would be a direct coil/circuit fault to that cylinder.
• Fuel and air system related: P0171/P0174 (fuel trim/deletion/lean condition) can accompany misfires if the engine is fighting a lean condition.
• Emission-related indicators: P0420/P0430 (catalyst efficiency) may show later if a persistent misfire causes catalytic damage.

REPAIR OPTIONS AND COSTS (2025 prices)

Prices can vary by region, shop, and exact Forester engine variant. The numbers below are typical ranges you might encounter in 2025 for a 2019–2024 Subaru Forester with a 4-cylinder engine.

• Spark plug replacement (cylinder 4)

  • Parts: $5–$20 (copper plugs on the inexpensive end) or $15–$45 (platinum/iridium)
  • Labor: $60–$140
  • Estimated total for cylinder 4 only: $70–$180
  • If replacing all spark plugs (common preventive maintenance): $120–$320 total (parts + labor)

• Ignition coil replacement (cylinder 4)

  • Parts: $40–$110
  • Labor: $60–$120
  • Estimated total: $100–$230
  • If coil and plug replacement are done together, you may see a combined total in the $180–$350 range.

• Fuel injector (cylinder 4) service

  • Injector cleaning (professional cleaning or redundant cleaners): $50–$140
  • Injector replacement (cylinder 4): $150–$350 for parts, plus 60–140 in labor
  • Total replacement: $210–$490

• Fuel system service and pressure check

  • Fuel pressure test: $60–$120 (labor typically included in a broader diagnostic)

• Vacuum leak repair (e.g., PCV hoses, intake manifold gasket)

  • Parts: $20–$120
  • Labor: $70–$180
  • Total: $90–$300

• Mechanical / compression related issues (valve, rings, head gasket)

  • Compression test: $60–$120
  • If low compression requires repair: $1,000–$3,500 (engine job range; major work or head gasket replacement)

• Full engine or catastrophic issues

  • Engine replacement: $4,000–$8,000+ (varies widely by model, OEM parts vs. used/remanufactured)

Notes on costs:

• Labor rates vary widely by region; Subaru-specific labor can be higher in some markets.
• If multiple cylinders are misfiring (P0300 family), the cost can be higher because more components may need replacement (plugs, coils, injectors) and labor multiplies.
• Always confirm parts compatibility with the exact Forester engine variant (e.g., 2.5L FB25 vs. newer engines that may be used in 2020s models).

DIY vs PROFESSIONAL

• Do-it-yourself (DIY) suitability:

  • Suitable tasks: Visual inspection of spark plug and coil, spark plug replacement, coil swap tests, basic ignition/fuel system checks, vacuum hose inspection, simple compression checks if you own a compression tester.
  • Tools typically needed: basic hand tools, torque wrench, spark plug socket, stethoscope or long screwdriver for injector/coil listening, compression tester.
  • Skills risk: Replacing spark plugs and coils is straightforward but requires correct torque and proper removal to avoid thread damage. Keep in mind cylinder 4 misfire could be caused by deeper issues that you might not resolve with DIY efforts.

• Professional maintenance:

  • Strongly recommended if:
    • You suspect compression or valve issues.
    • The misfire persists after replacing plugs and coils.
    • You don’t have diagnostic tools to monitor live data or to perform injector testing.
    • You need precise fuel pressure measurement or complex electrical testing.

• Safety and reliability considerations:

  • Misfires can damage the catalytic converter if not corrected promptly; avoid extended driving with a persistent misfire.
  • Working with fuel systems and high-energy ignition components requires caution and proper safety practices.

PREVENTION

• Follow Subaru’s maintenance schedule for spark plugs and ignition components. Replace spark plugs at the manufacturer’s recommended intervals and use the recommended plug type.
• Use high-quality gasoline and consider fuel system cleaning at reasonable intervals to prevent injector buildup.
• Maintain the air intake system: replace air filter on schedule; keep MAF sensor clean and free from contamination.
• Keep the PCV system clean and functional; replace defective PCV valves and hoses to prevent vacuum leaks.
• Inspect ignition wiring and coil boots for aging and cracking; replace worn boots promptly to prevent intermittent misfires.
• If you detect a misfire early, address it promptly with diagnostics to prevent downstream catalytic damage and broader engine wear.
• When performing maintenance, reset codes and monitor live data to verify the misfire is resolved after repairs.

Data limitations and transparency

• The provided NHTSA data shows no owner complaints and no recalls for this specific issue on 2019–2024 Foresters. This means there is no public complaint trend or recall history available in the given data to tailor this guide beyond general P0304 symptomology and standard diagnostic steps.
• Real-world results may vary by vehicle condition, driving style, maintenance history, and regional differences in service pricing.