P0304 Code: Subaru Impreza (2017-2023) - Causes, Symptoms & Fixes

No recalls found in NHTSA database.
No NHTSA complaints found for this make/model/year/issue combination.

Comprehensive diagnostic guide for OBD-II code P0304 on 2017-2023 Subaru Impreza

CODE MEANING AND SEVERITY

• Code meaning: P0304 = Cylinder 4 Misfire Detected. The engine control unit (ECU) has detected a misfire in cylinder 4 based on irregular ignition, fuel, or compression conditions.
• Severity: Can range from mild (rough idle, transient loss of power) to more serious (reduced performance, potential catalyst damage if the misfire is persistent). Repeated misfires can overheat and damage the catalytic converter and exhaust components and may cause elevated emissions.

COMMON CAUSES ON SUBARU IMPREZA

Note: In Subaru Impreza models from 2017–2023, the engines are typically a 4-cylinder horizontally opposed (boxer) setup. Misfires on cylinder 4 are commonly caused by issues in the ignition, fuel delivery, or mechanical condition of that cylinder. Typical culprits include:

• Ignition system
  • Worn or fouled spark plug on cylinder 4
  • Faulty ignition coil (coil-on-plug) for cylinder 4 or weak secondary/primary circuit
  • Poor ignition coil connection or damaged coil boot
• Fuel system
  • Clogged or failing fuel injector for cylinder 4
  • Low fuel pressure or intermittent fuel delivery to cylinder 4
  • Dirty or contaminated fuel
• Air and vacuum
  • Vacuum leak or intake leak near cylinder 4
  • Mass Air Flow (MAF) sensor or intake air issues affecting air/fuel ratio
• Mechanical/engine health
  • Low compression in cylinder 4 (valve, piston ring, or head gasket issue)
  • Timing issues (timing chain/belt tension or cam gear problems) in rare cases
  • Carbon buildup or oil fouling on cylinder 4 spark plug or combustion chamber
• Electrical/ECU
  • Faulty wiring or connectors to cylinder 4 ignition coil or injector
  • Secondary ignition misfire may show up with related codes such as P0354 (coil 4 control circuit) or P0304 with P0354

SYMPTOMS

• Ill or rough idle, especially at stoplights or idle transitions
• Noticeable loss of power or sluggish acceleration
• Misfire indication on the odometer with P0304 diagnostic trouble code
• Increased exhaust and fuel consumption; popup of check engine light (CEL)
• Possible spark plug fouling symptoms (oil/fuel fouling) or rough running under load
• In some cases, the vehicle may run normally for short periods and then misfire again

DIAGNOSTIC STEPS

A structured approach to diagnose P0304 on a 2017-2023 Subaru Impreza:

• Step 0: Confirm the issue
  • Retrieve the freeze-frame data and live data with a capable OBD-II scanner. Note engine RPM, load, fuel trim (short-term and long-term), MAF/MAP readings, fuel pressure if available, and misfire counts per cylinder.
  • Confirm that P0304 is the only cylinder misfire code or if there are related codes (P0300, P0301, P0302, P0303, P0354, etc.).
• Step 1: Visual inspection
  • Inspect spark plug on cylinder 4 for wear, fouling, oil fouling, or improper gap.
  • Inspect ignition coil/coil boot for damage, spark jump, or loose connection.
  • Check wiring harness and connectors to the coil and injector for corrosion, damage, or loose pins.
  • Inspect for vacuum leaks around the intake manifold, PCV lines, or intercooler hoses if equipped.
• Step 2: Ignition system testing
  • If spark plug appears worn or fouled, replace it with a fresh, properly gapped plug designed for your engine.
  • Swap test: move the coil or spark plug from cylinder 4 to another cylinder (e.g., cylinder 1) and clear the codes. If the misfire follows the spark plug/coil to the new cylinder, the component is at fault (plug or coil). If the misfire remains on cylinder 4, the issue is more likely injector, compression, or mechanical.
  • Inspect coil resistance (primary and secondary) with a multimeter if you have the tool and the knowledge. Compare to the service manual specifications.
• Step 3: Fuel system testing
  • Inspect and test the cylinder 4 injector: listen for injector operation with a stethoscope, measure injector resistance, or perform a balance test with a professional scanner.
  • Check fuel pressure and regulator function if the vehicle has accessible test ports; compare to the factory spec for your model/engine.
  • Check for signs of fuel leakage around the injector rail or injector seals.
• Step 4: Air and vacuum system checks
  • Use a smoke test or carb spray to detect vacuum leaks near the intake manifold and associated hoses around cylinder 4.
  • Inspect turbo/intercooler plumbing (if equipped in any trim) for leaks or cracks.
• Step 5: Mechanical inspection
  • Perform a compression test on cylinder 4 and compare with the rest of the cylinders. Low compression on cylinder 4 may indicate valve seating issues, head gasket problems, or piston/ring wear.
  • If compression is low, consider leak-down testing to determine the source (valve, piston rings, head gasket).
• Step 6: Data interpretation
  • If swapping the coil/plug follows the cylinder, fix that component.
  • If injector swap indicates the problem, replace injector (or clean/repair if serviceable).
  • If compression is significantly low on cylinder 4, plan for mechanical repair (valve train, head gasket, or more extensive engine work depending on damage).
• Step 7: Road test
  • After performing repairs, clear codes and perform a road test to verify that P0304 does not reappear and that normal idle and power are restored.

RELATED CODES

• P0300 – Random/Multiple Misfire Detected
• P0301 – Cylinder 1 Misfire
• P0302 – Cylinder 2 Misfire
• P0303 – Cylinder 3 Misfire
• P0354 – Ignition Coil/Primary/Secondary Circuit, Cylinder 4 (common when P0304 is present)
• P035x family – Ignition coil circuit faults for other cylinders
• P0171 / P0174 – Fuel trim issues that can accompany misfires
  Note: If P0354 or other ignition-related codes appear alongside P0304, they help pinpoint a coil or wiring issue as the likely cause.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Prices vary by region, shop, and the exact engine type on the Impreza (2.0L boxer variants). The following are typical ranges for common fixes. All prices are rough estimates for parts and labor combined.

• Spark plug replacement (cylinder 4 or all cylinders if desired)
  • Parts: $8–$15 per plug (premium or OEM plugs may be higher; 4 plugs total)
  • Labor: $60–$120
  • Total (single cylinder replacement): $70–$150
  • Total (all cylinders, preventive): $140–$260
• Ignition coil replacement (cylinder 4 coil, or coil pack on a non-COPE setup)
  • Parts: $60–$180 per coil
  • Labor: $40–$100
  • Total per cylinder: $100–$280
• Fuel injector replacement (cylinder 4)
  • Parts: $100–$250 for standard injector; OEM or high-performance injectors can be higher
  • Labor: $60–$150
  • Total: $160–$400
• Fuel system cleaning or injector service (temporary fix for sticky injector)
  • Parts: $0–$50 (chemicals, if DIY) or $50–$150 at shop
  • Labor: $0–$60
  • Total: $50–$210
• Fuel pressure problem (pump/regulator)
  • Parts: $150–$450 for pump; regulator $50–$150
  • Labor: $100–$250
  • Total: $250–$700
• Mechanical/Compression issues (head gasket, valve, piston)
  • Compression test: $100–$150 (diagnostic)
  • Head gasket/valve repair: widely variable; typical range $1,500–$4,000+, potentially more for labor-intensive jobs
  • Major engine work (timing chain/belt, cam/valve work): $1,200–$3,000+ depending on extent
• Vacuum leak repair (hoses, PCV, gaskets)
  • Parts: $10–$60
  • Labor: $50–$150
  • Total: $60–$210

Notes:

• If a single-cylinder misfire (P0304) is due to a faulty injector or ignition component, relatively low-cost fixes can restore performance. If the root cause is mechanical (low compression or head gasket), costs can rise substantially.
• Always confirm the exact engine variant and OEM part numbers before ordering parts, as Subaru Impreza engines vary by model year and trim.
• Prices are estimates for typical markets in 2025 and can vary by region, shop, and whether OEM or aftermarket parts are used.

DIY VS PROFESSIONAL

• DIY-friendly tasks (often appropriate for a motivated amateur with basic tools and safety practices):
  • Replacing a spark plug on cylinder 4 (and possibly all 4 plugs) if you have the right gap tools and torque specs.
  • Swapping ignition coils between cylinders to diagnose whether the coil is at fault.
  • Inspecting for obvious vacuum leaks and basic hose/PCV checks.
  • Cleaning or replacing a faulty injector if you have the proper injector rail tools and knowledge.
• Tasks typically better left to a professional:
  • Compression testing and leak-down tests, which require specialized tools.
  • Diagnosing low compression root causes (head gasket, valve seating, piston rings) and performing head work.
  • Fuel system pressure checks and injector testing/repair when there is a suspicion of pump/regulator issues.
  • Complex wiring or ECU-related fault diagnostics.
• Safety: When working with high-voltage ignition systems, fuel systems, or hot engine components, follow safety procedures, disconnect the battery when required, and use proper torque specs to avoid damage.

PREVENTION

• Regular maintenance: follow Subaru’s maintenance schedule for spark plugs, ignition coils, and fuel system components.
• Spark plugs: Replace per the recommended interval for your engine variant. If you regularly drive in harsh conditions, consider more frequent inspection or replacement.
• Ignition system: Inspect coil boots and coil wiring for cracks, corrosion, or poor connections; replace faulty coils promptly.
• Fuel system: Use quality fuel and consider an occasional fuel system cleaning or injector service if recommended by your technician.
• Vacuum and intake: Keep intake manifolds, hoses, and PCV systems in good condition; fix leaks promptly to prevent lean misfire conditions.
• Oil management: Maintain proper oil level and quality; oil fouling on plugs or valves can contribute to misfires over time.
• Monitor: If you notice rough idle, reduced power, or CEL illumination, address promptly to minimize potential catalyst damage and prevent further engine wear.

Data and limitations

• The provided data shows no recalls or official NHTSA complaints specifically for this issue and model combination in the given dataset. No recalls found in NHTSA database.
• This guide uses general automotive knowledge and typical Subaru Impreza behaviors to help diagnose P0304. Actual parts, procedures, and costs vary by model year, engine option, region, and shop. Always consult the vehicle’s service manual and use a diagnostic scan tool to confirm the root cause.