P0304 Code: BMW 3-Series (2019-2023) - Causes, Symptoms & Fixes

No recalls found in NHTSA database

Note: The data provided for this guide shows no owner complaints or recalls in the NHTSA database specifically for a 2019-2023 BMW 3-Series with code P0304. This guide uses general diagnostic best practices for P0304 on BMW 3-Series and notes where the data limitations apply.

CODE MEANING AND SEVERITY

• What P0304 means: P0304 is a cylinder 4 misfire detected by the OBD-II system. The engine management system has detected that cylinder 4 is not firing as expected, which can cause rough running, loss of power, and other drivability issues.
• Severity and potential consequences:
  • Moderate: Intermittent misfire can cause rough idle, reduced acceleration, and poor fuel economy.
  • High risk if left unmanaged: Repeated or severe misfires can overheat or damage the catalytic converter, downstream oxygen sensors, and downstream emissions systems. Prolonged misfires may also lead to engine damage if unaddressed.
• BMW-specific note: On modern BMW 3-Series with direct injection and turbocharged engines, a misfire can be caused by ignition, fuel delivery, or air management issues. Since cylinder 4 is involved, common causes often center on the coil, spark plug, injector, or intake/vacuum conditions around that cylinder.

COMMON CAUSES ON BMW 3-SERIES

• Ignition system
  • Faulty spark plug in cylinder 4 (worn gap, fouling, or damage)
  • Bad or weak ignition coil for cylinder 4 (coil-on-plug style common on BMWs)
  • Damaged or corroded coil connector or wiring to the cylinder 4 coil
• Fuel system
  • Faulty fuel injector for cylinder 4 (sticking, clogged, or poor spray pattern)
  • Low fuel pressure or fuel delivery issue affecting cylinder 4
  • Dirty or contaminated fuel affecting a single cylinder
• Air and vacuum management
  • Vacuum leak near cylinder 4 intake manifold runners or associated hoses
  • Mass Air Flow (MAF) sensor or related air-fuel sensor issues causing uneven fueling
  • PCV system problems or cracked vacuum lines affecting cylinder 4
• Mechanical and internal engine
  • Low compression in cylinder 4 due to valve, piston, or ring issues
  • Carbon buildup on intake valve (especially in direct-injected BMW engines)
• Electrical and control
  • Faulty injector driver or DME calibration issue (less common but possible)
  • Damaged wiring harness or connector insulation near cylinder 4 components
• Miscellaneous
  • Exhaust or catalytic converter issues can sometimes cause misfire symptoms to appear, though P0304 itself is a misfire indication
  • Wrong spark plug type or gap for the engine/spec

SYMPTOMS

• Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL) illuminated
• Rough idle or engine stumble, especially at idle or low speeds
• Noticeable loss of power or hesitation during acceleration
• Increased fuel consumption or rough running under load
• Occasional misfire during cold start or after a hot restart
• Possible exhaust smell or presence of unburned fuel odor in some cases
• If the problem persists, dashboard may show P0304 again after clearing codes

DIAGNOSTIC STEPS

Important: When working on modern BMWs, using a capable scan tool (BMW ISTA, OBD-II scanner with BMW enhancements) and live data is highly recommended.

Step 1 – Confirm the code and gather data

• Read P0304 and note any freeze-frame data (engine speed, load, fuel trims, misfire counters if available).
• Check for any related codes (P0300 for random/multiple misfires, P0301, P0302, P0303, P0305, etc.).

Step 2 – Inspect ignition components on cylinder 4

• Spark plug:
  • Remove and inspect cylinder 4 spark plug for wear, fouling, oil fouling, or electrode damage.
  • Check spark plug gap against BMW specification; replace if worn or fouled.
• Ignition coil and wiring:
  • Inspect coil boot and connector for corrosion or damage.
  • Check coil resistance if you have the tooling; compare with manufacturer spec.
  • Swap coil from cylinder 4 with another cylinder (e.g., cylinder 2) and clear codes to see if misfire follows the coil. If the misfire moves with the coil, the coil is the likely fault; if it stays on cylinder 4, the coil is likely OK and the fault lies elsewhere.

Step 3 – Inspect the fuel delivery for cylinder 4

• Fuel injector:
  • Check injector resistance/coil (specs vary by engine family; compare to service manual).
  • Swap injector 4 with another cylinder’s injector test (careful with wiring harnesses and vehicle security). If the misfire follows the injector, the injector is suspect.
  • Use a noid light or oscilloscope to verify injector signal and spray pattern if available.
• Fuel pressure:
  • Check rail pressure with the appropriate pressure gauge and compare to BMW spec. Low fuel pressure can cause misfires, especially under load.

Step 4 – Inspect air intake and vacuum around cylinder 4

• Leaks:
  • Inspect intake manifold runners, hoses, vacuum lines, PCV system, and gaskets for cracks or leaks near cylinder 4.
  • Spray with a quick air-leak test (or carb spray) while the engine is running to look for RPM changes indicating a vacuum leak.
• Sensors:
  • Inspect MAF sensor readings and intake tract cleanliness; a dirty MAF can affect fueling.

Step 5 – Check mechanical condition

• Compression test:
  • Perform a compression test on cylinder 4 and compare with the other cylinders. Low compression indicates internal engine issues (valve, ring, piston, head gasket, etc.).
• Leak-down test:
  • If compression is low, perform a leak-down test to pinpoint whether valves, rings, or head gasket are at fault.
• Carbon buildup:
  • For direct-injected engines, intake valve carbon buildup can cause misfires. Consider valve cleaning if indicated by test results.

Step 6 – Scan and verify after components replacement

• After replacing suspected coil, spark plug, or injector, re-scan for P0304 and perform a road test to verify misfire no longer occurs.
• Monitor live data (fuel trim, misfire counters) to confirm stability.

Step 7 – Consider software/ECU issues

• In some cases, BMW software calibrations or adaptions can influence fueling/ignition. If all hardware tests are clean and misfire persists, a software update or adaptation reset via ISTA or dealership tools may be advisable.

RELATED CODES

• P0300 – Random/Multiple Cylinder Misfire Detected
• P0301 – Cylinder 1 Misfire
• P0302 – Cylinder 2 Misfire
• P0303 – Cylinder 3 Misfire
• P0305 – Cylinder 5 Misfire
• P0306 – Cylinder 6 Misfire
• P0307 – Cylinder 7 Misfire (if applicable to engine variant)
  Note: If multiple cylinders show misfire codes, the issue may be fuel delivery, vacuum leaks, or a common electrical fault (e.g., DME, power supply, or main ground).

REPAIR OPTIONS AND COSTS (2025 PRICES)

Prices vary by region, dealership vs independent shop, and engine variant (4-cylinder B48/B46 vs 6-cylinder B58). The ranges below reflect typical U.S. labor rates and parts pricing as of 2025.

• Spark plug replacement (cylinder 4)

  • Parts: $5–$20 (each, BMW-appropriate plugs)
  • Labor: 0.5–1.0 hour
  • Estimated total: $150–$260

• Ignition coil replacement (cylinder 4)

  • Parts: $60–$150 for an OEM or high-quality aftermarket coil
  • Labor: 0.5–1.0 hour
  • Estimated total: $140–$280

• Fuel injector replacement (cylinder 4)

  • Parts: $150–$300 for injector (OEM or quality aftermarket)
  • Labor: 1–2 hours
  • Estimated total: $350–$700

• Fuel system cleaning or injector cleaning/additives

  • Parts: $0–$50
  • Labor: 0.5–1.0 hour (optional and not always conclusive)
  • Estimated total: $100–$150

• Vacuum leak repair ( hoses, PCV, gaskets)

  • Parts: $20–$150
  • Labor: 0.5–2.0 hours
  • Estimated total: $150–$400

• Intake manifold gasket or related gasket repair

  • Parts: $50–$200
  • Labor: 1.5–3.0 hours
  • Estimated total: $300–$900

• Compression test / leak-down diagnostic

  • Parts: minimal
  • Labor: 0.5–1.5 hours
  • Estimated total: $80–$250

• Mechanical valve/seat work or head gasket repair (if needed for persistent low compression)

  • Parts: $200–$2,000+ (depending on extent)
  • Labor: 4–12+ hours
  • Estimated total: $1,000–$6,000+ (often substantial)

• Software updates or reprogramming (if required)

  • Parts: typically none
  • Labor: 0.5–2.0 hours
  • Estimated total: $100–$400 (dealer vs independent)

Important note: If the misfire is caused by a mechanical issue (e.g., valve or piston problem) the cost can escalate quickly. The most cost-effective path is often diagnosing with a systematic swap-and-test approach to narrow down ignition vs fuel vs air vs mechanical causes.

DIY VS PROFESSIONAL

• Do-it-yourself (DIY) options
  • Safe and common DIY tasks: replace cylinder 4 spark plug and, if you’re comfortable, replace the ignition coil. A swap-test to verify the coil or injector is feasible with basic hand tools.
  • Tools you’ll want: torque wrench, appropriate spark plug socket, basic multimeter (for coil wiring checks), noid light or oscilloscope (optional but helpful), BMW-approved diagnostic tool or a capable OBD-II scanner with live data.
  • Cautions: High-pressure fuel system work requires care; avoid fuel leaks and follow safety procedures. If you’re not comfortable with fuel system components, consider professional service.
• Professional repair options
  • A shop can perform a thorough diagnostic (live data, fuel pressure test, injector testing, compression testing, leak-down test) and accurately pinpoint the failing component.
  • Pros: Access to BMW ISTA software, OEM parts, factory service procedures, and the ability to diagnose complex issues (e.g., injector driver faults, wiring harness corrosion, or ECU-related faults).
  • Cons: Higher initial labor costs, but often fewer misdiagnosis expenses.

PREVENTION

• Regular maintenance schedule
  • Spark plugs: Replace at the manufacturer-recommended interval (commonly around 60,000 miles for BMW four-cylinder engines, but check your exact engine code and owner's manual). Using OEM or OEM-equivalent plugs helps reliability.
  • Ignition coils: Replace only when there is confirmed failure or if the OEM coil shows weakness; avoid misfires by not pushing ignition components beyond service intervals.
  • Fuel system care: Use high-quality fuel and consider fuel injector cleaner treatments sparingly (not a substitute for mechanical cleaning if deposits are present). Consider periodic professional injector cleaning if you drive long distances or if you’re in a climate that deposits carbon.
  • Air intake and vacuum system: Inspect hoses and PCV lines for cracks or leaks; replace as needed to maintain proper air-fuel mixture.
  • Engine cooling and oil: Keep engine oil clean and at proper level; overheating can worsen misfire symptoms and engine wear.
• Driving habits
  • Short trips can contribute to carbon buildup and misfires in direct-injected engines; longer, steady drives help combustion and emissions control.
• Early action
  • If you notice rough idle, hesitation, or a CEL with P0304, address promptly to minimize potential damage to the catalytic converter and other exhaust sensors.

Recalls and TSBs note

• No recalls found in NHTSA database for this specific combination (2019–2023 BMW 3-Series with P0304), and no official recalls or TSBs are cited in the provided data. No recalls found in NHTSA database.

Data limitations

• The guidance above is based on general diagnostic practice for P0304 and common BMW 3-Series configurations. While it reflects typical industry approach, exact procedures and values can vary by engine variant (e.g., 4-cylinder B48/B46 vs. 6-cylinder B58), model year, and vehicle hardware. Always consult the vehicle’s service manual or a BMW specialist for model-specific specs, torque values, and procedures.