P0304 Code: Toyota Corolla (2020-2024) - Causes, Symptoms & Fixes

Below is a comprehensive diagnostic guide for OBD-II code P0304 (misfire detected in cylinder 4) specifically for 2020–2024 Toyota Corolla. Note on data sources: in the provided NHTSA data for 2020–2024 Toyota Corolla, the OWNER COMPLAINTS section lists a P0401 case (not P0304), and there are no recalls found in the database. No P0304 complaints are shown in that dataset. The guide blends model-specific considerations with general automotive diagnostics for P0304. Use it as a structured starting point, but treat any symptoms and data from your own vehicle as primary.

CODE MEANING AND SEVERITY

• What P0304 means: A misfire detected in cylinder 4 by the engine control module (ECM/PCM). The ECM uses sensors (MAP/MAF, crankshaft position, knock, oxygen sensors, etc.) to determine if cylinder 4 is not producing expected power and combustion.
• Severity:
  • Intermittent misfires may cause rough running, poor acceleration, and higher fuel consumption.
  • Frequent or constant misfires can overheat and damage the catalytic converter and unburned fuel can wash down the cylinder wall leading to more severe engine damage over time.
  • In some driving conditions (high load, acceleration, or towing) a misfire can cause unsafe handling if power delivery is degraded.

COMMON CAUSES ON TOYOTA COROLLA (2020–2024)

• Faulty spark plug in cylinder 4 (worn, fouled, or damaged)
• Faulty ignition coil on cylinder 4 or damaged coil wiring
• Faulty or leaking fuel injector on cylinder 4 or injector wiring
• Wiring or connector harness problems to the coil or injector for cylinder 4
• Vacuum leaks near cylinder 4 intake port or plenum (gaskets, hoses)
• Low fuel pressure or dirty fuel system affecting cylinder 4
• Mechanical issue in cylinder 4 (low compression due to valve seating, piston ring, or head gasket concerns)
• Dirty or faulty mass airflow (MAF) sensor or air intake system causing lean misfire
• Incorrect cam/crank timing in rare cases (timing belt/chain issue, though less common on modern Corollas)
• Misunderstanding of code: P0304 may occur with intermittent P0300 or P035x (coil circuit) symptoms if the coil or injector wiring is intermittent

SYMPTOMS YOU MIGHT SEE with P0304

• Check Engine Light (CEL) or MIL (malfunction indicator lamp) is on
• Rough idle or engine vibration at idle
• Noticeable misfire under load or during acceleration
• Decreased engine power, hesitation, or stumble
• Poor or irregular engine performance at highway speeds
• Increased fuel consumption or rough acceleration feel
• In some cases, temporary stalling during idle or at stoplights
• P0304 may be accompanied by other codes such as P0300 (random/multiple misfire) or P035x (ignition coil circuit faults)

DIAGNOSTIC STEPS (systematic approach)

Note: Start with a proper OBD-II scan to confirm P0304 and review freeze frame data and misfire counters.

A. Confirm and collect baseline data

• Scan for codes: Confirm P0304 and note any related codes (P0300, P0351–P0356, P0204, P0301–P0303, etc.).
• Review freeze frame data: Engine load, RPM, temperature, fuel trim (short/long), and misfire counts at the time of fault.
• Check live data (if your scanner supports it): Cylinder misfire counts, ignition coil primary/secondary voltage, injector pulse width, MAF, MAP, fuel pressure (if supported), and fuel trim.

B. Quick mechanical/visual checks

• Inspect cylinder 4 spark plug: condition (oil fouling, ash, wear), gap, and presence of cracks.
• Inspect coil pack and coil boot for cylinder 4: cracks, carbon tracking, heat damage, or loose connections.
• Inspect wiring to cylinder 4 coil and injector: damaged insulation, corrosion, loose connectors, and ground points.
• Inspect vacuum lines near cylinder 4 intake runner and gaskets for leaks or cracks.
• Check intake manifold and plenum for leaks around cylinder 4 region.

C. Component-level tests (swap and test to isolate)

• Spark plug test/relacement
  • If spark plug is worn or fouled, replace cylinder 4 plug (and consider replacing all 4 if plugs are near their service interval).
  • After replacement, clear codes and test drive to see if P0304 returns.
• Ignition coil test
  • If coil is suspected, swap coil from cylinder 4 with another cylinder (e.g., cylinder 2). If P0304 moves to the new cylinder, the coil is the likely culprit.
  • If you don’t have an extra coil, perform a resistance test on the coil primary/secondary with the service manual specs (compare to spec). Look for open circuits or high resistance.
  • Inspect coil connectors for corrosion or damage.
• Injector test
  • Swap or swap-test the cylinder 4 injector with another cylinder to see if the misfire code follows the injector.
  • Check injector resistance with a multimeter per service manual. Compare to spec.
  • Consider cleaning the injector if you don’t find a mechanical fault. A stuck/dirty injector can cause a misfire.
• Fuel rail and pressure
  • If fuel pressure is accessible, test to ensure adequate flow/pressure for cylinder 4. A weak injector or failing fuel pump/regulator can cause misfires at certain cylinders.
• Compression test and leak-down test
  • Perform a compression test on all cylinders; if cylinder 4 shows abnormally low compression, perform a leak-down test to identify if the issue is valve seating, piston rings, or head gasket.
  • If compression is low on cylinder 4, plan for a more thorough mechanical diagnosis.

D. Optional/advanced checks

• MAF/MAP sensor and air intake cleanliness
  • Clean MAF sensor (following Toyota service procedure) and inspect for contamination or damage.
  • Check for excessive air leaks after the MAF/MAF sensor and ensure correct intake pressure readings.
• Oxygen sensors / fuel trim
  • If long-term fuel trim is consistently rich/lean, inspect for stuck or slow oxygen sensor performance or a serious air leak.

E. Road test after service

• Clear codes, reset monitors, and take a test drive under varying loads (idle, light throttle, and steady highway speeds) to confirm the misfire is resolved.

RELATED CODES

• P0300: Random/Multiple misfire detected.
• P0301, P0302, P0303, P0305, P0306: Misfire in cylinders 1, 2, 3, 5, 6 respectively.
• P0351–P0356: Ignition coil secondary circuit faults for cylinders 1–6 (often seen with misfires due to coil issues).
• P0204: Injector fault for cylinder 4 (possible alternative cause if injector circuit is the issue).
• P0171/P0174: System too lean or rich (may interact with misfire if fuel delivery is inconsistent).
  Note: In practice, P0304 can coexist with related codes; diagnosing typically starts with the misfire cause (spark, injector, or compression) and then checks for secondary sensor/adaptor faults.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Prices vary by region, shop, and whether you choose OEM parts or aftermarket. The ranges below are ballpark figures for the United States in 2025.

• Diagnostic scan and fault confirmation (labor already included in most diagnostic shop visits): 60–120
• Spark plug replacement (cylinder 4; or all four if needed)
  • Parts: 8–20 each (iridium/platinum OEM-style plugs may be on the higher end)
  • Labor: 0.5–1.0 hours
  • Estimated total: 120–350 (often 180–250 if only cylinder 4)
• Ignition coil replacement (cylinder 4)
  • Parts: 40–120
  • Labor: 0.3–1.0 hours
  • Estimated total: 70–270
• Fuel injector (cylinder 4) replacement
  • Parts: 120–300
  • Labor: 0.5–1.0 hours
  • Estimated total: 170–420
• Fuel injector cleaning (non-replacement)
  • Parts: 0–60
  • Labor: 0.5–1.0 hours
  • Estimated total: 70–180
• Fuel pressure check/repair (pump, regulator)
  • Parts: 60–350
  • Labor: 0.5–1.0 hours
  • Estimated total: 120–500
• Cylinder 4 compression test (diagnostic)
  • Parts: 0–50
  • Labor: 0.5–1.0 hours
  • Estimated total: 60–150
• Mechanical issues (low compression from valve/piston/ring or head gasket)
  • Typical repair scope: top-end rebuild or head gasket job
  • Estimated total: 1,000–3,000+ (highly variable; often much higher if engine disassembly is required)
• General note on costs:
  • DIY parts may save labor, but misfire diagnostics can require specialized tooling.
  • Labor rates vary by region; dealerships tend to be higher than independent shops.

DIY vs PROFESSIONAL

• DIY-friendly steps (if you have basic tools and experience):
  • Swap coil from cylinder 4 with another cylinder to see if the misfire follows the coil.
  • Inspect and replace the cylinder 4 spark plug if found worn or fouled.
  • Clean or inspect the cylinder 4 injector; attempt a swap with a known-good injector if you have the tools to do so safely.
  • Inspect vacuum lines and intake hoses for leaks; fix cracks or loose clamps.
• When to seek a professional:
  • Misfire persists after spark plug/injector/coil checks.
  • Suspected mechanical fault (low compression) or suspected valve/piston/rig issues.
  • You don’t have access to an appropriate scan tool, test equipment (fuel pressure gauge, compression tester), or the expertise to perform higher-risk tasks.
  • If misfires reoccur in multiple cylinders or you see multiple DTCs tied to ignition or fuel delivery.

PREVENTION

• Follow Toyota’s maintenance schedule for spark plugs and ignition system components. Modern Toyota engines commonly use iridium/platinum plugs with longer intervals; confirm your exact plug type and recommended replacement interval in the owner’s manual.
• Replace spark plugs at the recommended interval (or sooner if you notice rough running or reduced performance). Ensure correct torque and plug gap per service manual.
• Use high-quality fuel and consider top-tier fuels to reduce deposits on injectors and intake components.
• Perform periodic fuel-system cleaning or injector treatment only as recommended by Toyota or a trusted technician. Do not overuse cleaners; improper use can damage components.
• Keep air intake system clean: replace air filter regularly, check for air leaks in the intake tract, and clean MAF sensor if contaminated (per the service manual).
• Address fuel system and vacuum leaks promptly; small leaks can cause lean conditions and misfires.
• Maintain proper engine oil level and use the recommended grade; poor lubrication can lead to rough operation and other faults.
• If you notice rough idle or intermittent misfires, address early to minimize potential catalytic converter damage.
• Regularly inspect electrical connectors and wiring harnesses for coil/injector circuits; corrosion or loose connections can cause intermittent misfires.

Important data note

• Based on the provided NHTSA data for 2020–2024 Toyota Corolla, there is no P0304 complaint in the dataset; there is a P0401 complaint in the owner records. There are no recalls listed for this model/year in the provided data. The diagnostic guidance above is informed by common P0304 causes in modern four-cylinder Toyota engines, plus general diagnostic principles and typical repair costs, and should be adapted to your vehicle’s exact symptoms and service data.

If you’d like, tell me your mileage, whether the check engine light is solid or blinking, and any symptoms you’ve observed (idle behavior, acceleration feel, fuel economy). I can tailor a step-by-step diagnostic checklist and estimate costs more precisely for your situation.