P0304 on 2006-2010 Lexus IS 250: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2006-2010 Lexus IS 250

The 2006-2010 Lexus IS 250 uses the 4GR-FSE engine, which features direct injection only. Unlike port injection, fuel is sprayed directly into the cylinder, completely bypassing the intake valves. The IS 350's engine, by contrast, uses a dual-injection system (D-4S) with both direct and port injectors, the latter of which spray fuel on the back of the intake valves, keeping them clean. On the IS 250, oil vapor from the PCV system bakes onto the hot intake valves, leading to significant carbon buildup that restricts airflow and causes misfires. This problem was so common that Lexus issued a Customer Support Program (CSP ZLG) and multiple Technical Service Bulletins (TSBs) to address it.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Rough or shaky idle, especially after a cold start or when coming to a stop.
• Flashing or solid Check Engine Light, often accompanied by a "Check VSC" message.
• Noticeable loss of power and poor acceleration.
🎬 Watch: What happens when misfires limit your speed to 35mph.
• Engine hesitation or stumbling, sometimes only at specific RPMs or under load on the highway.
• Increased fuel consumption.
• Smell of unburned fuel from the exhaust.

Most Likely Causes

1. Carbon Buildup on Intake Valves 🔴 High Probability The 4GR-FSE engine's direct-injection-only design doesn't wash the intake valves with fuel, leading to a high rate of carbon accumulation. This is a widely documented factory design issue for this specific vehicle, acknowledged by Lexus.
   How to confirm: After ruling out ignition issues by swapping coils and plugs, use a borescope to visually inspect the intake valves for heavy, black carbon deposits. The misfire will persist on cylinder 4 after swapping ignition components. The issue is often worse on the driver's side bank (cylinders 2, 4, 6) which requires removing the intake manifold for inspection.
   Typical fix: The intake manifold must be removed for cleaning. The most effective method is walnut shell blasting, which uses crushed walnut shells propelled by compressed air to safely scour the carbon off the valves. 🎬 See how walnut blasting effectively removes heavy carbon buildup. Chemical cleaning methods (like Seafoam or dealer "top engine cleans") are often less effective for heavy buildup. In severe cases covered under the Lexus CSP, the dealer repair involved replacing the pistons and piston rings with an updated design to reduce oil consumption, which contributes to the carbon buildup.
   Est. part cost: $20-$60 (for intake gaskets)
2. Failed Ignition Coil 🟡 Medium Probability
   How to confirm: The cylinders are numbered 1-3-5 on the passenger side (Bank 1) and 2-4-6 on the driver's side (Bank 2), front to back. Swap the ignition coil from cylinder 4 with cylinder 2 (the front-most, easiest to access coil on the same bank). Clear the codes and drive. If the code changes to P0302, the ignition coil is faulty.
   Typical fix: Replace the failed ignition coil. It is often recommended to replace all three coils on the driver's side bank at the same time since accessing them requires removing the upper intake manifold. 🎬 Watch this complete walkthrough for replacing plugs and coils. Use OEM or Denso coils for best results. The original part number 90919-02250 has been superseded by updated numbers like 90919-A2002.
   Est. part cost: $70-$170
3. Worn or Fouled Spark Plug 🟡 Medium Probability
   How to confirm: After confirming the ignition coil is good, swap the spark plug from cylinder 4 with another cylinder (e.g., cylinder 2). If the misfire code follows the plug, the spark plug is the issue. Visually inspect the plug for wear, oil, or heavy carbon fouling.
   Typical fix: Replace the faulty spark plug. It is best practice to replace all six spark plugs at the same time with high-quality Iridium plugs (e.g., Denso FK20HBR11) as recommended by the manufacturer. Accessing the driver's side plugs requires removing the intake manifold.
   Est. part cost: $15-$35 per plug
4. Clogged Fuel Injector ⚪ Low Probability
   How to confirm: This is more difficult to diagnose as the injectors are located under the valve covers. If ignition and carbon issues are ruled out, the injector may be the culprit. A professional can perform a flow test. Swapping injectors is a labor-intensive job.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $100-$250

Rare But Worth Checking

• Low Cylinder Compression: If all other causes are ruled out, a mechanical issue like a worn piston ring or leaking valve could be the cause. This was a known issue related to the carbon problem, sometimes requiring piston replacement by Lexus. A compression test can confirm this.
• Vacuum Leak: A leak in the intake manifold gasket or a vacuum hose near cylinder 4 could lean out the mixture and cause a misfire. This can be found using a smoke machine. Improperly re-installing the intake manifold after a spark plug job is a common cause.
• Faulty Engine Control Module (ECM): In very rare cases, after all mechanical, ignition, and fuel possibilities have been exhausted, a faulty driver in the ECM could be the cause of the misfire signal. This was mentioned as a last resort in forum discussions.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0304 and any other related codes. Note any freeze-frame data which shows engine conditions when the code was set.
2. Access the driver's side of the engine. The cylinders are numbered 2, 4, 6 from front to back. This requires removing the upper intake manifold.
3. Swap the ignition coil from cylinder 4 (middle) with cylinder 2 (front). Clear codes, drive the vehicle, and re-scan. If the code is now P0302, replace the faulty ignition coil (part # 90919-A2002 or equivalent).
4. If the code remains P0304, swap the spark plug from cylinder 4 with cylinder 2. Clear codes, drive, and re-scan. If the code is now P0302, replace all six spark plugs (Denso FK20HBR11).
5. If the misfire still remains on cylinder 4, the most likely cause is carbon buildup. With the intake manifold already off, use a borescope camera to inspect the intake valves of cylinder 4 for heavy, crusty deposits.
6. If significant carbon is found, schedule a walnut blasting service or perform a manual valve cleaning. This service can cost between $300 and $1,000 at an independent shop.
7. If no significant carbon is visible, further professional diagnostics are needed to check the fuel injector, wiring harness, and cylinder compression.

Parts You'll Likely Need

• Ignition Coil (OEM #90919-02250 (superseded by 90919-A2002, 90919-02255)) — Ignition coils are a common failure point and a primary cause of misfires. Accessing Bank 2 coils requires significant labor, so replacing them preventatively is wise.
  Trusted brands: Denso (673-1309), NGK (48977), Delphi
  OEM price range: $100-$135
  Aftermarket price range: $50-$100
• Spark Plug (OEM #90919-01249 (Denso FK20HBR11)) — Spark plugs are a regular maintenance item. It is critical to replace all six since the labor to access the driver's side is high.
  Trusted brands: Denso (FK20HBR11), NGK
  OEM price range: $15-$20
  Aftermarket price range: $12-$18
• Intake Plenum Gasket (OEM #17176-31050) — These three gaskets must be replaced whenever the upper intake manifold (plenum) is removed for spark plug replacement or carbon cleaning.
  Trusted brands: Lexus (OEM), Fel-Pro (MS 97084), Mahle
  OEM price range: $6-$10 per gasket (x3 needed)
  Aftermarket price range: $15-$30 (set of 3)

Related Codes That Often Appear With This One

• P0300 — If the carbon buildup issue becomes severe enough to affect multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire) code will often appear alongside cylinder-specific codes.
• P0301, P0302, P0303, P0305, P0306 — As carbon buildup is not isolated to one cylinder, it's common to see misfire codes for other cylinders as well, especially P0302 and P0306 which are on the same bank as P0304.
• Check VSC — The Vehicle Stability Control (VSC) system is often disabled by the ECM as a safety precaution when a significant engine fault like a misfire is detected, triggering this warning light on the dash.

Technical Service Bulletins (TSBs) & Recalls

• L-SB-0029-12: This is the key Service Bulletin. It details the repair procedure for the carbon buildup issue, which includes the replacement of pistons and piston rings with redesigned parts.
• L-TT-0192-15: A Tech Tip bulletin confirming that IS250 and GS300 vehicles may exhibit MIL 'ON' with DTCs P0300-P0306 and run rough, pointing technicians toward the known carbon issue.
• L-TT-0093-12: An earlier Tech Tip also noting that IS250 and GS300 vehicles may exhibit MIL 'ON' with DTCs P0300-P0306 and have intermittent rough running conditions.
• Customer Support Program (CSP) ZLG: Not a TSB, but a formal letter sent to owners extending the warranty for carbon buildup repairs to 9 years/unlimited miles.

Platform-Specific Known Issues

• The 2.5L 4GR-FSE engine is notorious for carbon buildup on the intake valves, leading to misfires, rough idle, and hesitation. This was so prevalent that Lexus issued Customer Support Program ZLG, extending coverage for this specific issue to 9 years with unlimited mileage.

Mechanic-Grade Diagnostic Values

• Low-Pressure Fuel System at Idle — expected: 196 to 588 kPa (28 to 85 psi). Failure: Pressure lower than specified can indicate a failing in-tank fuel pump.
• High-Pressure Fuel System at Idle — expected: 3.5 to 4.5 MPa (508 to 653 psi). Failure: Pressure outside this range can indicate issues with the high-pressure fuel pump or fuel pressure sensor.
• Crankshaft Position Sensor Resistance (Cold) — expected: 1,630 - 2,740 Ω. Failure: Resistance outside this range can indicate a faulty sensor, which is critical for misfire detection.
• Crankshaft Position Sensor Resistance (Hot) — expected: 2,065 - 3,225 Ω. Failure: Resistance outside this range when the engine is warm points to a sensor failing under heat.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $53 (or similar): This is not a DTC, but a monitor within the OBD-II system that tracks raw misfire counts for each cylinder before a P030x code is set. Viewing this data can reveal intermittent misfires on cylinder 4 that haven't yet triggered the check engine light. (see via A professional scan tool like Toyota's Techstream or an advanced OBD-II scanner with Mode $06 capabilities.)

Scan Tool Commands That Help

• Toyota Techstream: Power Balance Test (Active Test) — This function allows a technician to sequentially disable the fuel injector to each cylinder. By observing the RPM drop for each cylinder, you can confirm if cylinder 4 is contributing less power than the others. A small or no RPM drop when cylinder 4 is disabled confirms it's the source of the misfire.
• Toyota Techstream: Misfire Monitor (Data List / Monitor) — Use the 'Monitor' screen in Techstream to get a more accurate snapshot of misfire counts per cylinder, especially under load when the standard data list may not update correctly. This can help confirm that cylinder 4 is the primary or only cylinder misfiring.
• Toyota Techstream: Control the Fuel Pump Speed (Active Test) — This test can be used to verify the operation of the low-pressure in-tank fuel pump, helping to rule out a system-wide fuel delivery issue if multiple misfires were present.

Wiring & Ground Locations

• Engine Ground Points — Key ground points for the engine harness are located on the left and right sides of the engine compartment.. A corroded or loose ground connection can cause erratic behavior in the ignition system, including weak spark or incorrect signals from sensors, potentially leading to misfire codes. Ensuring clean and tight grounds is a crucial step in diagnosing electrical faults.
• Junction Connectors (J/C) — Multiple junction connectors are located in the kick panels on both the driver's and passenger's sides (e.g., J1, J4, J10) and behind the dash.. The wiring for the ignition coils and fuel injectors passes through these connectors. A poor connection at one of these points could specifically affect the circuit for cylinder 4, causing a misfire.

Real Owner Repair Stories

• ClubLexus user 'noblesenga' (2007 IS 250, 175k km) — Cylinder 4 misfire (P0304) only at highway speeds (100kmph+). No misfire in city driving or on cold starts. Restarting the car temporarily clears the issue.
  ❌ Tried (didn't work) Swapped ignition coil from cylinder 4 to cylinder 1; problem did not follow the coil., Swapped spark plug from cylinder 4 to cylinder 5; problem did not follow the plug.
  ✅ What actually fixed it The user sold the car before a definitive fix was confirmed, but the final suspicion after ruling out ignition components was a failing fuel injector on cylinder 4.
• Reddit user 'Ok-Potential-3022' (2008 IS 250) — Cylinder 5 misfire (P0305), more frequent in cold or rainy weather. Blinking or solid check engine light, limp mode when blinking. A whining sound during acceleration.
  ❌ Tried (didn't work) Replaced ignition coil on cylinder 5., Replaced spark plug on cylinder 5.
  ✅ What actually fixed it The thread resolution pointed strongly to the classic carbon buildup issue on the intake valves. A former Lexus technician in the thread described the full repair as replacing piston rings and cleaning the valves, and noted that holding the RPM at 3k and then stabbing the throttle could often induce a misfire by dislodging carbon flakes.

OEM Part Supersession History

• 13011-31120, 13011-31121, 13011-31122 → 13011-31220 — Updated piston ring set design to reduce oil consumption and mitigate the carbon buildup problem.
  Heads up: The updated rings were part of the official Lexus repair for the carbon issue and were reportedly used in production for the 2010 model year onward.
• 90919-02250 → 90919-A2005, 90919-02256 — Standard part revision and update by the manufacturer.

Model Year Variations Within This Range

• 2010: Starting with the 2010 model year, Lexus reportedly revised the engine with updated piston rings to help combat the oil consumption that leads to the severe carbon buildup issue. While not a complete fix, 2010 models are considered less prone to the issue than 2006-2009 models.