P0303 on 2006-2010 Lexus IS 250: Cylinder 3 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 a direct injection fuel system. Unlike traditional port injection systems that spray fuel over the intake valves and keep them clean, direct injection sprays fuel directly into the combustion chamber. This design allows oil vapors from the PCV system to bake onto the intake valves and piston tops, leading to heavy carbon deposits. This accumulation disrupts airflow and can cause the valves to hang open, which is the leading cause of misfires, rough idling, and the P0303 code on this specific vehicle. This issue is so prevalent that Lexus issued Technical Service Bulletins (TSBs) and a Customer Support Program (CSP), internally labeled ZLG, to address it. The official fix often involved a top engine clean or, in severe cases, a complete engine teardown to replace the pistons and piston rings with redesigned parts.

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 when stopped at operating temperature. [Bulletin #L-TT-0192-15]
• Flashing or solid Check Engine Light
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power.
• Reduced fuel economy.
• Engine stalling, particularly after coming to a stop from highway speeds.
• A 'choked' feeling from the engine during initial acceleration.

Most Likely Causes

1. Excessive Carbon Buildup 🔴 High Probability The 4GR-FSE engine's direct-injection design does not wash carbon deposits off the intake valves, leading to accumulation that disrupts airflow and causes misfires. This is a widely recognized issue for this model, acknowledged by Lexus through a warranty extension program.
   How to confirm: After ruling out ignition components, a technician can use a borescope to visually inspect the intake valves and piston tops in cylinder 3 for heavy carbon deposits. The intake manifold must be removed for this inspection.
   Typical fix: Professional cleaning is required. This can involve top-engine chemical cleaning (often seen as a temporary fix), media blasting (such as walnut blasting) of the intake valves, or a manual scraping after removing the intake manifold. In severe cases under the now-expired warranty program, Lexus dealers replaced the pistons and rings with an updated design. Some owners perform a 'piston soak' with solvents like Berryman B-12 or SeaFoam to dissolve carbon on the piston tops and free stuck rings.
   Est. part cost: $50-$200 for chemicals/gaskets, or significantly more if parts are replaced.
2. Failed Ignition Coil 🟡 Medium Probability → Shop Ignition Coil Ignition coils are a common wear item on most modern vehicles and can fail with age and heat cycles.
   How to confirm: Swap the ignition coil from cylinder 3 with another cylinder (e.g., cylinder 1, which is on the same, easily accessible bank). Clear the codes and drive the car. If the code changes to P0301, the ignition coil is faulty. This is the most common and effective first diagnostic step.
   Typical fix: Replace the failed ignition coil. It is often recommended to replace them in sets if they are original and high-mileage. Use OEM (Toyota/Denso) or a reputable brand like NGK.
   Est. part cost: $70-$130 for one OEM-quality coil.
3. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug
   How to confirm: Remove the spark plug from cylinder 3 and inspect it for wear, oil, or carbon fouling. An oil-fouled plug can indicate other issues like a leaking valve cover gasket. You can also swap it with another cylinder's spark plug to see if the misfire code follows.
   Typical fix: Replace the spark plug. It is best practice to replace all six spark plugs at the same time with the correct type (Iridium Long-Life).
   Est. part cost: $60-$120 for a set of six high-quality spark plugs.

Rare But Worth Checking

• Clogged or Faulty Fuel Injector: → Shop Fuel Injector Direct injectors operate under high pressure and can become clogged or fail. One owner on ClubLexus replaced all injectors after other fixes failed, which improved the condition but didn't fully resolve their specific high-mileage issue.
• Low Engine Compression: In severe cases, carbon buildup on piston rings can cause them to stick, leading to a loss of compression in the cylinder. A compression test can verify this. One forum member reported excellent compression (210 PSI on cylinder 3) yet still had a persistent misfire, making this a less likely, but still possible, cause.
• Damaged Ignition Coil Connector or Wiring: → Shop Ignition Coil The plastic connectors can become brittle over time and break, leading to a poor connection and misfire. This is especially common on older, high-mileage vehicles where the plastic has been exposed to years of heat cycles.
• Leaking Valve Cover Gasket / Spark Plug Tube Seal: → Shop Engine Valve Cover A common failure is the spark plug tube seal, which is integrated into the valve cover gasket. When it fails, oil leaks into the spark plug well, fouling the plug and potentially shorting the ignition coil. Several owners on Reddit and ClubLexus have confirmed this as the fix for their P0303 after finding oil on the spark plug.

Diagnosis Steps

1. Read the code using an OBD-II scanner to confirm P0303 and any other related codes.
2. Identify cylinder 3. On the IS 250's V6, the passenger side bank (Bank 1) contains cylinders 1, 3, and 5 from front to back. The driver's side (Bank 2) has 2, 4, and 6. Cylinder 3 is the middle cylinder on the passenger side.
3. Swap the ignition coil from cylinder 3 with the coil from cylinder 1. Clear the codes and run the engine to see if the misfire code moves to P0301. If it does, the coil is bad. This is the easiest and most conclusive first step.
4. If the coil is not the issue, inspect the spark plug from cylinder 3. Look for signs of wear, damage, or fouling (carbon or oil). Replace if necessary. Consider replacing all six if they are due.
5. If the ignition system components are good, the next step is to suspect carbon buildup. This requires removing the upper and lower intake manifold to gain access to the intake ports.
6. Use a borescope to visually inspect the intake valves and piston top of cylinder 3. Heavy, black, crusty deposits confirm the common issue.
7. If significant carbon is found, the fix is cleaning. Options include media blasting (walnut shells) or manual scraping with brushes and chemical solvents.
8. If no significant carbon is found, proceed with more advanced diagnostics, such as testing the fuel injector and performing an engine compression test for cylinder 3.

Parts You'll Likely Need

• Ignition Coil (OEM #90919-02250 (discontinued), 90919-A2002, 90919-02251) — A common failure point for misfires. Easy to diagnose by swapping between cylinders. The original part number has been superseded.
  Trusted brands: Denso (OEM Manufacturer), NGK
  OEM price range: $99.
  Aftermarket price range: $50-$80
• Iridium Spark Plug (OEM #Denso FK20HBR11) — A necessary maintenance item that can cause misfires when worn or fouled. The carbon issue on this engine can foul plugs prematurely.
  Trusted brands: Denso, NGK
  OEM price range: $15-$22 per plug.
  Aftermarket price range: $10-$15 per plug
• Intake Manifold Gasket Set — Required if removing the intake manifold for carbon inspection or cleaning.
  Trusted brands: Fel-Pro, Mahle
  OEM price range: $40-$60
  Aftermarket price range: $20-$40

Related Codes That Often Appear With This One

• P0300 — If the carbon buildup issue is affecting multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside cylinder-specific codes.
• P0301, P0302, P0304, P0305, P0306 — It's common for other cylinder misfire codes to be present, as the underlying carbon issue is not isolated to a single cylinder. The TSBs from Lexus specifically list all cylinder misfire codes as related symptoms. [Bulletin #L-TT-0093-12 Rev]
• P0420 — A chronic misfire can send unburnt fuel into the exhaust, damaging the catalytic converter and eventually triggering a P0420 (Catalyst System Efficiency Below Threshold) code.

Technical Service Bulletins (TSBs) & Recalls

• L-TT-0192-15: Notes that some IS250 and GS300 vehicles may exhibit MIL ON with various P030x codes and run rough intermittently, especially after stopping with the engine warm.
• L-TT-0093-12 Rev: Describes the same conditions of MIL ON with P030x codes and intermittent rough running at operating temperature. It provides a procedure for technicians to duplicate the symptom.
• L-SB-0029-12 (and related versions): While not directly cited in the search, TSBs in this family often relate to engine mechanical repairs. The actual repair procedure for piston replacement would have been detailed in a similar Service Bulletin for technicians performing the CSP work.

Platform-Specific Known Issues

• Carbon Buildup Customer Support Program (CSP ZLG): Lexus acknowledged the carbon buildup problem and issued CSP ZLG, which extended the warranty for this specific issue to 9 years with unlimited mileage from the vehicle's in-service date. The program covered the cost of cleaning the engine and, if necessary, replacing the pistons and piston rings with redesigned parts. This program has now expired for all 2006-2010 models, but its existence confirms the prevalence of the problem.
• TSBs for Rough Idle and Misfires: Lexus issued multiple Technical Service Bulletins, including L-TT-0192-15 and L-TT-0093-12, specifically mentioning DTCs P0300-P0306 and intermittent rough running at operating temperature, directly related to the carbon buildup problem. [Bulletin #L-TT-0192-15, Bulletin #L-TT-0093-12 Rev] These bulletins guided technicians on how to replicate and diagnose the condition.

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel System — expected: 3.5 to 4.5 MPa (508 to 653 psi) at idle. Failure: Pressure outside the specified range may indicate a problem with the high-pressure fuel pump or sensor.
• Low-Pressure Fuel System — expected: 196 to 588 kPa (28 to 85 psi). Failure: Incorrect pressure could point to an issue with the in-tank fuel pump or injectors.
• Crankshaft Position Sensor Resistance — expected: 1,630 - 2,740 Ω (Cold), 2,065 - 3,225 Ω (Hot). Failure: Resistance outside of these ranges can indicate a faulty sensor, which is critical for misfire detection.
• Mode $06 Misfire Counts — expected: A count of 0 for each cylinder during normal operation.. Failure: Any significant count for cylinder 3 (Test ID $A3, Component ID $53 on some systems) indicates intermittent misfires are occurring, even if a code is not yet set. This data is crucial for diagnosing intermittent issues.

Hidden / Shadow Codes Worth Checking

• Mode $06 Misfire Data: Professional scan tools can access Mode $06 data, which shows raw misfire counts for each cylinder from the last 10 drive cycles (EWMA) and the current drive cycle. For P0303, a technician would look for counts specifically on cylinder 3. This can reveal an intermittent misfire long before it's frequent enough to set a persistent P0303 code. (see via A professional scan tool or software like Toyota Techstream is required to view Mode $06 data.)

Scan Tool Commands That Help

• Toyota Techstream: Misfire Monitor (Data List) — Use this to view live and historical misfire counts for each individual cylinder (e.g., CYL #1, CYL #2, etc.). This is the most direct way to confirm which cylinder is misfiring and how frequently, without relying solely on the generic P030x code.
• Toyota Techstream: Active Test: Control the Injector Cut — This function allows a technician to individually disable the fuel injector for each cylinder. If you cut the injector to cylinder 3 and there is NO change in engine idle, it confirms that cylinder 3 was not contributing power, pinpointing the misfire location.
• Toyota Techstream: Active Test: Control the Fuel Pump Speed — This allows for testing the functionality of the low-pressure fuel pump by commanding it to run, helping to diagnose fuel delivery issues before the high-pressure pump.

Wiring & Ground Locations

• Ground Points EA, EB — On the 4GR-FSE, these are the primary ground points for the engine harness, located on the cylinder heads. Specifically, ground point 'EA' is on the front of the Bank 1 (passenger side) cylinder head, near cylinder 1. 'EB' is in a similar location on Bank 2.. The ignition coils are grounded via the engine harness to these points. A loose or corroded ground at EA could cause erratic ignition performance for cylinders 1, 3, and 5, leading to intermittent misfires.
• Ground Points A1-A5, B1-B2 — These are various chassis ground points located in the engine bay on the inner fenders and front support.. The Engine Control Module (ECM) and other critical sensors rely on clean chassis grounds. A compromised ground at one of these locations can introduce electrical noise and voltage drops, potentially causing the ECM to misinterpret sensor data and incorrectly flag a misfire.

Real Owner Repair Stories

• Reddit user in r/LexusIS (2009 Lexus IS 250) — Persistent P0303 misfire on cylinder 3.
  ❌ Tried (didn't work) The user had already replaced all spark plugs and ignition coils, but the misfire on cylinder 3 remained.
  ✅ What actually fixed it The user found oil on the spark plug for cylinder 3. The final fix was replacing the valve cover gasket, which includes the spark plug tube seals. This stopped the oil from leaking into the spark plug well and fouling the plug.
• ClubLexus forum user (2006 Lexus IS250 AWD, 147k miles) — Crank no-start, followed by multiple cylinder misfires (including on cylinders 4 & 6) detected by Techstream after getting the car to run.
  ❌ Tried (didn't work) New battery, New spark plugs
  ✅ What actually fixed it The initial no-start issue was resolved by replacing a failed fuel pressure sensor. The misfires that occurred afterward were intermittent and appeared to be related to the carbon buildup issue, exacerbated by fuel system instability.

OEM Part Supersession History

• 13011-31120, 13011-31121, 13011-31122 → 13011-31143 — The original piston rings were part of the design that contributed to excessive oil consumption and carbon buildup. The updated part numbers represent a revised design intended to mitigate this issue, which was implemented as part of the Lexus Customer Support Program (CSP) ZLG.

Model Year Variations Within This Range

• 2006-2010: These model years were equipped with the original piston and ring design that was highly susceptible to carbon buildup, leading to the P030x series of codes. Lexus issued a Customer Support Program (CSP ZLG) to address this for these specific years.
• 2011+: Starting in the 2011 model year, Lexus implemented a revised piston and ring design from the factory to reduce oil consumption and subsequent carbon buildup. While not completely immune, these later models are significantly less likely to experience severe carbon-related misfires compared to the 2006-2010 range.