P0305 on 2011-2015 Lexus RX Hybrid: Cylinder 5 Misfire Causes and Fixes

What's Unique About the 2011-2015 Lexus RX HYBRID

For this specific generation of the RX 450h (which uses the 2GR-FXE engine), misfire codes, including P0305, are specifically mentioned in a Lexus Technical Service Bulletin (TSB L-SB-0045-12 Rev). This TSB addresses complaints of a rough running engine immediately after a cold start. While standard ignition component failure is common, this TSB points to a more complex potential issue involving carbon buildup on pistons and rings, leading to low compression when the engine is cold. This makes a proper diagnosis, including a cold compression test, more critical than on other vehicles. Furthermore, cylinder 5 is on the rear bank (against the firewall), requiring removal of the intake plenum for access, which significantly increases labor time and cost for what would otherwise be a simple repair.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light is on (may be solid or flashing)
• Rough running on initial start-up, especially after a cold soak.
• Engine stalling at low speeds or when put into neutral after a cold start.
• Rough or vibrating idle
• Engine hesitation or stumbling during acceleration
• Reduced engine power
• Poor fuel economy
• A smell of fuel from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coils are common wear-and-tear items. Heat and vibration, especially on the tightly-packaged rear bank, eventually cause the internal windings to fail, leading to a weak or no spark. TSB L-SB-0044-19 for a later generation also points to ignition coils as a primary cause for misfires, suggesting a pattern for this engine family.
   How to confirm: The standard method is to swap the ignition coil from cylinder 5 with an adjacent cylinder (like cylinder 3 or 1 on the same bank). However, since accessing the rear bank requires removing the intake manifold, this is not a simple task. A more practical approach for this engine is to replace all three rear coils and plugs at the same time to avoid duplicate labor costs in the near future. Before disassembly, a technician can use a scan tool to monitor misfire counts.
   Typical fix: Replace the faulty ignition coil. Due to the high labor cost to access the rear bank, it is strongly recommended to replace all three rear ignition coils (cylinders 1, 3, 5) and all six spark plugs at the same time.
   Est. part cost: $50-$120 per coil
2. Worn Spark Plug 🔴 High Probability → Shop Spark Plug
   How to confirm: Remove the spark plug from cylinder 5 after taking off the intake manifold and inspect it. Look for a worn or fouled electrode, cracks in the porcelain, or an incorrect gap. Compare it to a plug from a working cylinder. Given the labor involved, visual inspection is secondary to proactive replacement. The standard electrode gap is between 1.0 mm and 1.1 mm (0.039 in. and 0.043 in.).
   Typical fix: Replace the spark plug in cylinder 5. It is best practice to replace all six Iridium spark plugs at the same time to ensure even performance and avoid repeating the labor-intensive intake manifold removal. Use high-quality Iridium plugs as specified by the manufacturer (e.g., Denso or NGK).
   Est. part cost: $10-$25 per plug
3. Internal Engine Issue (per TSB L-SB-0045-12 Rev) 🟡 Medium Probability Lexus issued a TSB for 2010-2017 RX 450h models that exhibit misfires (P030x) and rough running on cold starts. The underlying cause can be related to carbon buildup on pistons and rings, leading to low compression when the engine is cold.
   How to confirm: Perform a compression test on all cylinders when the engine is completely cold (after sitting for 8+ hours). If cylinder 5 shows significantly lower compression than the others, this TSB may apply. A follow-up cylinder leak-down test can further pinpoint the issue. This must be done before replacing ignition parts if the main symptom is cold-start roughness. A Techstream scan tool can also perform a relative compression test without removing spark plugs.
   Typical fix: The TSB procedure involves first checking if an ECM software update is available and applying it. If the problem persists, the more involved repair requires engine disassembly to replace the pistons and piston rings with updated parts. This is a major, expensive repair for a professional technician only.
   Est. part cost: $2000-$5000+ for internal engine work
4. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: After ruling out spark issues, the fuel injector is a possible culprit. A technician can perform a fuel injector balance test to measure flow rates. A simpler DIY method is to listen to the injector with a mechanic's stethoscope; it should have a consistent, audible clicking sound. Like the coils, the rear injectors are under the intake manifold.
   Typical fix: Replace the faulty fuel injector. It's also wise to clean the fuel rail and consider replacing other injectors if they are original and the manifold is already removed.
   Est. part cost: $150-$300 per injector

Rare But Worth Checking

• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the intake runner for cylinder 5 can lean out the air/fuel mixture and cause a misfire. This can be diagnosed with a smoke test. Since the manifold must be removed for other repairs, replacing these gaskets is a mandatory step.
• Low Fuel Pressure: A weak fuel pump or clogged fuel filter can cause a lean condition and misfires, but this would typically affect multiple cylinders and set a P0300 (random misfire) code as well.
• Faulty ECM: In very rare cases, the Engine Control Module itself or the wiring between the ECM and the ignition coil can be faulty. A ClubLexus forum post discusses checking the IGT/IGF signal wires between the coil and ECM as a diagnostic step.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner and confirm P0305 is present. Note any other codes and check freeze frame data to see when the misfire occurred (e.g., engine temp, RPM).
2. If the Check Engine Light is flashing, minimize driving to prevent catalytic converter damage.
3. Identify cylinder 5. On the transverse-mounted 2GR-FXE engine, the bank closer to the firewall is Bank 1. From passenger to driver side, the cylinders are 1, 3, 5. Cylinder 5 is on the rear bank, closest to the driver's side.
4. If symptoms are ONLY present on a cold start, prioritize a cold engine compression test before any disassembly. A significant drop in cylinder 5 points towards the mechanical issue described in TSB L-SB-0045-12 Rev.
5. For all other cases, proceed with disassembly. This involves removing the plastic engine covers, wiper arms, cowl tray, and finally the upper intake manifold/plenum. A detailed video guide for the RX350 (which is nearly identical) shows this process.
6. Once the intake manifold is off, you have access to the rear coils (1, 3, 5). At this point, swapping coils for diagnosis is inefficient. The best practice is to replace all three rear ignition coils and all six spark plugs.
7. While the manifold is off, inspect the intake gaskets for any signs of damage and replace them. The OEM part number for the intake plenum gasket is around 17176-31040.
8. Inspect the wiring and connectors for the ignition coils and fuel injectors for any damage, corrosion, or brittleness.
9. If replacing coils and plugs does not resolve the issue, the next steps involve professional diagnosis of the fuel injector, wiring harness, or potentially the ECM.

Parts You'll Likely Need

• Ignition Coil (OEM #90919-A2013 (supersedes 90919-A2007)) — This is the most frequent point of failure for a single-cylinder misfire. The part number has been superseded, indicating revisions. It's critical to replace all three on the rear bank due to labor costs.
  Trusted brands: Denso (OEM), NGK
  OEM price range: $90-$140
  Aftermarket price range: $50-$90
• Iridium Spark Plug (OEM #90919-01247 (Denso FK20HR11)) — Spark plugs are a routine maintenance item and a common cause of misfires. It is best practice to replace all six while the intake manifold is removed.
  Trusted brands: Denso, NGK
  OEM price range: $15-$25
  Aftermarket price range: $10-$18
• Intake Plenum Gasket (OEM #17176-31040) — This gasket must be replaced anytime the upper intake manifold is removed to access the rear bank of cylinders. It is non-reusable and critical for preventing vacuum leaks.
  Trusted brands: Lexus/Toyota (OEM), Fel-Pro
  OEM price range: $15-$30
  Aftermarket price range: $10-$20
• Intake Manifold to Head Gasket (OEM #17178-31030) — If the lower intake manifold is removed for injector or other service, these gaskets are also required. There are two.
  Trusted brands: Lexus/Toyota (OEM)
  OEM price range: $10-$20 each
  Aftermarket price range: $5-$15 each

Related Codes That Often Appear With This One

• P0300 — If the condition causing the misfire is not isolated to one cylinder (e.g., a vacuum leak or low fuel pressure), a P0300 'Random/Multiple Cylinder Misfire' code may appear alongside P0305.
• P0301, P0303 — The rear bank of cylinders on the 2GR-FXE engine (cylinders 1, 3, 5) are subjected to similar heat and access challenges. It's common to see misfires on multiple cylinders on this bank, often pointing to components that have aged similarly.

Technical Service Bulletins (TSBs) & Recalls

• L-SB-0045-12 Rev: Addresses MIL ON with DTCs P0300-P0306 and rough running on initial start-up after a cold soak for 2010-2017 RX 450h vehicles.

Platform-Specific Known Issues

• A known issue documented in TSB L-SB-0045-12 Rev describes rough running and misfire codes (P0300-P0306) on cold starts due to potential carbon buildup causing low compression. The fix can range from an ECM update to a full piston and ring replacement.
• Access to the rear bank of cylinders (1, 3, 5) is significantly obstructed by the intake manifold, wiper cowl, and other components, turning a simple coil/plug replacement into a multi-hour job.

Mechanic-Grade Diagnostic Values

• Spark Plug Electrode Gap (used plug) — expected: Up to 1.4 mm (0.055 in.). Failure: Gap exceeds 1.4 mm, indicating excessive wear.
• Spark Plug Electrode Gap (new plug) — expected: 1.0 to 1.1 mm (0.039 to 0.043 in.). Failure: Gap is outside this range.

Hidden / Shadow Codes Worth Checking

• Mode 6, Test ID $A2-$A7: Mode 6 data contains raw misfire counts for each cylinder (Cyl 1 to Cyl 6) during the current and last ten driving cycles. This data can show misfires that haven't yet reached the threshold to set a P030x code. (see via A professional scan tool or software like Toyota Techstream can access and display Mode 6 data.)

Scan Tool Commands That Help

• Techstream: Control the Select Cylinder Fuel Cut — This active test allows a technician to individually shut off fuel to any cylinder. When cylinder 5 is commanded off, if there is no change in engine RPM or smoothness, it confirms that cylinder 5 was not contributing, pointing to a problem with that specific cylinder.
• Techstream: Check the Cylinder Compression — This is a relative compression test. The tool cranks the engine and measures the RPM fluctuation as each cylinder compresses. A cylinder with low compression will show a higher RPM during its compression stroke because there is less resistance. This can quickly identify a mechanical issue without removing spark plugs for a manual test.
• Techstream: Misfire Count (Data List) — Under the Data List menu, a technician can monitor live misfire counts for each cylinder. This is useful for confirming the misfire is happening in real-time and seeing the conditions (RPM, load) under which it occurs.

Wiring & Ground Locations

• C55 (Connector B) — On the Engine Control Module (ECM).. Pin 36 on connector C55 is the IGT5 signal wire for the cylinder 5 ignition coil. A technician can back-probe this pin to check for the square wave signal from the ECM that commands the coil to fire. The wire color is typically Pink (P).
• Engine Ground Points — There are multiple ground points, but a key one for the ignition system is located on the left rear of the engine compartment.. A poor ground connection for the engine or ignition system can cause a weak spark, leading to intermittent or persistent misfires. Ensuring these grounds are clean and tight is a crucial diagnostic step.

Real Owner Repair Stories

• ClubLexus Forum (1998 Lexus GS 300 (similar P0305 diagnosis)) — Flashing check engine light, P0305 code.
  ❌ Tried (didn't work) Replaced all spark plugs with OEM Denso parts., Replaced all ignition coils and wires.
  ✅ What actually fixed it The spark plug in cylinder 5 was inspected and found to be perfectly clean, with no signs of combustion. This indicated a lack of fuel. The final cause was determined to be a bad fuel injector for cylinder 5.
• ClubLexus Forum (Lexus RX300 with 150k miles (similar rear bank access)) — Rough idle, blinking CEL, P0305 code.
  ✅ What actually fixed it The owner swapped the ignition coil from cylinder 5 (rear bank) to cylinder 4 (front bank). The code changed to P0304, confirming the ignition coil was faulty. Replacing the single bad coil resolved the issue.

OEM Part Supersession History

• 90919-02251, 90919-A2002, 90919-A2004, 90919-A2007 → 90919-A2013 — Standard part evolution for improved reliability or manufacturing changes.
  Heads up: The new part number (90919-A2013) is the correct replacement for the various older numbers used on the 2GR-FXE engine.