P0301 on 2016-2019 Lexus RX Hybrid: Cylinder 1 Misfire Causes and Fixes

What's Unique About the 2016-2019 Lexus RX HYBRID

The 2016-2019 RX Hybrid, with its 2GR-FXS engine, has a known history of misfire codes, as documented in multiple Lexus Technical Service Bulletins (TSBs). TSB L-SB-0044-19 specifically outlines a repair procedure for P0301-P0306 codes that involves replacing the faulty ignition coil and reprogramming the ECM with updated software to prevent future occurrences. 🎬 Watch: Step-by-step TSB repair for RX450h misfire codes. Due to the engine's transverse layout, accessing the rear three cylinders (1, 3, and 5) is extremely labor-intensive, requiring the removal of the wiper cowl and the entire upper intake manifold. This significantly increases the cost of repairs like spark plug replacement, making it a 4+ hour job for a professional.

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 or flashing
• Rough or shaky idle, especially on a cold start
• Engine hesitation or stumbling during acceleration
• Reduced engine power
• Decreased fuel economy
• A smell of unburned gasoline from the exhaust

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coils are a common failure point leading to misfires on many modern engines. Lexus issued TSB L-SB-0044-19 which explicitly identifies a malfunctioning ignition coil as the primary component to be replaced for this issue. Forum discussions on ClubLexus frequently point to coil failure as the root cause for P030x codes on this platform.
   How to confirm: Swap the ignition coil from cylinder 1 with another cylinder (e.g., cylinder 3, which is also on the difficult-to-access rear bank, or cylinder 2 on the accessible front bank). Clear the codes and drive the vehicle. If the code changes to P0303 or P0302, the ignition coil is faulty.
   Typical fix: Replace the faulty ignition coil. It is recommended to use OEM (Denso) or equivalent quality brands like NGK. Per TSB L-SB-0044-19, an ECM reprogram may also be required.
   Est. part cost: $75-$135 for one OEM Denso coil.
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a regular maintenance item with a finite lifespan. The recommended replacement interval is every 60,000 miles. An old or fouled plug can't produce a strong enough spark to ignite the air-fuel mixture consistently. Given the high labor cost to access the rear bank, it's crucial to address this during any related service.
   How to confirm: After confirming the ignition coil is good, remove the spark plug from cylinder 1. Inspect it for wear (worn electrode), carbon buildup, or oil/fuel fouling. Compare it to a new plug.
   Typical fix: Replace the spark plug. It is highly recommended to replace all six spark plugs at the same time with high-quality Iridium plugs from Denso or NGK to ensure consistent performance and avoid repeating the high-labor repair in the near future.
   Est. part cost: $90-$120 for a set of six OEM-quality plugs.
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Fuel injectors can become clogged with deposits over time, preventing the correct amount of fuel from reaching the cylinder. An older TSB for the previous generation RX 450h (L-SB-0045-12 Rev) specifically points to replacing fuel injectors to solve cold-start misfires, indicating it's a known, though less common, issue in the model line.
   How to confirm: This is more difficult to diagnose for a DIYer. A professional can perform a fuel injector balance test. A DIY method involves listening to the injector with a mechanic's stethoscope to ensure it's clicking, though this doesn't confirm flow rate.
   Typical fix: Clean or replace the faulty fuel injector. Sometimes running a high-quality fuel system cleaner can resolve minor clogs. If replacement is needed, it is often recommended to replace them as a set on a high-mileage vehicle.
   Est. part cost: $150-$250 for one injector.

Rare But Worth Checking

• Vacuum Leak: A leak in the intake manifold gasket near cylinder 1 can lean out the air-fuel mixture and cause a misfire. This can be checked with a smoke test. A user on Lexus Owners Club noted a loose vacuum hose connection caused a similar issue.
• Low Cylinder Compression: This is a serious mechanical issue, such as a worn piston ring, a problem with the valves, or a head gasket failure. It's rare on these reliable engines but would require a compression test to confirm and likely lead to major engine repair. Scuffing on cylinder walls has been noted on some 2GR engines.
• ECM Software: Lexus TSB L-SB-0044-19 explicitly states that an ECM software update is a required part of the repair procedure after replacing a faulty ignition coil to fully resolve the misfire condition. The dealer uses a Techstream tool to apply the new calibration file.

Diagnosis Steps

1. Read the trouble code with an OBD-II scanner to confirm P0301 and check for any other codes.
2. Note the freeze frame data to understand the engine conditions (speed, temperature, load) when the misfire occurred.
3. Identify cylinder 1. On the 2GR-FXS engine, the rear bank (closest to the firewall) contains cylinders 1, 3, and 5 from right to left (passenger to driver side). The front bank has 2, 4, and 6.
4. Since cylinder 1 is on the difficult rear bank, first test the theory by swapping an accessible front bank coil (e.g., cylinder 2) with another front bank coil (e.g., cylinder 4) to ensure your testing method is sound. If you are committed to the full job, proceed with accessing the rear bank.
5. To access the rear bank, you must remove the plastic wiper cowl and the wiper motor assembly, followed by the upper intake manifold. This is a lengthy process.
6. Once accessed, swap the ignition coil from cylinder 1 with the coil from cylinder 3. Reassemble enough to run the engine, clear the codes, and drive.
7. If the code returns as P0303, the ignition coil is bad and needs replacement.
8. If the code remains P0301, the coil is likely good. Proceed to inspect the spark plug in cylinder 1.
9. Replace the spark plug if it is worn, fouled, or damaged. It is best practice to replace all six plugs (both banks) at this time due to the high labor involved.
10. If the plug and coil are good, the issue may be the fuel injector. This step may require professional diagnosis.
11. If all else fails, check for vacuum leaks (smoke test) or perform an engine compression test.

Parts You'll Likely Need

• Ignition Coil (OEM #90919-A2013) — This is the most common cause of a single-cylinder misfire and is the primary replacement part listed in Lexus TSB L-SB-0044-19. This part number supersedes 90919-02280.
  Trusted brands: Denso, NGK
  OEM price range: $95-$135
  Aftermarket price range: $40-$80
• Iridium Spark Plug (OEM #90919-01263 (Denso FK20HBR8)) — Spark plugs are a maintenance item and the second most likely cause. Using OEM-spec Iridium plugs is critical for performance and longevity, especially given the high labor cost for replacement. An alternative OEM part number is 90919-01247 (Denso FK20HR11).
  Trusted brands: Denso, NGK
  OEM price range: $15-$20 per plug
  Aftermarket price range: $10-$15 per plug

Related Codes That Often Appear With This One

• P0300 — If misfires are detected on multiple random cylinders, P0300 (Random/Multiple Cylinder Misfire Detected) will be stored alongside the specific cylinder code.
• P0302, P0303, P0304, P0305, P0306 — If other cylinders are also misfiring, their respective codes will appear. The Lexus TSBs for this issue list all cylinder misfire codes as being related.

Technical Service Bulletins (TSBs) & Recalls

• LSB004419: MIL ON with misfire DTCs P0301-P0304 on 2018-2019 models.
• L-SB-0044-19: MIL ON with misfire DTCs P0301-P0306 on 2016-2019 models, pointing to potential ignition coil failure and a mandatory ECM update.
• L-SB-0045-12 Rev: Notes MIL ON with misfire codes and rough running on cold start-up for 2010-2017 models, suggesting fuel injectors as a possible cause.

Platform-Specific Known Issues

• Lexus has issued multiple Technical Service Bulletins (TSBs) related to misfire codes on these vehicles. Bulletin L-SB-0045-12 Rev mentions that some 2010-2017 RX 450h models may exhibit a MIL ON with P0301 and experience "Rough running on initial start-up after a cold soak".
• Bulletin L-SB-0044-19 (and its revision LSB004419) covers 2016-2019 models, outlining a procedure that requires replacing the faulty ignition coil AND reprogramming the ECM.

Mechanic-Grade Diagnostic Values

• Ignition Coil Connector — expected: 4-pin connector.. Failure: Damaged pins or connector housing.
• Ignition Coil Primary/Secondary Resistance — expected: Specific resistance values (in ohms) are not readily available in public-facing documents. This data typically requires access to the official Toyota Technical Information System (TIS).. Failure: A reading outside of the manufacturer's specified range.
• Fuel Pressure — expected: Specific pressure values (in PSI or kPa) are not readily available in public-facing documents and require access to the official service manual.. Failure: Pressure below the specified range.
• Engine Compression Ratio (for calculation) — expected: 13.0:1. Failure: This is a design specification, not a direct test value. A compression test (measuring PSI) is needed to diagnose mechanical issues.

Hidden / Shadow Codes Worth Checking

• Mode $06 Misfire Data: The ECU tracks misfire events for each cylinder even if they are not frequent enough to set a CEL or a standard pending code. This data is crucial for diagnosing intermittent misfires. It provides a 'misfire rate' for the current drive cycle and an 'EWMA' (Exponentially Weighted Moving Average) which represents the misfire history over the last 10 drive cycles. (see via Use the Toyota Techstream software and navigate to the 'Monitor' screen. Select 'Misfire' from the list and then click the 'Details' button. This is more reliable than watching the live data list, which may not accurately count misfires under load.)

Scan Tool Commands That Help

• Techstream: Utility -> Misfire Monitor — Use this function to view detailed, reliable misfire counts per cylinder when diagnosing an intermittent or no-code misfire. It helps pinpoint the offending cylinder when standard live data is inconclusive or shows zero misfires despite symptoms being present.

Wiring & Ground Locations

• Valve Cover Grounds — The 2GR engine family typically has ground wires running from the top of each valve cover to the vehicle's frame, often near the strut towers.. A poor engine ground can cause a host of electrical issues, including weak spark and erratic sensor readings, leading to misfires. Ensuring these main grounds are clean and tight is a critical step in diagnosing any electrical fault.
• Harness Grounds — The main engine wiring harness has ground points that attach directly to bosses on the side of the cylinder heads, often under the intake or throttle body area.. These grounds are essential for the proper function of the ignition coils and fuel injectors. A loose or corroded harness ground can directly cause a misfire on one or more cylinders.

Real Owner Repair Stories

• ClubLexus Forum User (2006 Lexus RX350 (2GR-FE engine, similar architecture)) — Persistent misfire on cylinder 1, low compression.
  ❌ Tried (didn't work) Confirmed PCV valve was good., Confirmed fuel injectors, ignition coils, and spark plugs were good.
  ✅ What actually fixed it The root cause was diagnosed as severe cylinder wall scuffing and carbon deposits on the pistons, found after a borescope inspection. This was on a rebuilt engine, indicating a serious internal mechanical failure was the cause, not a simple bolt-on part.
• Speedkar99 on YouTube (Lexus ES350 with 2GR-FE engine) — Difficulty starting, horrible engine noise, persistent misfire on multiple cylinders.
  ❌ Tried (didn't work) Initial diagnosis might point towards standard ignition or fuel issues.
  ✅ What actually fixed it The final diagnosis after teardown was that the rear bank's intake camshaft VVT gear actuator bolts had backed out. This caused a loss of timing, leading to piston-to-valve collision, bending all the intake valves on that bank. The misfire was a direct result of the valves not seating and a total loss of compression.

OEM Part Supersession History

• 90919-02280 → 90919-A2013 — Updated part design to improve reliability and address the misfire condition outlined in TSB L-SB-0044-19.
  Heads up: The new part number 90919-A2013 also supersedes older numbers 90919-02251, 90919-A2002, 90919-A2004, and 90919-A2007. Always use the latest part number for replacement.

Model Year Variations Within This Range

• 2018-2019: The TSB for this issue, L-SB-0044-19, also applies to the longer wheelbase RX 450hL model, which was introduced for the 2018 model year.
• 2016-2019: The TSB repair requires an ECM software update. The specific calibration ID for the new software varies by model year. For example, a 2016 model may be updated to calibration 348Q4400, while a 2019 may receive a different file. The technician must select the correct file using Techstream based on the vehicle's VIN.