P0301 on 2016-2019 Lexus RX450h: Cylinder 1 Misfire Causes and Fixes

What's Unique About the 2016-2019 Lexus RX450h

On the 2GR-FXS engine in the RX450h, cylinder 1 is located on the rear cylinder bank (cylinders 1, 3, 5), tucked against the firewall. Access is extremely labor-intensive, requiring removal of the wiper cowl and upper intake manifold. Lexus has issued a specific Technical Service Bulletin (L-SB-0044-19) for this issue, which often involves not only replacing the faulty ignition coil but also reprogramming the ECM with an updated calibration file to prevent future misfires. 🎬 Watch: A complete walkthrough of the TSB repair and ECM update. Manufacturer TSB Bulletin #LSB004419 further confirms that these vehicles may exhibit a MIL ON condition specifically for DTC P0301 through P0304.

Symptoms You May Notice

• Rough or vibrating idle, especially on cold starts
• Hesitation or stumbling during acceleration
• Reduced engine power
• Solid or flashing Check Engine Light
• Decreased fuel economy
• Smell of unburned gasoline from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Lexus TSB L-SB-0044-19 specifically identifies a malfunctioning ignition coil as a primary cause for misfires on this platform and recommends an ECM update alongside replacement. Forum discussions and mechanic videos confirm this is the most common failure point for a P030x code on this engine.
   How to confirm: After accessing the rear bank, swap the ignition coil from cylinder 1 with the coil from another cylinder (e.g., cylinder 3). If the trouble code changes to P0303, the coil is confirmed bad.
   Typical fix: Replace the faulty ignition coil with an OEM Denso or equivalent part. Per TSB L-SB-0044-19, the ECM must also be reprogrammed with updated software by a dealer or qualified shop to complete the repair. The TSB allocates 2.8 hours of labor for this job under warranty.
   Est. part cost: $95-$135
2. Worn or Fouled Spark Plug 🔴 High Probability Spark plugs are a maintenance item with a recommended replacement interval of 60,000 miles. If they are original or past their service life, they are a likely cause for a misfire, especially if the ignition coil tests good.
   How to confirm: After confirming the ignition coil is good, inspect the spark plug in cylinder 1. Look for a worn electrode, heavy carbon deposits, oil fouling, or cracks in the porcelain.
   Typical fix: Replace the spark plug. Due to the high labor cost to access the rear bank, it is highly recommended to replace all three rear spark plugs 🎬 See how to access the rear bank for spark plug replacement. (cylinders 1, 3, 5) and their corresponding intake manifold gaskets at the same time.
   Est. part cost: $15-$25 per plug
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability The 2GR-FXS engine uses a D-4S system with both port and direct injection. Carbon buildup or electronic failure can cause an injector to deliver an improper amount of fuel. A related TSB for earlier models (L-SB-0045-12 Rev) points to injector issues causing cold-soak misfires.
   How to confirm: This is more difficult to diagnose and is usually investigated after ignition issues are ruled out. It requires checking injector resistance (expected: 11.6 to 12.4 Ω at 68°F) or performing a flow test. A professional can also use a scan tool to monitor fuel trims per bank.
   Typical fix: Replace the faulty fuel injector. This is also a labor-intensive job that requires intake manifold removal.
   Est. part cost: $150-$250

Rare But Worth Checking

• Intake Manifold Gasket Leak: A vacuum leak isolated to the intake runner for cylinder 1 could lean out the mixture and cause a misfire. This can be diagnosed with a smoke test after the intake manifold is removed for other diagnostics.
• Low Engine Compression: This is a worst-case mechanical engine issue (e.g., bad valve, worn piston rings, or cylinder wall scuffing). It is diagnosed with a compression test and a leak-down test after all other causes are eliminated. While rare on these reliable engines, it's not impossible.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0301 and check for any other codes. Note the freeze frame data.
🎬 Watch: How to diagnose and fix a P0301 cylinder misfire.
2. Gain access to the rear cylinder bank by removing the wiper cowl assembly and the upper intake manifold. This is a significant job requiring several hours.
3. Swap the ignition coil from cylinder 1 with the coil from cylinder 3 (another rear cylinder).
4. Reassemble enough to safely run the engine. Clear the codes and perform a test drive, attempting to replicate the conditions from the freeze frame data.
5. Re-scan for codes. If P0303 appears, the ignition coil is faulty and must be replaced.
6. If P0301 returns, the coil is likely good. Proceed to inspect the spark plug in cylinder 1. Replace it if it is worn, fouled, or past its 60,000-mile service interval. It is highly recommended to replace all three rear plugs at this time.
7. If the coil and plug are good, the issue may be the fuel injector, wiring, or a mechanical problem requiring professional diagnosis, such as an injector resistance test or engine compression test.
8. After replacing the faulty component, ensure the ECM is reprogrammed per TSB L-SB-0044-19 by a qualified shop to prevent recurrence.

Parts You'll Likely Need

• Ignition Coil (OEM #90919-A2013) — This is the most common failure part for a P0301 code on this vehicle, as identified by Lexus in TSB L-SB-0044-19. The TSB specifies this as the updated part number, replacing 90919-02280.
  Trusted brands: Denso (OEM), NGK
  OEM price range: $95-$135
  Aftermarket price range: $60-$90
• Iridium Spark Plug (OEM #90919-01263) — This is a standard maintenance item and the second most likely cause of a misfire if the ignition coil is good. The specific Denso part number is FK20HBR8.
  Trusted brands: Denso (FK20HBR8), NGK
  OEM price range: $15-$25
  Aftermarket price range: $10-$18
• Upper Intake Manifold Gasket Set — These gaskets must be replaced anytime the upper intake manifold is removed to access the rear cylinder bank for spark plug or ignition coil service.
  Trusted brands: Fel-Pro, Mahle, OEM
  OEM price range: $30-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0300 (Random Misfire)
• P0303, P0305 (Misfires on other rear-bank cylinders)

Technical Service Bulletins (TSBs) & Recalls

• L-SB-0044-19 Rev1: Addresses MIL ON with misfire DTCs P0301-P0306. The procedure involves confirming the coil is faulty, replacing it with updated part number 90919-A2013, and reprogramming the ECM with a new calibration file.
• Bulletin #L-SB-0019-16: While issued for a different model, this bulletin notes that a "runs rough" condition and P0301 can occur during light acceleration while the Exhaust Gas Recirculation (EGR) system is active.

Platform-Specific Known Issues

• Access to the rear cylinder bank (1, 3, 5) is extremely difficult, significantly increasing labor costs for what would otherwise be a simple repair.
• A specific TSB (L-SB-0044-19) exists for this issue, indicating it's a known problem that requires both a part replacement and a software update for a complete fix.
• The repair is covered under the Lexus Federal Emission Warranty for 96 months or 80,000 miles, whichever comes first.

Mechanic-Grade Diagnostic Values

• Port Fuel Injector Resistance — expected: 11.6 to 12.4 Ω at 20°C (68°F). Failure: Resistance outside of the specified range indicates a faulty injector coil.
• Ignition Coil Primary Resistance — expected: 0.5Ω ± 0.1Ω. Failure: A reading outside this range suggests an internal fault in the coil's primary winding.
• Ignition Coil Secondary Resistance — expected: ~12 kΩ. Failure: A significantly different reading indicates a fault in the coil's secondary winding.
• Spark Plug Gap (for new DENSO FK20HBR8) — expected: 0.7 – 0.8 mm (0.028 – 0.031 in.). Failure: An incorrect gap can lead to a weak spark and misfires. The gap on used plugs should not exceed 1.4mm.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $A2-$A7: Provides misfire counts for each cylinder (A2 for Cyl 1, A3 for Cyl 2, etc.) for the last/current driving cycle. A technician can see misfire events happening in real-time, even if they are not yet severe enough to set a P0301 code and turn on the check engine light. A healthy engine should have zero or very few counts. (see via An advanced OBD-II scan tool or a mobile app like OBD Fusion that can access Mode $06 test results.)
• Techstream Misfire Monitor (EWMA): Within the Toyota/Lexus Techstream software, the dedicated 'Misfire Monitor' provides more accurate data than the standard live data list, which can stop counting misfires under load. The EWMA (Exponentially Weighted Moving Average) value shows a history of misfires over the last 10 trips, helping to identify intermittent issues. (see via Requires Toyota's Techstream diagnostic software and a compatible vehicle interface.)

Scan Tool Commands That Help

• Techstream: Misfire Monitor — Use this instead of the standard Data List to get a more accurate count of misfires, especially under load. It provides current misfire rates and historical data (EWMA) to diagnose intermittent problems.
• Techstream: ECU Flash Reprogramming — This function is required after replacing a faulty ignition coil, as per TSB L-SB-0044-19. It updates the ECM with a new calibration file to prevent the misfire from recurring.
• Techstream: Active Test — Allows the technician to individually test components. For example, an active test could be used to check the function of a fuel injector or VSV without removing it from the vehicle.

Wiring & Ground Locations

• Ignition Coil Connector (Cylinder 1) — On top of the ignition coil for cylinder 1, located on the rear cylinder bank, driver's side.. Testing at this 4-pin connector can confirm if the coil is receiving the correct signals. Pin 1: +12V power from IG2 relay. Pin 2: IGT1 (Ignition Trigger) signal from the ECM. Pin 3: IGF1 (Ignition Feedback) signal to the ECM. Pin 4: Ground. A failure in wiring or a bad ground can mimic a failed coil.
• Engine Ground Points — Multiple points exist. Key grounds are typically located on the cylinder heads and at various points on the engine block, secured to the chassis. One diagram shows a ground point at the 'left rear of engine compt'.. A poor engine ground can cause a weak spark, erratic sensor readings, and phantom misfires. Always check that main ground straps are clean, tight, and free of corrosion when diagnosing ignition issues.

OEM Part Supersession History

• 90919-02280 → 90919-A2013 — Updated part designed to address the malfunction causing misfires, as specified in TSB L-SB-0044-19.
  Heads up: While the old part number will physically fit, it is prone to the original failure. Using the updated 90919-A2013 part is critical for a permanent repair. 90919-A2013 also replaces other prior part numbers like 90919-A2007 and 90919-02251.

Model Year Variations Within This Range

• 2016-2019: There are no significant variations within this year range regarding the 2GR-FXS engine or the causes of a P0301 code. The primary differences between model years were related to infotainment (4G LTE added in 2019) and safety features, not the powertrain. The TSB L-SB-0044-19 applies to all years from 2016-2019.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• 12V Battery Drains Prematurely 🟠 Medium — Commonly reported if the vehicle is left sitting for a few weeks. The hybrid system prevents traditional jump-starting; a trickle charger is required.
• Sticking Fuel Filler Door 🟡 Low — The electronic lock actuator or the mechanical pop-open spring can fail, causing the door to be stuck closed or not spring open when pushed. An emergency manual release is located in the cargo area.
• CAN Bus Security Vulnerability 🔴 High — Like many modern Toyota/Lexus vehicles, thieves can gain access to the vehicle's CAN bus system through the headlight wiring to start the car and steal it. This is a widely known issue in the owner community.
• Timing Cover Oil Leak 🟠 Medium — While more prevalent on the earlier 2GR-FE engines (pre-2010), the 2GR-FKS (non-hybrid version of this engine) can still experience oil leaks from the timing chain cover. This is a very expensive repair as it often requires dropping the engine. Reports suggest the issue was largely resolved in production by the 2018 model year. (Ref: L-SB-0027-18 (for a related leak on RX350))

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this specific repair, using a used part is strongly discouraged. The primary cause is a faulty ignition coil with a known defect addressed by an updated OEM part. A used coil from a salvage yard is highly likely to be the older, failure-prone part number, leading to a repeat failure.

What to inspect on the donor part:

• Not applicable, as used ignition coils are not recommended for this repair.

OEM-only on this vehicle (don't cheap out):

• Ignition Coil: It is critical to use the updated OEM part number (90919-A2013) or a direct replacement from the original manufacturer, Denso, to ensure the fix is permanent.
• Spark Plugs: Due to the high labor cost of accessing the rear bank, use only high-quality Iridium plugs from OEM suppliers like Denso or NGK to maximize service life.

Aftermarket brands forum-validated for this vehicle:

• Denso (OEM for ignition coil and spark plugs)
• NGK (OEM equivalent for ignition coils and spark plugs)

Brands owners have reported issues with on this vehicle:

• Unbranded, low-cost ignition coils from online marketplaces. While no specific brand was cited in research for this vehicle, these parts are notorious for high failure rates and are a poor choice given the extreme labor required for replacement on the rear bank.

Documented NHTSA Reports

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0303 — A companion misfire code for cylinder 3; often used during diagnosis to confirm a faulty coil by swapping the cylinder 1 coil to cylinder 3.
• P0305 — A misfire code for cylinder 5, which is also located on the difficult-to-access rear bank alongside cylinder 1.
• P0171 — Bank 1 Lean code; reported by owners to appear alongside P0301 when fuel delivery issues or vacuum leaks are present.
• P117B — Fuel performance/direct injector code; can appear with P0301 on the 2GR-FXS D-4S system when an injector is malfunctioning.
• P0300 — Random or multiple cylinder misfire code that may trigger if the ignition coil failure or ECM software issues affect more than just cylinder 1.

Frequently Asked Questions