P0300 on 2016-2017 Lexus RX Hybrid: Causes and Fixes for Random Misfires

What's Unique About the 2016-2017 Lexus RX HYBRID

For the 2016-2017 RX 450h, a P0300 code is specifically addressed by Lexus Technical Service Bulletin (TSB) L-SB-0045-12 Rev. This TSB targets an issue where the vehicle runs rough and sets misfire codes (P0300-P0306) particularly on the first start-up after a cold soak. Unlike a simple tune-up, the official diagnostic path for this specific condition involves checking for an ECM software update and potentially replacing fuel injectors, making it a more complex issue than on other vehicles. The problem is prevalent enough that owners on forums like ClubLexus frequently discuss the cold start misfire and the TSB as the primary path to a solution.

Diagnostic Flowchart

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

Symptoms You May Notice

• Rough idle or engine shaking, especially on a cold start.
• Engine knocking sound for the first 30-60 seconds after a cold start.
• Hesitation or stumbling during acceleration.
• Flashing or solid Check Engine Light.
• Reduced engine power and a 'gutless' feeling.
• Noticeable decrease in fuel economy.
• A smell of raw fuel from the exhaust.
• Engine may stall at low speeds or when put into neutral while cold.

Most Likely Causes

1. Worn Iridium Spark Plugs 🔴 High Probability The rear three spark plugs are extremely difficult to access, requiring the removal of the intake manifold and plenum. 🎬 Watch this detailed step-by-step guide for replacing V6 spark plugs. This high labor cost (4+ hours) leads many owners to delay this critical maintenance item, which is due every 60,000-120,000 miles. Old plugs are a primary cause of misfires.
   How to confirm: Inspect plugs for wear, fouling, or incorrect gap (standard is 1.0-1.1 mm). Given the high labor cost, if the vehicle is near its service interval, it's often best to replace them preventatively. A ClubLexus user reported resolving a P0300 with a plug and coil change.
   Typical fix: Replace all six spark plugs with OEM-spec (Denso FK20HR11) iridium plugs. The intake plenum gaskets must also be replaced during this job.
   Est. part cost: $70-$150
2. Faulty Ignition Coil(s) 🔴 High Probability Coil-on-plug systems are a common failure point. While one failing coil usually sets a specific cylinder code (e.g., P0301), multiple weak or intermittently failing coils can contribute to a P0300. It is highly recommended to replace the rear three coils when doing the spark plugs due to the labor involved.
   How to confirm: Use a scanner with live data (like Techstream) to check misfire counts per cylinder. Swap a suspected bad coil with a known good one and see if the misfire code follows the coil.
   Typical fix: Replace the faulty ignition coil(s). It is often recommended to replace all six if they are original and high-mileage, especially given the labor to access the rear bank.
   Est. part cost: $50-$130 per coil
3. ECM Software Update Needed 🟡 Medium Probability Lexus issued TSB L-SB-0045-12 Rev specifically for cold start misfires on 2010-2017 RX 450h models. Another TSB, L-SB-0044-19, also calls for an ECM update 🎬 Watch this walkthrough of the TSB repair procedure for faulty coils. when replacing a faulty coil on 2016-2019 models.
   How to confirm: A Lexus dealer or a qualified shop with a Techstream scanner can check the current calibration ID against the latest version available in the TSB. For a 2016 RX 450h, the TSB indicates new calibrations are 348Q4100, 348Q4200, or 348Q4300, superseding 348Q4000.
   Typical fix: Flash the ECM with the new software calibration. The ECM itself should not be replaced for this issue.
   Est. part cost: $0 (software only, but dealer labor applies)
4. Clogged or Faulty Fuel Injectors 🟡 Medium Probability The same TSB (L-SB-0045-12 Rev) that addresses cold start misfires also points to fuel injector replacement as the final step if the ECM update does not resolve the issue, suggesting they are a known source of the problem on this platform. A ClubLexus member noted their dealer replaced an injector and high-pressure fuel pump to resolve a rough idle.
   How to confirm: Diagnosis requires testing fuel pressure and injector flow rates. Resistance should be between 11.6 to 12.4 Ω at 68°F (20°C). A technician can also perform an injector balance test using a tool like Techstream.
   Typical fix: Clean or replace the faulty fuel injector(s). Some owners on forums have had success after the dealer replaced injectors per the TSB.
   Est. part cost: $150-$300 per injector
5. Dirty Mass Air Flow (MAF) Sensor ⚪ Low Probability
   How to confirm: A dirty MAF sensor can provide incorrect airflow readings to the ECM, upsetting the air-fuel ratio. A forum member on ClubLexus reported fixing a cold-start P0300 on their Lexus by cleaning the sensor with dedicated MAF sensor cleaner.
   Typical fix: Carefully remove the MAF sensor and clean it with a dedicated MAF sensor cleaner spray. Do not touch the sensor wires. Let it air dry completely before reinstalling.
   Est. part cost: $10-$15 for cleaner

Rare But Worth Checking

• Vacuum Leak: A leak in a vacuum hose or intake manifold gasket can introduce unmetered air, leaning out the air-fuel mixture and causing random misfires. A smoke test is the most effective way to find a leak.
• Low Fuel Pressure: A weak fuel pump or clogged fuel filter can starve the engine of fuel, leading to misfires across all cylinders, especially under load. One owner reported a high-pressure fuel pump failure causing severe misfires.
• Low Engine Compression: This is a serious mechanical issue (e.g., bad valve, worn piston rings, head gasket failure). Carbon buildup on intake valves is a known issue on direct-injected engines that can cause a temporary loss of compression. A compression test should be performed if all ignition and fuel-related causes have been ruled out.
• Clogged PCV Valve or Hose: A clogged PCV (Positive Crankcase Ventilation) system can alter crankcase pressure and affect the air-fuel mixture, potentially leading to misfires. It's an inexpensive part to check and replace, located on the valve cover.

Diagnosis Steps

1. Read all stored trouble codes with an OBD-II scanner. 🎬 See how to diagnose and fix a random P0300 engine misfire. Note any codes other than P0300, as they can provide critical clues.
2. Check the freeze frame data to see the engine conditions (RPM, temperature, load) when the code was set. This is crucial to determine if it's a cold-start issue related to TSB L-SB-0045-12 Rev.
3. If the misfire happens primarily on cold starts, this is a strong indicator of the TSB-related issue. Contact a Lexus dealer or qualified independent shop to inquire about TSB L-SB-0045-12 Rev and check if an ECM calibration update is available.
4. As a low-cost first step, try cleaning the Mass Air Flow (MAF) sensor with dedicated cleaner.
5. Inspect the ignition system. If the vehicle has over 60,000 miles, plan to replace all six spark plugs and the intake gaskets. While the intake is off, inspect the ignition coils (especially the rear three) for any signs of damage or corrosion and consider replacing them as a preventative measure.
6. Use a scanner with live data to monitor misfire counts per cylinder (Mode $06 data). This can help identify if the misfire is truly random or concentrated on specific cylinders.
7. Check for vacuum leaks. A professional smoke test is the most reliable method. Pay attention to the PCV hose and intake manifold area.
8. If other steps don't resolve the issue, have a professional test fuel pressure and injector function, as faulty injectors are the final step in the TSB repair procedure.
9. As a final step for persistent issues, a mechanical compression test can rule out internal engine problems like valve or ring issues.

Parts You'll Likely Need

• Spark Plugs (OEM #90919-01247 (Denso FK20HR11)) — These are a primary cause of misfires and a regular maintenance item. Accessing the rear bank is very labor-intensive, so it's critical to use high-quality OEM parts to avoid repeat repairs.
  Trusted brands: Denso (OEM), NGK
  OEM price range: $15-$25 per plug
  Aftermarket price range: $10-$20 per plug
• Ignition Coil (OEM #90919-A2008) — Ignition coils are a very common failure point leading to misfires. Due to the labor to access the rear bank, it's wise to replace the rear coils during a spark plug job.
  Trusted brands: Denso (OEM), NGK, Delphi
  OEM price range: $100-$140
  Aftermarket price range: $40-$90
• Intake Manifold Gasket Set (OEM #17177-0P030 (replaces 17177-31070)) — These gaskets are single-use and must be replaced anytime the intake manifold is removed, which is required for spark plug or ignition coil replacement on the rear cylinder bank.
  Trusted brands: Lexus (OEM), Fel-Pro, Victor Reinz
  OEM price range: $30-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0301, P0302, P0303, P0304, P0305, P0306 — These codes indicate a misfire on a specific cylinder. They often accompany P0300 when the misfire is happening on multiple, but identifiable, cylinders. The TSB for this vehicle lists all these codes alongside P0300.
• P0420, P0430 — These codes indicate 'Catalyst System Efficiency Below Threshold.' They can appear if the P0300 misfire condition is ignored for too long, allowing unburnt fuel to damage the catalytic converters.

Technical Service Bulletins (TSBs) & Recalls

• L-SB-0045-12 Rev: 'MIL ON DTC P0300, P0301, P0302, P0303, P0304, P0305, and/or P0306 Present With a Rough Run on Initial Startup After a Cold Soak'. This is the primary TSB for this issue, covering 2010-2017 RX 450h models. The procedure involves checking for an ECM update and, if necessary, replacing fuel injectors.

Platform-Specific Known Issues

• A known issue documented in Lexus TSB L-SB-0045-12 Rev can cause a rough run condition and set codes P0300-P0306 specifically after a cold soak (sitting overnight). The official repair procedure may involve an ECM software update or, in some cases, replacement of the fuel injectors.

Mechanic-Grade Diagnostic Values

• Low-Side Fuel Pressure — expected: 44 to 50 psi (304 to 343 kPa). Failure: Pressure below this range suggests a weak fuel pump or clogged filter.
• Fuel Injector Resistance — expected: 11.6 to 12.4 Ω at 20°C (68°F). Failure: Resistance outside this range indicates a faulty injector coil.
• Long Term Fuel Trim (LTFT) at Idle — expected: -10% to +10%. Failure: Sustained positive values above +10% suggest the ECM is compensating for unmetered air (vacuum leak).
• Spark Plug Electrode Gap — expected: 1.0 mm to 1.1 mm (0.039 in. to 0.043 in.). Failure: Gap larger than specified indicates wear; plugs should be replaced, not adjusted.

Hidden / Shadow Codes Worth Checking

• Mode $06 (TID $A2-$AD, CID $01-$0C): Provides raw misfire count data for each individual cylinder. The ECM tracks these counts even if they aren't high enough to set a cylinder-specific code (like P0301). This allows a technician to see which cylinders are contributing to the P0300 count. (see via A professional OBD-II scanner capable of displaying Mode $06 test results.)

Scan Tool Commands That Help

• Techstream: Misfire Count (Cylinder #1-#6) — Used to monitor live misfires per cylinder at idle or during a test drive to see if the misfire is truly random or biased toward certain cylinders.
• Techstream: Active Test — Allows the technician to command specific components to operate, such as individual fuel injectors or solenoids, to test their function without the engine running. This helps isolate a faulty component.
• Techstream: ECU Flash Reprogramming (Calibration Update) — This is a required step per TSB L-SB-0044-19 and L-SB-0045-12 Rev. It updates the ECM's logic to prevent future misfires after a repair, such as replacing an ignition coil or injectors.
• Techstream: Put Engine in Inspection Mode — This function forces the gasoline engine to run continuously on hybrid models, which is necessary for performing many diagnostic tests that would otherwise be interrupted by the engine shutting off at idle.

Real Owner Repair Stories

• ClubLexus Forum User (Lexus RX Hybrid (year not specified but in 4th gen forum)) — Check Engine Light on, engine knocking sound for ~30 seconds on cold start, significant decrease in fuel economy from ~28 MPG down to ~22 MPG over time.
  ❌ Tried (didn't work) Initially accepting the cold start knock as 'normal' based on forum discussions.
  ✅ What actually fixed it A diagnostic check revealed code P0300. The owner's mechanic proceeded to replace all spark plugs and all OEM Lexus ignition coils to resolve the issue. The owner explicitly states to other forum members that the cold start knock is NOT normal and is an indicator of this problem.

OEM Part Supersession History

• Unknown → 90919-A2005 — Part update/revision.
• N/A → N/A — TSB L-SB-0044-19, which addresses misfires on 2016-2019 models, lists two different part numbers for the ignition coil assembly: 90919-02280 and 90919-A2013. This indicates that either part may be used depending on the specific vehicle production or as an updated part.
  Heads up: A dealer or parts supplier should confirm the correct part using the vehicle's VIN.