P0304 on 2016-2019 Lexus RX HYBRID: Cylinder 4 Misfire Causes and Fixes

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

The 2016-2019 RX Hybrid uses the reliable 2GR-FXS engine, a 3.5L V6 featuring both direct and port fuel injection (D-4S) and operating on an Atkinson cycle for efficiency. While generally robust, this platform has a known issue documented in Lexus Technical Service Bulletin L-SB-0044-19. The TSB indicates that a misfire code, including P0304, is frequently caused by a faulty ignition coil, which may also lead to a rough running condition on initial start-up after a cold soak. The official fix involves replacing the faulty coil with an updated part and reprogramming the Engine Control Module (ECM) to adjust its logic.

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
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Rough running on initial start-up, especially after a cold soak
• Smell of unburnt fuel from the exhaust
• Decrease in fuel economy

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Lexus has issued Technical Service Bulletin L-SB-0044-19 specifically for this issue on 2016-2019 RX450h models. It indicates a known problem with ignition coils that can be resolved with a replacement part (New P/N 90919-A2013 replaces Old P/N 90919-02280) and an ECM reprogram. The repair is covered under the Lexus Federal Emission Warranty for 96 months or 80,000 miles.
   How to confirm: Swap the ignition coil from cylinder 4 with another easily accessible cylinder (e.g., cylinder 2 or 6 on the front bank). Clear the codes and drive the vehicle. If the code changes to P0302 or P0306, the ignition coil is confirmed to be faulty.
   Typical fix: Replace the faulty ignition coil. Per the TSB, the ECM should also be reprogrammed by a dealer or qualified shop with Toyota's Techstream software. It is often recommended to replace all six spark plugs at the same time if they are near their service interval.
   Est. part cost: $50-$120
2. Worn or Fouled Spark Plug 🟡 Medium Probability Spark plugs are a regular maintenance item with a recommended replacement interval of 60,000 miles. High-mileage vehicles that are overdue for a spark plug change are prone to misfires. Using non-OEM (Denso or NGK) plugs can also lead to issues.
   How to confirm: After confirming the ignition coil is good, remove the spark plug from cylinder 4. Inspect it for wear (worn electrode), carbon fouling, or oil deposits. The OEM plugs are Iridium type; ensure the correct plug is installed. Compare it to a new plug.
   Typical fix: Replace the spark plug in cylinder 4. It is best practice to replace all six spark plugs as a set, especially given the labor required to access the rear bank.
   Est. part cost: $15-$30 per plug
3. Clogged or Faulty Fuel Injector ⚪ Low Probability The 2GR-FXS engine has both port and direct injectors (D-4S). While reliable, injectors can become clogged or fail over time, leading to a poor fuel spray pattern and a lean misfire. This is less common than ignition system faults.
   How to confirm: This is more difficult to diagnose. A professional can perform a fuel injector balance test or use a scan tool active test to cut fuel to individual cylinders. Swapping injectors between cylinders can also help isolate the issue, but this is significantly more labor-intensive than swapping coils.
   Typical fix: Replace the faulty fuel injector. The system may need to be de-pressurized before service.
   Est. part cost: $150-$300

Rare But Worth Checking

• Vacuum Leak: A leak in a vacuum hose or intake manifold gasket can introduce unmetered air, throwing off the air-fuel ratio and causing a misfire. This would typically affect multiple cylinders and may set a P0171/P0174 lean code, but can sometimes manifest as a single cylinder misfire.
• Low Engine Compression: Internal engine issues like a burnt valve, worn piston rings, or a leaking head gasket can cause a loss of compression in cylinder 4. This is a serious mechanical issue and is the least likely cause on a well-maintained engine. A compression test can confirm this.
• Carbon Buildup: As a direct-injected engine, the 2GR-FXS can be susceptible to carbon buildup on the back of the intake valves over time. The port injectors help wash the valves, but heavy buildup could still potentially cause airflow issues to a cylinder, leading to a misfire. This is more likely on very high mileage engines or those used frequently for short trips.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0304 is present and check for any other codes.
2. Locate cylinder 4. On the transverse 2GR-FXS engine, the cylinders against the firewall (rear bank, Bank 1) are 1, 3, 5 (from passenger to driver side). The cylinders at the front of the vehicle (front bank, Bank 2) are 2, 4, 6. Cylinder 4 is the middle cylinder on the front bank, which is easily accessible.
3. Swap the ignition coil from cylinder 4 with the coil from cylinder 2 (the front-most cylinder on the passenger side). This requires a 10mm socket.
4. Clear the fault codes using the scanner.
5. Start the engine and let it run for a few minutes. Perform a short test drive. Re-scan for codes.
6. If the code has moved to P0302 (Cylinder 2 Misfire), the ignition coil is the culprit. Replace the bad coil with the updated part number.
7. If the code remains P0304, the coil is likely good. Proceed to inspect the spark plug.
8. Remove the spark plug from cylinder 4 (requires a 14mm thin-wall magnetic spark plug socket). Check for signs of wear, fouling, or damage. If it looks worn or is past its 60,000-mile service life, replace it. It's best to replace all six.
9. If the coil and plug are good, the issue may be with the fuel injector, wiring, or, more rarely, engine compression. These steps typically require professional diagnosis, which may include a compression test or injector balance test.

Parts You'll Likely Need

• Ignition Coil (OEM #90919-A2013) — This is the most frequent cause of P0304 on this vehicle, as identified by Lexus TSB L-SB-0044-19.
  Trusted brands: Denso (OEM), NGK
  OEM price range: $90-$140
  Aftermarket price range: $50-$90
• Iridium Spark Plug (OEM #90919-01263 (Denso FK20HBR8)) — Spark plugs are a primary wear item in the ignition system and a common cause of misfires if they are old or fouled. The service interval is 60,000 miles.
  Trusted brands: Denso (OEM), NGK
  OEM price range: $20-$30
  Aftermarket price range: $15-$25

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire Detected) may be stored alongside P0304.
• P0301, P0302, P0303, P0305, P0306 — If other cylinders are also misfiring, their respective codes will be present. The Lexus TSB L-SB-0044-19 for this vehicle covers all six cylinder misfire codes (P0301-P0306).
• P0354 — This code indicates a malfunction in the ignition coil 'D' primary or secondary circuit (cylinder 4). It can appear alongside P0304 and more definitively points towards an electrical issue with the coil or its wiring harness.

Technical Service Bulletins (TSBs) & Recalls

• LSB004419 / L-SB-0044-19: Addresses MIL ON with misfire DTCs P0301-P0306, recommending ignition coil replacement (updated P/N 90919-A2013) and ECM reprogramming. The repair is covered under the 96-month/80,000-mile Federal Emission Warranty.
• L-SB-0045-12 Rev: Mentions MIL ON with misfire codes and rough running on initial start-up after a cold soak for 2016-2017 models.

Platform-Specific Known Issues

• Lexus issued TSB L-SB-0044-19 for 2016-2019 RX450h models that exhibit a MIL ON with misfire DTCs. The bulletin outlines a repair procedure that involves replacing the faulty ignition coil with an updated part (P/N 90919-A2013) and reprogramming the Engine Control Module (ECM).
• TSB L-SB-0045-12 Rev also notes that 2016-2017 models may experience rough running on initial start-up after a cold soak in conjunction with misfire codes.
• The labor to replace the rear bank of spark plugs (cylinders 1, 3, 5) is high, as it requires removal of the upper intake manifold, cowl, and wiper assembly. This makes it wise to replace all plugs if accessing the rear bank for any reason.

Mechanic-Grade Diagnostic Values

• Low-Pressure Fuel System Pressure — expected: 28 to 121 psi (196 to 833 kPa). Failure: Pressure below this range could indicate a weak low-pressure fuel pump, which was subject to a recall on some models.
• High-Pressure Fuel System Pressure (at 3000 rpm, warmed up) — expected: 348 to 2828 psi (2400 to 19500 kPa). Failure: Pressure outside this range may indicate an issue with the high-pressure mechanical fuel pump or its control circuit.
• Injector Pulse Width (at idle) — expected: 1.2 to 2.4 ms. Failure: Values significantly outside this range may suggest a vacuum leak (higher ms) or other fueling issues. This value changes dynamically with engine load.
• Ignition Coil Primary Resistance (General Reference) — expected: Approximately 0.49 - 0.57 Ω. Failure: A reading outside this range (e.g., an open circuit) indicates a faulty coil. Note: This is a general value and may not be exact for the OEM Denso coil.

Hidden / Shadow Codes Worth Checking

• Mode $06 Misfire Counters (e.g., TID $A5, CID $04): Advanced scan tools can access Mode $06 data, which shows raw misfire counts for each cylinder from the current and previous drive cycles. This allows a technician to see small numbers of misfires that haven't yet reached the threshold to set a P0304 code. Comparing the count for cylinder 4 to other cylinders can confirm an intermittent issue before it becomes a hard fault. (see via A professional scan tool with Mode $06 capabilities, such as Toyota's Techstream software.)

Scan Tool Commands That Help

• Techstream: Control the Select Cylinder Fuel Cut — After verifying spark, this active test allows you to command the ECM to stop fuel delivery to a specific cylinder (e.g., cylinder 4). If the engine's idle roughness does NOT change when cylinder 4 is cut, it confirms that cylinder was not contributing, pointing strongly to a fuel injector or compression issue. If the idle gets worse, the injector was likely working.
• Techstream: ECM Reprogramming (Calibration Update) — This is a required step of the TSB L-SB-0044-19 repair after replacing the faulty ignition coil. It updates the ECM's logic for misfire detection and ignition control. This requires a specific calibration file for the vehicle's model year and maintaining battery voltage at 13.5V or higher during the process.
• Techstream: Control the Injection Mode — For diagnosing complex issues on the D-4S system, this test allows a technician to manually switch between port and direct injection. This can help isolate whether a fueling problem is related to one side of the system. A specific procedure of raising engine RPM between tests is required to prevent setting false rich codes (P0172/P0175).

Wiring & Ground Locations

• Certification ECU Ground — Behind the glove box, at the top white 27-pin plug for the Certification ECU, pin 18.. While not directly for the engine ECU, this identified ground point (White/Black wire) is an example of a critical ground in the dashboard area. Poor grounds in this vicinity can cause a variety of difficult-to-diagnose electrical issues, including problems with control modules. Checking the integrity of known grounds is a valid step when chasing intermittent electronic faults.

OEM Part Supersession History

• 90919-02280 → 90919-A2013 — The original part number is associated with the failures leading to TSB L-SB-0044-19. The new part is an updated design to resolve the issue.
  Heads up: 90919-A2013 also supersedes other previous numbers like 90919-A2004 and 90919-A2007. Always use the latest part number for replacement.

Model Year Variations Within This Range

• 2016-2019: While the mechanical components are largely the same, the ECM calibration files are specific to the model year and whether the vehicle is a standard (RX450h) or long-wheelbase (RX450hL) model. When performing the TSB repair, the correct updated software must be flashed to the ECM; a file for a 2016 model cannot be used on a 2019 model.