P0505 on 2018-2022 Lexus LC 500: Idle Control System Causes and Fixes

What's Unique About the 2018-2022 Lexus LC

The Lexus LC 500 uses a sophisticated 'drive-by-wire' electronic throttle body to manage idle speed. Unlike older systems with a separate IAC valve, the entire function is handled by minute adjustments of the main throttle plate. This design is precise but sensitive to carbon buildup or vacuum leaks, which can prevent the throttle plate from regulating the small amount of air required for a stable idle. A Technical Service Bulletin (L-SB-0032-25) for the 2UR-GSE engine family points towards issues with the throttle body assembly and related ventilation hoses, indicating a known sensitivity in this system. The bulletin states that modified parts were introduced to address the condition.

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
• Rough, fluctuating, or unstable idle (e.g., dipping to 300-400 RPM).
• Engine stalls when coming to a stop or at idle.
• Idle speed is consistently too high (e.g., 1200 RPM).
• Hesitation upon acceleration from a stop.
• VSC and/or Traction Control lights may also illuminate.
🎬 Watch: A quick breakdown of P0505 causes, symptoms, and fixes.

Most Likely Causes

1. Dirty Throttle Body 🔴 High Probability → Shop Fuel Injection Throttle Body Carbon and oil vapor from the PCV system can accumulate around the throttle plate and bore, restricting the precise, small movements needed for idle control in the ETCS. This is the most common cause reported by owners.
   How to confirm: Remove the air intake tube from the throttle body and visually inspect the throttle bore and plate for a black, sticky film of carbon. The buildup is often heaviest on the back side of the throttle plate.
   Typical fix: Clean the throttle body bore and plate thoroughly using a dedicated throttle body cleaner and a soft, lint-free cloth. 🎬 Watch: How to properly clean a Toyota or Lexus throttle body. An idle relearn procedure is mandatory afterward.
   Est. part cost: $10-$20
2. Vacuum Leak 🟡 Medium Probability TSB L-SB-0032-25 specifically mentions checking ventilation hoses (No. 2 and No. 3). Aged or cracked rubber hoses, particularly in the PCV system or the main intake tube, can introduce unmetered air, which confuses the idle control system and can cause P0505.
   How to confirm: Visually inspect all vacuum and PCV hoses for cracks or loose connections. A smoke test is the most definitive method to locate hard-to-see leaks in the intake system.
   Typical fix: Replace the cracked or leaking hose(s). Ensure all connections are secure.
   Est. part cost: $20-$100
3. Failed Throttle Body Assembly ⚪ Low Probability → Shop Fuel Injection Throttle Body While less common than being dirty, the internal motor or position sensor within the electronic throttle body can fail. TSB L-SB-0032-25 indicates that for a persistent issue, the remedy is to replace the throttle body assembly with an updated part.
   How to confirm: After confirming no vacuum leaks and that the throttle body is clean, if the problem persists, a diagnostic scan tool can be used to monitor throttle angle commands vs. actual position. If they don't match, or if the Closed Throttle Position (CTP) switch doesn't read 'ON' at idle, the unit is likely faulty.
   Typical fix: Replace the entire throttle body assembly and perform an idle relearn.
   Est. part cost: $650-$930

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is extremely rare. The ECM should only be considered after all other possibilities, including wiring, have been exhaustively ruled out by a professional.
• Wiring or Connector Issue: Damage to the wiring harness or corrosion in the connector for the electronic throttle body can cause intermittent signal loss. The plastic locking tab on the connector can become brittle and break, leading to a poor connection.

Diagnosis Steps

1. Read all stored DTCs with an OBD-II scanner. Note if any other codes, especially P2111, are present, as this points strongly to the throttle body.
2. Visually inspect the air intake system for any obvious cracks or loose connections between the air filter box and the throttle body.
3. Carefully inspect all vacuum lines and PCV hoses (especially Ventilation Hoses No. 2 and No. 3 mentioned in the TSB) connected to the intake manifold for cracks, brittleness, or disconnection.
4. Remove the intake duct from the throttle body. Inspect the throttle bore and the edges of the throttle plate for black carbon buildup.
5. If carbon is present, clean the throttle body thoroughly. Use a dedicated electronic throttle body cleaner and a soft cloth. Do not use abrasive brushes. To open the plate for cleaning, have a helper turn the ignition to 'ON' (engine off) and press the gas pedal.
6. After cleaning and reassembly, perform an idle relearn procedure. This is a critical step.
🎬 See this video for a step-by-step idle relearn walkthrough.
7. Idle Relearn Procedure: Warm the engine to normal operating temperature. Turn off all accessories (A/C, radio, lights). Turn the engine off. Turn the ignition ON (don't start) for 2-3 seconds, then OFF. Repeat once more. Start the engine. With the parking brake engaged and foot on the brake, shift into Drive and let the engine idle for 5-10 minutes as the ECM adjusts.
8. If the code returns, use a smoke machine to test the intake system for vacuum leaks that are not visually apparent.
9. If no leaks are found and the problem persists after cleaning and relearn, inspect the throttle body electrical connector for corrosion or damage. Ensure the locking tab is intact and the connection is secure. If wiring is good, the throttle body assembly itself is likely faulty and requires replacement.

Parts You'll Likely Need

• Electronic Throttle Body Assembly (OEM #22030-38050) — This is the primary component that controls idle speed. It is replaced if cleaning does not resolve the issue or if the internal motor/sensor has failed, as suggested by TSB L-SB-0032-25.
  Trusted brands: Lexus (OEM)
  OEM price range: $650-$930
  Aftermarket price range: N/A
• Throttle Body Gasket (OEM #22271-38040 (Verify with VIN)) — Should be replaced whenever the throttle body is removed to ensure a proper seal and prevent vacuum leaks.
  Trusted brands: Lexus (OEM), Mahle, Fel-Pro
  OEM price range: $15-$30
  Aftermarket price range: $5-$15
• Ventilation / PCV Hoses (OEM #Varies (e.g., 12261-XXXXX, 12262-XXXXX). Must verify with VIN.) — TSB L-SB-0032-25 points to these hoses as a potential cause. They can crack with age, causing vacuum leaks that lead to idle control problems. The TSB specifically mentions Ventilation Hose No. 2 and No. 3.
  Trusted brands: Lexus (OEM)
  OEM price range: $20-$100
  Aftermarket price range: $10-$50

Related Codes That Often Appear With This One

• P2111 — TSB L-SB-0032-25 explicitly links P0505 with P2111 (Throttle Actuator Control System – Stuck Open). This strongly indicates the ECM has detected a mechanical issue with the throttle plate's movement, often caused by heavy carbon buildup or an internal motor failure.

Technical Service Bulletins (TSBs) & Recalls

• L-SB-0032-25: Notes that certain vehicles with 2UR-GSE engines (including 2018-2025 LC 500) may show code P0505 and/or P2111. The fix involves replacing the throttle body assembly and ventilation hoses No. 2 and No. 3 with modified parts.

Platform-Specific Known Issues

• TSB L-SB-0032-25: Idle and Throttle Actuator Codes: A Technical Service Bulletin (L-SB-0032-25) exists for a wide range of Lexus models with the 2UR-GSE engine (including the LC 500, RC F, GS F, and IS 500) that may exhibit a MIL ON condition with DTC P0505 and/or P2111. The bulletin states that the throttle body assembly and ventilation hoses (No. 2 and No. 3) have been modified to address these conditions, and directs technicians to replace these parts if the issue is confirmed.

Mechanic-Grade Diagnostic Values

• Mass Airflow (MAF) Rate at Idle — expected: ~2.5 g/s. Failure: A reading around 3.0 g/s or higher at idle can indicate the throttle plate is not fully closing, possibly due to carbon buildup or another fault.
• Closed Throttle Position (CTP) Switch Status — expected: 'ON' at idle with foot off the accelerator.. Failure: If the status is 'OFF' at idle, the ECM does not recognize that the throttle is closed, which will disrupt idle control and can set P0505.
• ISC Airflow Rate (Learning Value) — expected: Within a specific range (e.g., not 1.3 L/sec or less, and not 8.5 L/sec or more).. Failure: If the learned value hits its minimum or maximum limit, it means the ECM is unable to achieve the target idle and has run out of adjustment range, triggering a DTC.
• ECM Ground Circuit Resistance — expected: Less than 0.1 Ω. Failure: Higher resistance indicates a poor ground connection, which can cause erratic behavior from the ECM and connected components like the throttle body.

Hidden / Shadow Codes Worth Checking

• Pending DTC / Permanent DTC: Before a code becomes 'active' and turns on the Check Engine Light, it may be stored as a 'pending' DTC. After a repair, even if the active code is cleared, a 'permanent' DTC may remain until the ECM's readiness monitors have successfully run. (see via Toyota Techstream software or an advanced OBD-II scanner. Techstream provides a 'DTC judgment result' (Normal, Abnormal, Incomplete) which gives more insight than a simple code reader.)

Scan Tool Commands That Help

• Toyota Techstream: Data List Monitoring — Use to monitor live data parameters like 'Engine Speed', 'ISC Feedback Value', 'ISC Learning Value', 'MAP', and 'Throttle Idle Position' to see if they are within expected ranges during idle.
• Toyota Techstream: Utility -> All Readiness — After a repair, use this function and input DTC P0505 to check the status of the related readiness monitor. This helps confirm if the ECM has verified that the idle control system is functioning correctly.

Wiring & Ground Locations

• Throttle Body Connector to ECM — The throttle body assembly is connected to the Engine Control Module (ECM) via a dedicated harness.. A wiring diagram for the 2UR-GSE shows the specific pins on the ECM connectors (A58 and G42) that correspond to the throttle body motor and sensors (e.g., EDT1, EMR1, EMD1). For example, pins 78, 79, 80, and 81 on connector G42(B) are for the throttle motor. This allows a technician to perform pin-point voltage and resistance tests to isolate a wiring fault from a component failure.

Real Owner Repair Stories

• ClubLexus user 'JaredAU' (2006 Lexus IS250 (similar ETCS logic)) — After changing spark plugs and cleaning the throttle body, the car threw code P0505 and had a slightly rough idle, but only when shifting into Park at operating temperature. It would idle fine when stopped in Drive.
  ❌ Tried (didn't work) Resetting ECU via fuse pull., Multiple idle relearn attempts.
  ✅ What actually fixed it The final resolution was not posted by the original author, but expert advice in the thread pointed to two likely causes: a small, hard-to-find vacuum leak introduced during the service (e.g., a hardened hose cracking at the nipple) or a MAF sensor reading slightly high (3.0 g/s), indicating the throttle plate was not fully closing, requiring a more thorough cleaning or inspection.

When the Usual Fixes Don't Work

• In several documented cases, owners have replaced the throttle body assembly—the most likely fix after cleaning fails—only to have the P0505 code persist with a high idle. Forum discussions among experienced owners frequently point to these situations being caused by an overlooked vacuum leak from a cracked hose or a poor electrical connection at the throttle body connector, rather than a faulty new part. This underscores the importance of performing a smoke test and carefully inspecting the wiring harness connector before spending significant money on a new throttle body.

OEM Part Supersession History

• Unknown → 22030-38050 (or related parts) — TSB L-SB-0032-25, dated July 2025, states that the throttle body assembly and related ventilation hoses were 'modified' to address conditions leading to P0505 and P2111.
  Heads up: While public parts catalogs show 22030-38050 as the correct part for the entire 2018-2024 range, the TSB implies that a revised version exists. When replacing, it is critical to use the latest part specified by a Lexus dealer to ensure the updated design is received.

Model Year Variations Within This Range

• 2021-2022: For the 2021 model year refresh, Lexus updated the shift logic for the 10-speed automatic transmission to improve response in the 50-70% throttle range. While not a direct change to idle control, this represents a software difference in the powertrain control system compared to 2018-2020 models.