RX 350h Engine Roaring Downhill at High SOC TSB L-SB-0012-26 Explained
Lexus has issued Technical Service Bulletin L-SB-0012-26 (Canada: L-TCI-6101) addressing an alarming, loud engine revving/roaring noise when coasting or braking downhill with high hybrid battery State of Charge (>80% SOC) on 2023-2025 Lexus RX 350h models. The condition is resolved via an official Hybrid Control Computer (HV ECU) software recalibration.
Good News: Your Engine is NOT Damaged & Repair is 100% Covered Under 8-Year Hybrid Warranty
Experiencing sudden 4,000+ RPM engine roaring while coasting downhill can terrify any driver into thinking their engine or transmission is about to blow up. However, this is purely an ECU control logic issue—no fuel is burning and no mechanical damage is occurring. Under Lexus TSB L-SB-0012-26 (Canada: L-TCI-6101), this repair is 100% covered under the Lexus Hybrid Control Module Warranty for 96 Months / 100,000 Miles in the US (96 Months / 160,000 km in Canada) at zero cost to you.
Symptoms & Conditions
- • Loud engine roar and high engine RPM (3,500 – 4,500+ RPM) when coasting downhill or braking with foot completely off the gas pedal.
- • Vehicle hybrid battery State of Charge (SOC) is high (over 80% on the battery display gauge).
- • Occurs predominantly in mountainous, high-elevation, or hilly driving conditions (e.g. descending mountain passes or long highway declines).
- • No Check Engine Light (MIL) or DTC trouble codes stored; vehicle drives normally once the downhill grade ends or battery SOC drops.
The Root Cause (Master Tech Diagnosis)
On Lexus hybrid systems, regenerative braking converts kinetic energy into electricity via Motor-Generator 2 (MG2) to charge the High-Voltage (HV) hybrid battery. However, during continuous downhill descents, the hybrid battery rapidly reaches full capacity (>80% SOC). To prevent catastrophic battery overcharging, cell degradation, and thermal runaway, the hybrid battery cannot accept further charge. To maintain braking deceleration without overheating the friction brake pads, the Hybrid Control Computer spins the gas engine (2.5L A25A-FXS) using Motor-Generator 1 (MG1) at high RPM. Crucially, NO FUEL IS INJECTED AND NO IGNITION OCCURS. The engine is operated purely as a kinetic air compressor (compression brake) to pump air and dissipate kinetic energy. The loud roaring sound is the acoustic resonance of unpowered compression pumping.
Master Tech Insider Warnings
Crucial information every Lexus owner needs to know before going to the dealership
Zero Fuel Injection During High-RPM Roar
When this happens, many owners panic thinking their engine is burning gallons of gas or about to explode. In reality, fuel injector pulse width is 0.0 milliseconds and ignition timing is off. The engine is being spun mechanically by MG1 as an air pump. Your fuel efficiency and engine longevity are not harmed.
DO NOT Replace the Hybrid Control Computer ($2,500+ Saved)
The TSB explicitly states in bold text: 'The Hybrid Vehicle Control Computer Assembly should NOT be replaced as part of the Repair Procedure.' If an independent garage or uninformed mechanic suggests replacing your hybrid computer or inverter, show them this TSB. It is 100% resolved via a software reflash.
100% Covered Under 8-Year Hybrid Warranty
This reflash is covered under the Lexus Hybrid Control Module Warranty for 96 months or 100,000 miles (160,000 km in Canada). When visiting your Lexus dealer, simply state: 'My RX 350h makes an extremely loud engine revving noise when braking downhill in mountainous terrain.' Reference TSB L-SB-0012-26 (or L-TCI-6101 in Canada) to receive the free calibration update.
💻 Official Factory ECU Calibration Details
Your dealership technician will connect Toyota/Lexus GTS+ (Techstream) and flash reprogram the Hybrid Vehicle Control Module (OFP 89980-0E###, OP Code EE2602) to the following updated calibration IDs:
| Engine Application | Calibration Number | Previous (Buggy) Calibration ID | New (Updated) Calibration ID |
|---|---|---|---|
| 2023 RX 350h (AWD) | MAIN: 8A2520E08200 / SUB: 899854806100 | 8A2520E08100 / 899854806000 | File: L-0003-26.cuw (9 MB) |
| 2024 RX 350h (AWD) | MAIN: 8A2520E10100 / SUB: 899854815100 | 8A2520E10000 / 899854815000 | File: L-0004-26.cuw (9 MB) |
| 2025 RX 350h (AWD) | MAIN: 8A2520E16100 / SUB: 899854824100 | 8A2520E16000 / 899854824000 | File: L-0005-26.cuw (9 MB) |
🔧 Recommended Hybrid Diagnostic Scanners & 12V Support
Verify hybrid battery SOC, monitor inverter temperatures, and maintain flash stability
Autel MaxiAP AP200 Bluetooth OBD2 Scanner
Reads proprietary Lexus hybrid live data parameters including High-Voltage battery State of Charge (SOC %), individual block voltages, and inverter temperatures directly from your smartphone.
NOCO Genius5 6V/12V 5-Amp Smart Battery Charger & Maintainer
The TSB strictly requires maintaining steady 13.5V vehicle power supply mode during ECU flash reprogramming. Ideal for maintaining auxiliary 12V battery health on modern hybrid Lexus vehicles.
BlueDriver Bluetooth Pro OBD2 Scan Tool
Professional wireless OBD2 scan tool with live data logging. Graph engine RPM, vehicle speed, and hybrid braking events in real time while driving downhill.
❓ Frequently Asked Questions About TSB L-SB-0012-26
Why does my Lexus RX 350h engine rev loudly when going downhill?
When descending a hill, regenerative braking charges the hybrid battery. Once the battery reaches ~80% State of Charge (SOC), it cannot absorb more electricity without risking severe damage. To provide engine braking and protect the friction brakes from overheating, the hybrid computer spins the engine without fuel as an air compressor. TSB L-SB-0012-26 (Canada: L-TCI-6101) updates the ECU logic to eliminate this alarming roaring noise.
Is my engine damaged when it revs to 4,000+ RPM downhill?
No. The engine is simply pumping air with zero combustion and zero fuel injection. While mechanically safe, the acoustic noise is alarming to drivers, which is why Lexus engineered this revised software update.
How much does the TSB L-SB-0012-26 update cost at a Lexus dealer?
It is 100% FREE under the Lexus Hybrid Control Module Warranty (96 months / 100,000 miles in the US, or 96 months / 160,000 km in Canada). The repair requires approximately 1.1 hours of labor (OP Code EE2602).
Does this TSB apply to the gas-only RX 350 or RX 500h?
No. Non-hybrid RX 350 models use an 8-speed automatic transmission and regular engine braking, while the RX 500h uses a different turbocharged hybrid powertrain with a 6-speed automatic transmission. This specific bulletin applies exclusively to the 2023-2025 RX 350h with the eCVT and A25A-FXS engine.