Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1991 Lexus LS 400FACTORY OEM SERVICE PROCEDURE
Engine Controls - System/Component Tests
- Introduction
- Control Unit
- Engine Sensors & Switches
- Relays
- Fuel Control System
- Idle Control System
- Ignition Coils System
- Timing Control Systems
- Exhaust Gas RECIRCULATION (EGR)
- Fuel Evaporation
- Positive Crankcase Ventilation (PCV)
- Throttle Controls
- A/C Clutch Control
- Cooling Fan Control System
- Notes
- LS400 A/C Pressure Switch
- Es250 Cooling Fan Ecu
- Es250 Water Temperature Sensor
- LS400 Water Temperature Switch
- Es250 & LS400 Engine Main & LS400 Cooling Fan Relay No. 3
- Cooling Fan Relay No. 1 (Es250 & LS400) & No. 2 (LS400)
- Es250 Cooling Fan Relay No. 2
- Es250 Cooling Fan Relay No. 3
- Cooling Fan Motors No. 1 & No. 2
- Theft Deterrent System
- Failure Symptoms & Circuit Tests
- Es250 Theft Deterrent Ecu Power Circuit Test
- LS400 Theft Deterrent Ecu Power Circuit Test
- Es250 Theft Deterrent Ignition Switch Circuit Test
- LS400 Theft Deterrent Ignition Switch Circuit Test
- Es250 Theft Deterrent Key Unlock Warning Switch Circuit Test
- LS400 Theft Deterrent Key Unlock Warning Switch Circuit Test
- Es250 Theft Deterrent Starter Cut System Circuit Test
- LS400 Theft Deterrent Starter Cut System Circuit Test
- LS400 Theft Deterrent Ecu Operation Test
- LS400 Theft Deterrent Ecu Ground Circuit Test
- Circuit Diagnosis Charts (LS400)
Factory Service Tools
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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