Why is my Lexus LS400 not starting?

The most common reasons are a dead or weak battery, poor electrical connections, a faulty starter, or an immobilizer/anti-theft system preventing ignition. A methodical check of power, fuel, ignition, and security systems usually reveals the culprit.

Electrical and starting system

Electrical issues that prevent cranking or ignition are the most frequent no-start problems on the LS400. Start here to rule out simple, fixable causes.

• Weak or dead battery


• Corroded or loose battery terminals or cable connections


• Blown main fuses or fusible links (EFI, IGN, or battery fuse)


• Bad or damaged engine ground strap

If you find a dead battery, corroded terminals, loose cables, or blown fuses, address those first. Only after those checks should you test the starting circuit and related components.

Symptom cues to help triage

Use these signs to gauge likely causes: no crank, slow crank, a single click, or a continuous battery warning light.

Fuel delivery problems

Without adequate fuel pressure and delivery, the LS400 can crank but fail to start or run poorly. These items are common culprits to verify.

• Empty or very low fuel level


• Faulty fuel pump or failed fuel pump relay


• Clogged or dirty fuel filter


• Fuel pressure insufficient due to issues with lines or injectors

If the fuel system checks out or you repair a faulty pump/relay, re-test starting. If fuel delivery is confirmed, proceed to ignition and sensor checks.

Engine management, sensors, and security

Engine sensors and the anti-theft immobilizer can prevent starting even when power and fuel are present. Look for warning lights, irregular engine codes, or immobilizer indicators.

• Crankshaft position sensor or camshaft position sensor failure


• Faulty or degraded engine control unit (ECU) or wiring harness


• Immobilizer/key transponder recognition issues


• Blown fuses or faulty relays related to EFI/ignition


• Vacuum leaks or intake leaks affecting starting quality

If immobilizer or security faults are indicated, you may need dealer assistance or a replacement key/transponder; a technician can guide you through the proper reset or programming steps.

Diagnostics and interpretation

Interpreting codes from an OBD scanner (and the LS400’s built-in diagnostic codes) helps pinpoint the exact sensor or control module involved.

Step-by-step diagnostic guide

Walk through this checklist in order to triage a no-start condition safely. Do not attempt to bypass immobilizer or hot-wire the car.

1. Check the battery: with the key off, measure voltage at the battery. It should be around 12.6 volts. If it's significantly lower, charge or replace the battery before proceeding.


2. Inspect battery connections: clean corrosion from terminals and ensure cables are tight and free of damage.


3. Attempt to start and listen: does the engine crank, is there a loud click, or is there no sound at all? This helps differentiate electrical vs. mechanical issues.


4. Check fuses and relays: inspect EFI/main relay/fuel pump fuse; replace any blown fuses and reseat relays.


5. Check the security/immobilizer indicator: if the security light is blinking or on, the key may not be recognized; try another key if you have one or wait for the system to reset as per manual.


6. Test for fuel delivery: when the key is in ON, you should hear the fuel pump priming for a second or two; if not, test or replace the fuel pump relay or pump assembly.


7. Verify spark and ignition timing: if you have the tools and knowledge, check for spark at the spark plugs; otherwise, consult a professional to avoid dangerous handling.


8. Check for engine codes: connect an OBD scanner (if available) and pull any fault codes; document them for diagnosis.


9. Assess mechanical conditions: if the engine cranks but does not start, check compression or consult a professional for a deeper diagnosis.


10. Decision point: if you cannot identify or fix the issue, arrange a professional diagnostic service with the vehicle’s VIN, year, and symptom description.

These steps cover the core likely causes and guide you toward the right fix. If you encounter immobilizer or diagnostic code complexities, a dealership or qualified technician is best.

Summary

For an LS400 that won’t start, begin with the basics: battery and connections, then the starting circuit, fuel delivery, and engine management/security systems. Systematic testing and, where possible, code retrieval will usually reveal the culprit—ranging from a dead battery to a faulty starter, a failed fuel pump, or an immobilizer fault. When in doubt, professional service is advised to avoid accidental damage or misdiagnosis.

What problems does a Lexus LS400 have?

Common problems with the Lexus LS400 include issues with the power steering pump, which can fail and leak, as well as the alternator and starter, which can fail prematurely. Electronics like the Engine Control Unit (ECU) and instrument cluster backlighting can have problems due to bad capacitors, and mechanical parts like the timing belt water pump and suspension components (shocks, struts, bushings) are common areas for wear and tear.
 
Mechanical problems

• Power steering: The power steering pump is known to leak and can damage the alternator. The steering rack may also develop leaks. 
• Starter: The starter motor is in a difficult-to-reach location and can fail, sometimes multiple times. 
• Timing belt/water pump: This is a major maintenance item that needs to be replaced around every 100,000 miles. For 1995 and newer models, the engine is an interference engine, meaning a failed timing belt can cause catastrophic engine damage. 
• Oil leaks: Oil pan and other oil leaks are common, especially in higher-mileage vehicles. 
• EGR pipe: The EGR pipe is prone to cracking and is difficult to access for replacement. 

Electrical and electronic problems

• ECU: The Engine Control Unit (ECU) is prone to failure from bad capacitors, which can cause various running issues.
• Instrument cluster: The backlighting in the instrument cluster often fails due to bad capacitors.
• Radio/HVAC backlighting: Backlighting for the radio and climate controls can fail due to bad transistors. 

Suspension and other issues

• Suspension: Suspension components like shocks, struts, and bushings are subject to wear and tear due to the car's weight. 
• Door lock actuators: These can go bad over time. 
• Seatbelt retractors: These can also fail. 
• Sunshade: The rear window sunshade can fail and make grinding noises. 

What causes a Lexus car to be on and not start?

If your Lexus won't start, it could be a bad starter, a low/dead battery, or one of these other reasons: Jammed steering lock. Bad timing belt. Broken distributor.

What would cause a crank but no start?

A car that cranks but won't start is often caused by a lack of spark, fuel, or compression. Common issues include a faulty fuel pump, crankshaft position sensor, or bad spark plugs, as well as an empty gas tank or a dead battery. Less common causes can include a blown fuse, a problem with the engine's timing, or issues with the ignition switch or immobilizer system. 
Fuel system issues

• Empty fuel tank: A simple check that is sometimes overlooked. 
• Faulty fuel pump: Prevents fuel from getting from the tank to the engine. 
• Clogged fuel filter: Blocks fuel flow. 
• Blown fuel pump fuse or bad relay: Prevents power from reaching the fuel pump. 
• Faulty fuel injectors: Can prevent fuel from being delivered correctly. 

This video explains how to check for a bad fuel pump: 58sChrisFixYouTube · Oct 7, 2018
Ignition system and electrical issues

• Bad spark plugs: Worn or fouled plugs can prevent a proper spark. 
• Faulty ignition coil: A failing coil can stop the spark from being generated. 
• Faulty crankshaft or camshaft sensor: These sensors provide crucial timing information to the engine's computer. If they fail, the engine won't know when to fire the spark plugs. 
• Faulty ignition switch: This component signals the engine to start. 
• Weak battery: A battery that has enough power to crank the engine may not have enough to start it. 

This video shows how to use a spark tester to check for spark: 59s1A Auto: Repair Tips & Secrets Only Mechanics KnowYouTube · Dec 10, 2022
Compression and timing issues

• Broken or jumped timing belt/chain: This can cause a catastrophic engine failure, preventing it from starting because the valves won't be timed correctly. 
• Low engine compression: Caused by problems like a blown head gasket, warped cylinder head, or worn piston rings. 

Other potential causes

• Faulty starter motor: While the car "cranks," a worn starter may not be able to provide enough power to start the engine. 
• Immobilizer/Anti-theft system: A malfunctioning key or anti-theft system can prevent the car from starting. 
• Engine Control Unit (ECU) malfunction: A problem with the vehicle's computer can cause a no-start condition. 

This video discusses common causes of a cranking no-start condition: 57sDr. Cool YouTube · Dec 8, 2023

Why won't my LS400 start?

If your LS 400 won't start after shutting off, despite a good battery, check the starter relay and ignition switch for faults. A failing starter relay can cut power, preventing cranking. Also inspect battery cables and ground connections for corrosion or looseness. Test for parasitic battery drain using a multimeter.