If your Lexus doesn’t start after replacing or disconnecting the battery, the problem in most cases is due to the security system (anti-theft) getting activated, a loose battery terminal clamp or the key fob no longer being recognized. 

If your Lexus doesn’t turn on at all meaning the lights don’t come on and ignition doesn’t turn on, there is a good chance you connected the battery terminal backward or you have a blown fuse. Your Lexus won't start because of blown fuses. 

In this guide, we look at common problems that will prevent a Lexus from starting after replacing the battery or disconnecting the battery.

Symptoms

Lexus won't start after replacing the battery
  • Lexus won’t start after changing the battery
  • Lexus won’t start after disconnecting the battery
  • Security light comes on, won’t turn off
  • Engine won’t turn over
  • The alarm goes off when connecting the battery

Common Causes

This is a list of the most common causes that will prevent your Lexus from starting after you disconnect or replace the battery. These problems may also affect Toyota vehicles. 

Immobilizer Activated

lexus won't start due to immobilizer enabled

The most likely reason why your Lexus is not starting after you change the battery is that the security/immobilizer / anti-theft feature disabled the engine start. 

Possible Solution

  1. Get out of the vehicle. 
  2. Lock the car with the key fob or lock the driver’s door with the key.
  3.  Leave the vehicle parked for at least 5 minutes. 
  4. Press unlock on the keyfob to unlock the car.  Don’t open any doors yet. 
  5. Remove the emergency key form the key fob and use it to lock/unlock the driver’s door three times. 
  6. Now enter the vehicle and start the engine. If your car has Start/Stop button, press the button with your keyfob, not your finger. 

This will in most cases reset the immobilizer. If this procedure doesn’t work, it may be time you need to take your Lexus to the dealer or local auto repair garage.

The alarm may or may not go off as soon as you reconnect the battery. The security light on the dashboard stays on or may flash. 

Key Not Recognized

Lexus will not start because key is not detected

Your Lexus may not start after changing the battery or disconnecting it has lost synchronization between the key and the vehicle. 

Possible Solution

start lexus after replacing battery

Instead of stating the vehicle by pressing the Start/Stop button, take the keyfob and press the Start button with the keyfob. 

Loose Battery Terminal

loose battery terminal

It is possible that the battery terminal may be loose. A loose battery terminal will allow the car lights, ignition, and instrument cluster to turn on but due to weak connection at the battery post won’t allow you to start the engine. You may hear a click or two but that is it. 

Possible Solution

To fix this problem, check the battery cables to make sure they are not loose. Use a 10mm wrench to faster the terminal nut.

Make sure the battery terminal clamp is tight and does not rotate on the post. If needed, clean the battery terminals with a battery brush. 

Battery Connected Backwards

lexus won't start after connecting battery backwards

Connecting the battery backward will also prevent you from start the car.

The red cable needs to be connected to the battery post with the + next to it. The black cable needs to be connected to the battery post with the (-) symbol next to it.  

If you install the battery backward, the car will be completely bead and lights will not turn on at all.

Connecting the battery backward will blow one of the larger ampere fuses or the fusible link.

Possible Solution

Lexus won't start due to blown fuse

To be able to start your Lexus again, you will need to check all the fuses and replace fuses that have blown.

Check all the fuses in all fuse panels, typically most Lexuses have three fuse panels. Two under the dashboard and one in the engine bay. 

The larger fuses are not always located in the fuse panel, so check your Lexus wiring diagram to find out where the 30A, 40, 50A, 120A fuse are located depending on the model. Check all the fuses and especially the main fuses, starter, solenoid fuse, and alternator fuses. 

If all the fuses are good, there is a small chance the Engine Control Unit (ECU) may have been damaged when connecting the battery backward. See your Lexus dealer or a local auto repair shop near you. 

Aftermarket Alarm

Lexus won't start due to aftermarket alarm

If your Lexus has an aftermarket alarm or remote start it may be reset disabling the engine start on your car. 

Check if you have an aftermarket alarm and if you do find out who the manufacturer is and call them up. 

Conclusion

Your Lexus may not start after you disconnect the battery for a number of reasons as listed above. Also, make sure the battery is fully charged.  Other possible issues include a bad brake light switch, fuel pump, bad ignition switch, starter relay and etc. 

Unless you are familiar with troubleshooting car problems, it may be wise to tow your car to a mechanic or Lexus dealer to have them troubleshoot the problem further. 
 

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