There could be several reasons why your Lexus GX470 is not starting with the flat key and transponder chip:
Wrong Key: You might be attempting to use a key for a different vehicle1.
Locked Steering Wheel: If the steering wheel is locked, you can unlock it by turning the steering wheel from left to right while simultaneously turning the ignition key on1. If this doesn’t work, it’s recommended to have the vehicle towed to a Lexus dealership as the key may be worn/damaged or there may be a problem with the ignition switch itself1.
Battery Issue: If you recently changed or disconnected the battery, your Lexus may not start because it has lost synchronization between the key and the vehicle2. You can try pressing the Start button with the keyfob2.
Key Battery: The battery in your key might need to be replaced3.
Error Code: If an error code is preventing the vehicle from starting, you can try clearing the code with a code reader or by disconnecting the negative terminal for 5 minutes4.
If none of these solutions work, it’s best to consult with a professional mechanic or your local Lexus dealership for further assistance.