I recently replaced the battery for both my 2005 Lexus LS430 Key FOBs. The first Key [that I had been using till the battery died and it stopped working] now works great: All the buttons [lock, unlock and trunk release] all work and the door automatically unlocks when I am near it and I can start the car without having to insert the key into the key socket. However on the second Key FOB, the buttons [lock, unlock and trunk release] work, but the automatic door unlock when I am near the door does not and worse is that when I try to start the car without the key inserted it displays 'Key Not Detected" on the dash and will not turn unless I insert the Key FOB into the Key Socket and start it that way!

I tried to Google if there is something more I need to do to get the Auto-detect functions to work on the second Key FOB! Like pair the Key FOB or turn on some switch in he car but could not find anything relevant! Can someone help with what may be wrong or what I need to fix with [almost new] second Key FOB that does not work, while the much used older one does. Any pointers or help would be much appreciated!

you might have to use Techstream to basically (learn) tell the car that key is a valid key , its fairly easy to do once you have Techstream setup

Thanks for the tip! I don't know how the car unlearned this Key FOB [probably because it was rarely used], but what is Techstream and is this something I can access and do myself. Any links to instructions or videos would be appreciated!

My terminology is probably incorrect, but Techstream is proprietary Toyota diagnostic system that interfaces with the vehicle. The system interface is essentially a scan tool. Most all scan tools can do generic diagnostics and customization. However, things like key registration, etc. typically require a proprietary system.


Old Toyota Hand-Held Tester

Sounds like the 2nd fob was not programmed for the SmartKey functionality.
Your Lexus dealer can sort that or if you are IT literate get TechStream and do it yourself.

If it worked before changing battery try a different name brand battery like energizer. When battery is out also press all buttons to delete voltage. I have seen where cheap batteries even when voltage is perfect do not work properly in some devices even measuring voltage with buttons pushed. Strange but I have seen that happen. It is also possible that with that fob that when battery is removed it looses its memory and has to be reprogramed.