No doubt the security light has legitimate uses like that ^^^ I think Soft Ride was just referring to the times the light comes on when it SHOULDN'T be.

It worked! Security light off on startup after I re-synched the keyfob. Hopefully it'* a permanent fix... thanks!

And I must have been having a flashback to my '99 Aurora ... these '00 LeSabre keys do NOT have chips so I don't suppose the key cleaning method would do anything. I assume the "PK3" engraving means Passkey 3 system, so I'll have to read up on that... my previous GM'* were the older security system.

talking about in addition to what he said.

and yes the pk3 does not have a chip. the ones that have the plus on it have them

That is excellent news! I am glad that I was able to Help!
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Any time you disconnect the cars battery, without using a battery minder, you should re-synchronize the the remote. Problems can also appear when you are too far away from the car and are out of range. If you hit the buttons on the fob in those conditions, a security feature may now kick in, also causing synchronization. Weak batteries in the FOB can also cause it too.

125 presses away from the car will completely disable the fob... 50 or so and it will cause the security issue. i had my car and laurens car programmed to my remote. came back from the trip to alabama, and the impala wont respond to my remote. time for a relearn for me.

Already did before you asked! I'll change the batteries in both fobs if I have the right size button cells.

I think its a 2032 battery in there, and be careful not to pull up on the battery!
That little metal clip that holds the battery is delicate & can break off from the PC board easy. (dont ask me how I know)
To prevent this, always place Fob with the back off, on a flat surface.
With a finger hold down apply light pressure to the rectangular rear area of the clip, while sliding battery out.
I just used my finger to slide out the battery. The owners manual states to use only a pencil to remove it, no steel in there.

Update: Security light came back on yesterday, just solid with the chime and no other symptoms. I replaced batteries in both fobs (2032 3-volt battery is correct)... the two I removed both tested at 3.2 volts so they were probably fine. But I replaced them anyway.

This morning I re-synched both fobs. Key #2 (the one I've been using as primary) still had Security light. I replaced with #1 in the ignition and no light, drove to work and it stayed off. After arriving I put key #2 back in just to check it... no light now. If #2 is the problem, is that the fob or the key itself? I don't know which is primarily responsible for confirming the security signature (maybe both).

I need to read up on the Passkey 3 system. It'* a minor intermittent problem at this point, but I'd like to figure out the issue if I can.

Apparently the PK3 (RFA) system is able to differentiate between different fobs. It can recognize up to 4 fobs.
If a trouble code (DTC) is present the light illuminates. It should clear it self on the next ignition duty cycle.
A scan tool that is capable, that can read any stored PCM messages and possibly any RFA module related codes, should be utilized if the the light remains on.
Hope this helps.

I noticed when the light is on, regardless of key #, if I turn the ignition off and back on then it clears (doesn't come back on). Same result as re-synching the fobs when the light is on... it clears. After being away from the car for awhile with the ignition off, the light is tending to pop back on again. I'll get codes scanned sometime here in the near future and see if anything is stored.