Engine not starting

How exactly is the theft system not working? Does the light stay on or flashing or does it not come on at all?

Does the dash come on when you are turning the key? Does the engine even crank or does it do nothing when you turn the key?

Let's start with all these questions and can you find out if there are codes.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Let's start here and go from there. Thanks

Okay. This means then key ID is not getting to the ECM to turn off the system. When you put the key in and it is a good key, it should blink twice and then turn off. If it is a wrong key it will blink twice and then blink rapidly.

The fact that it doesn't come on at all, points to an immobilizer issue. Did this just happen one day or is there more to the story before this started? Did you just buy the vehicle? Do you have another key that you can try?

Okay. I suspect the immobilizer is the issue but let's just rule out a weak or drained battery. Sometimes this can cause some odd issues especially since it was sitting for a couple days.

As long as the battery tests okay, I would suspect the immobilizer is not picking up the key.