My girlfriend has a 2000 catera. She lost the transmitter to the factory alarm but has had no problems using the car. Just recently when you get in the car, by unlocking the doors with the key, and go to turn the car on the alarm starts sounding the horn. This has never been a problem. Any ideas what could be going on?

 

It could be the tamper switch in the drivers door handle.
It could be a problem with the body control module.
It could be a wiring problem somewhere in between.

It's impossible to know for sure without testing the system step by step.
A Tech II, DVOM, and wiring schematic will be needed to properly dig this problem.

 

I was thinking it was the Body Control Module. When you lock the doors manually and then open them with the key nothing goes crazy. As soon as you turn the ignition the alarm goes off. The car still turns on though. That's the interesting part. So what ever it is it is not affecting the immobilzer.