If you are a member of CICCI you can get to the service manuals from their home page; Click on the technical library. DTC for the anti theft system is explained. They will lead you to the proper trouble shooting sequence, without spending money on trial & error fixes. I really don't know if the auto parts store OBD tester will come up with anything, but it is the best place to start. My siren stopped working about a year ago. I think the internal battery has died. I have left it alone just because the alarm/siren can be more than just awkward when it goes off at the wrong time.