Q: Antitheft system causing car to not start

Depending on where and how the security system was installed, this can change the possibilities of various issues and fixes. An aftermarket system put in by a private party could have been installed in different ways that could cause issues with the starting system or other sources depending on what the power is being drawn from. As where an OEM security system that came with the vehicle will have wiring diagrams and more diagnostic specified information about where the problem is coming from. Also there is a possibility that the dealer installed an aftermarket security system which falls in the middle of the other two. Possibly because the dealer may have had the schematics for the installation or in some cases they may have not. The best plan of action would be to have a qualified technician like the ones here at YourMechanic, perform an electric diagnostic on the security system in order to map out how the siren was installed. If the system wasn’t installed properly the car might not start. In this situation it can only be started with the supplied key fob with an auto start. In some bad cases it’s possible for the vehicle to not shut off at all.