Vehicle security system

I am developing a vehicle security system that has two primary functions both of which can be controlled by the owner by a simple text message from his cell phone. A GSM modem, interfaced with an uno would sit inside the vehicle and would receive the messages from the owner and perform the functions as per the message received.

The 2 primary functions that I am thinking about are:

  1. Immobilization of the vehicle
  2. Setting a kind of an alarm so that if the vehicle is ignited after the alarm is set (through a text message), the owner will be notified by an SMS on his phone.

I dont have a sound practical knowledge about the working of car engines so I am not sure what I should be using to activate the immobilization. There are too many things going on so i am looking for a starting point basically. The target would be to keep it simple and universal so that it can be installed in any car without too much trouble. It should of course be something that can be controlled by the arduino. And it would be preferable if the immobilization action can be reversed by the arduino through another message (though this is not a must as of now). The immobilization should be strong enough so that the principle cant be figured out easily and reversed by petty thieves.

For the second function I am thinking of tapping from an electrical line that lights up when the engine is ignited. I would have the arduino monitor this line. But I think there are plenty of lines like that. Is there a line that would be easier to tap than the others?

If anyone has any knowledge/ideas about how these 2 functions can be implemented, I would be very grateful.

Disable the vehicle with a triggered relay on the power wire for the fuel injecrors.

Check for a running car by checking the state of the ignitiin power circuit from the fuse box or key switch.

Im on q cell phone so excuse the simplistic responses

In order to "ignite the vehicle/engine" you or the thief would have to connect the ignition wire to +12v. You could monitor this via a relay. The coil will be connected to ground and the ignition wire. The closer you monitor the ignition wire to the engine the better chance that the thief has not cut the monitor relay out of the circuit when he cuts the ignition wire to hot-wire your car.

I am sure you could also use some "voltage sense" type logic. If you read voltage over 13v start the alarm or sms or whatever.
Car batteries sit idle at about 12.5v or less. I would take a voltmeter and verify on and off batter voltages. This is how aftermarket remote starter kits determine the engine is running and stop cranking. (or they use a preset crank time, this is usually a last resort or if lazy, choice is on the installer).

You might want to think about a relay shield or read up on how to interface a relay with an UNO.