It really depends on the car. My car, for example, is a Chrysler 300 and there is no real connection between the key and what provides power to the engine. He

It really depends on the car. My car, for example, is a Chrysler 300 and there is no real connection between the key and what provides power to the engine. Here[1] is a diagram showing how the CAN bus works on the Chrysler/Dodge LX platform vehicles. The SCM/SKREEM module on that diagram is where the ignition switch and logic that talks to the keyfob is and makes sure you have a valid key. Here's a recall related to the module which hints a little bit about what it is and does[3]. Note on the recall that this applies to a whole bunch of Dodge/Chrysler platforms. Based on the wiring diagram[2] (for Dodge trucks, which is one of the platforms that [3] applies to), the SCM/SKREEM (called Node-Wireless Ignition in one diagram and WCM in another) uses a 2-wire CAN bus for its communication. Once the SCM/SKREEM validates the key it sends a message on the CAN bus to the engine controller to tell it to start the engine. In these vehicles, whether the engine is on or off is purely under software control. That said, I'm not going to try turning my car off while I'm driving. [1] http://the8thlayerof.net/2013/11/10/canbus-vehicle-network-data-capture-tapping-the-can-network-wires-chrysler-lx-platform-2006-dodge-charger-srt8/ http://the8thlayerof.net/2013/11/10/canbus-vehicle-network-d... [2] http://tucrrc.utulsa.edu/DodgeCAN.html http://tucrrc.utulsa.edu/DodgeCAN.html [3] http://project-jk.com/images/tsb/TSB_08-007-08.pdf http://project-jk.com/images/tsb/TSB_08-007-08.pdf