Hotwiring is the process of bypassing an automobile's ignition interlock and thus starting it without the key. Although it is legal if performed with the consent of the car owner, the activity is usually performed by criminals who intend to steal the vehicle. Also hotwiring can compromise the safety of the car, making it illegal to drive a hotwired car in some countries.

The classical method for hotwiring cars involves two people:

• one completing the ignition circuit by locating the ignition wires attached to the ignition switch under the dashboard: this can be done by detaching them and crossing (tying together) the exposed ends
• the other momentarily completing the starter motor circuit, so that the starter motor turns over the engine. This is done by detaching the starter motor wire from the ignition wire and touching it to a powered wire on the other side of the ignition switch.

It is possible to undertake both of these operations from within the engine compartment rather than from the dashboard, by making the same circuits as described above by direct links from the positive pole of the battery to the ignition and starter motor circuits.

To hotwire a car just run an insulated wire with both ends exposed from the negative terminal on the car battery to the solenoid on the starter motor. There should be an out-sticking connector, just touch the wire to this for a few seconds and this should start the motor which inturn will start the car. There is known to be a risk of electrocution whilst carrying out this process.

Those who lack the necessary skills and knowledge of electrical engineering to hotwire a car in the classical way sometimes use a brute-force method: simply smashing the key mechanism to reveal the rotation switch, which is operated by the key's tumbler. In a criminal context, this could be accomplished with the same tool that may have been used to smash the vehicle's windows to gain entry. The rotary switch can then be operated by a screwdriver or similar tool.

Some more modern vehicles are designed to overcome the relative ease with which classical ignition systems can be hotwired. This is primarily accomplished through the use of engine immobiliser devices which disable the engine in some manner. Such systems require a code key to disengange. The code key could be analog (such as the precise resistance of the physical key) or digital (such as a passcode embedded in a USB or similar electronic key). As automobile electronics advance, hotwiring is expected to become increasingly difficult. Both hotwiring methods described above are defeated by the use of an immobilization device.

However, there are ways to bypass these safety mechanisms; since most engine immobilisers are actually implemented in a subroutine of the electronic engine control unit, in the process of stealing a car some criminals simply replace said control unit with a reprogrammed one which they brought along with them. This method of hotwiring is typically carried out on more expensive automobiles and by thieves scouting for a specific model. Some automakers have responded to this problem by making it increasingly difficult to remove the unit from a car, either by riveting it to the body or by thoroughly shrink-wrapping it and the wires connected to it.

It is possible that the increased difficulty of hotwiring has led to the rise in carjacking in other ways, such as stealing car keys. However, security and insurance experts agree that most auto thefts can be deterred by employing common sense safety methods, such as never leaving spare keys in plain sight (or keeping spare keys inside the vehicle).

The ignition circuit is different from one car to another, there is no set standard; Sure older cars were similar but not identical. Just electrifying the starter isn't going to get you anywhere, the car still needs to think there is a key and that is in the "ON" position. The ignition circuit typically uses 4-6 wires which, if connected in the correct order, can be used to start the engine and trick the car. The downside to this, however, is that you can't steer the car. Even older cars lock the steering wheel after its is turned a certain distance, and will not release it until the key is physically turned. This is mechanical and is very difficult to disassemble and cannot be over-ridden by electronic methods. Sometimes the best way is to use brute-force to turn the ignition. Of course, you end up destroying it in the process but at least its drivable for the time being. Hotwiring is not straightforward and should be used only in emergencies.

See also

• Lockpicking

Car theft | Security exploits