Abstract: A vehicle anti-theft system includes a four-node bridge network having a removable resistor. The network includes a first resistive segment having a resistance R.sub.1 between a first interface node and a first intermediate node, a second resistive segment having a resistance R.sub.2 between the first intermediate node and the second interface node, a third resistive segment between said second intermediate node and said second interface node having a resistance R.sub.3 and including a removable resistor and a fourth resistive segment having a resistance R.sub.4 between the second intermediate node and the first interface node. The resistances R.sub.1, R.sub.2, R.sub.3 and R.sub.4 satisfy the equation R.sub.3 R.sub.1 =R.sub.2 R.sub.4. The first interface node and the second interface node are electrically connected to the ignition system of the vehicle on either side of the ignition switch.