An immobilizer monitors the ignition status of a motor vehicle, in a low power sleep mode or while performing other tasks, and is responsive to a change in ignition status between an off state and an on state to become active from the sleep mode or to interrupt the other tasks. If the change is to an on state, the immobilizer initiates ignition control module interrogation of an ignition control module to check for a valid key in the ignition, authorizes or prevents engine operation according to the result and returns to the sleep mode when finished. If the change is to the off state, the immobilizer again initiates ignition control module interrogation of the ignition control module to check for a valid key in the ignition and sets a key in status datum true or false according to the result. Either the immobilizer itself or a separate body computer responsive to the key in status datum provides a key in ignition warning when a key in status datum of true is detected simultaneously with an open driver door. When the key in status datum is set false, the immobilizer returns to sleep mode or its other tasks.