The presence of a powered CAN bus is a good indicator of whether an aftermarket telematics device powered by a vehicle OBD port should enter a low-power mode to prevent unwanted battery drain, as the vehicle will remove power from the CAN bus when it determines the vehicle is not in use. Circuitry in the telematics device monitors the voltage on the CAN bus wires of the OBD port to determine whether the CAN voltage is above or below some threshold, such as 1V. This enables the vehicle ON/OFF status to be determined reliably without any additional wiring in the vehicle. The circuitry may include a simple, inexpensive, and low-power integrated circuit comparator, an operational amplifier, or an ADC to determine whether the vehicle is ON or OFF based on the CAN voltage. A very simple embodiment uses a single MOSFET and resistor to detect the vehicle ON/OFF state.