Abstract: A circuit is provided for a tractor/trailer combination to provide a warning in the tractor of an event such as low tire pressure occurring at the trailer utilizing only conventional wiring connections between the tractor and trailer. When low tire pressure occurs, a flasher circuit on the trailer flashes the trailer marker lamps which are normally energized so that the flashing lamps are visible to the operator. The electrical pulses in the marker lamp circuits are sensed by a detector in the tractor to illuminate an indicator lamp in the tractor. A time delay circuit controlling the flasher circuit for providing a self-check function, and for allowing manual flashing of the lamps without triggering the self-check operation.