Calterm 66331 LED Indicator SENSITIVITY ADJUSTMENT DIAL is a Calterm EXCLUSIVE feature that allows isolation of a specific plug wire and eliminates false signals from wires routed in close proximity. Green LED indicates that the tester and batteries are operating properly. Red LED flashes each time voltage is released from the coil through the spark plug wire. Rugged high impact ABS housing resists oil and endures shop conditions. Works with DIS and conventional ignition systems.