Caspers Electronics Knock Gauges contain 9 LEDs in three colors to indicate the severity of detonation in real-time: Green (mild); yellow (moderate); and red (severe). The center green LED is illuminated during "standby," and when detonation occurs. Designed to fit into a standard 2-1/16" gauge opening or gauge pod. Simple 3-wire hookup. Requires the factory ESC module to operate.