Thermocouple is a critical safety device in gas water heaters, functioning as a flame failure detection component. This heat-sensitive sensor generates a small millivoltage (typically 15-35mV) through the Seebeck effect when exposed to the pilot flame. The electrical current maintains the gas control valve in an open position during normal operation. If the flame extinguishes unexpectedly, the thermocouple rapidly cools, interrupting the current flow and triggering an automatic gas shut-off within 2-3 seconds to prevent hazardous gas accumulation.