The Siemens FDT421 is an intelligent, addressable thermal fire detector designed for ceiling or wall mounting. It uses a thermistor and microprocessor-based signal analysis to detect abnormally high temperatures instead of smoke. The detector has seven selectable temperature thresholds, ranging from approximately 135 °F to 174 °F, and also supports a rate-of-rise mode for rapid detection of sudden heat increases. It features a 360° tri-color LED status indicator and performs self-diagnostics every 10 seconds to ensure sensor and electronics health. The FDT421 is compatible with many addressable fire alarm control panels and supports two-wire, polarity-insensitive loop wiring.