We investigate the defect-dependent electronic structure and gas-sensing potential of cubic α-CsPbI₃ using first-principles density functional theory and nonadiabatic molecular dy-namics simulations. Among the intrinsic defects, interstitials, vacancies, antisites, and switches studied, the IPb and PbI antisite defects exhibit transition energy levels near the middle of the band gap, thus functioning as deep traps. Short-term adsorption of ammo-nia selectively modifies the electronic structure, coordinating with Pb at PbI sites and Cs at IPb sites, significantly altering recombination pathways. Detailed analysis reveals that NH3 reduces anharmonicity at IPb defects, enabling enhanced recombination at elevated tem-peratures, while trap-assisted recombination dominates at room temperature. Other ana-lytes, including CH3NH2 and NO2, show negligible impact on the band gap or recombina-tion dynamics, highlighting the selectivity of NH3 interactions. Ab initio nonadiabatic mo-lecular dynamics simulations at 300 K and 600 K further demonstrate tempera-ture-dependent modulation of carrier lifetimes, with NH3 accelerating recombination at ambient conditions and suppressing certain pathways at higher temperatures. These findings suggest that α-CsPbI₃ can serve as a selective and sensitive ammonia sensor over a broad temperature range and offer insights for ammonia detection under industrially relevant conditions.