Metal oxide chemiresistors (MOCs) with a low optimal operating temperature, high sensitivity and fast response/recovery are highly promising for various applications, but remain challenging to realize. Herein, we demonstrate that SnO₂ nanofibers after being co-doped with Cu²⁺ and Au show considerably enhanced sensing performances at an unexpectedly decreased operating temperature. A synergistic effect occurs when the two dopants are introduced together. Co-doping may form a novel strategy to the development of ultrasensitive MOCs working at a low optimal temperature.