A carbon monoxide shutoff system for an engine of a portable electrical generator has a carbon monoxide gas sensor, a microcontroller, and an output indicator. The carbon monoxide gas sensor generates an output electrical current proportional to a detected concentration of ambient carbon monoxide. The microcontroller has an input connected to the carbon monoxide gas sensor, and an output connected to an operational control of the engine. A deactivation signal generated by the microcontroller in response to detection of a deactivation condition is based upon the output electrical current from the carbon monoxide gas sensor matching predefined value and duration thresholds. The deactivation signal is operative to stop the engine.