Abstract: A system is provided for automatically shutting down an engine of a portable or handheld device in response to the engine operating while in an enclosed space, such as a garage, shed, room, etc. to prevent dangers associated with carbon monoxide accumulating in the enclosed space. The engine has an oxygen sensor in its exhaust that is configured to detect the presence or absence of oxygen in the exhaust. A fuel injector injects fuel for combustion within the combustion chamber. The oxygen sensor may be a switch producing an associated frequency indicating the presence or absence of oxygen. This frequency can control the fuel injection duration such that as the frequency of switching decreases, the injector injects less fuel. If the frequency continues to decrease even while injection is reduced, the controller can assume the engine is operating in confined spaces and can shut down the engine.