What component of a vehicle is primarily responsible for starting a fire when left running over grass?

The catalytic converter is primarily responsible for starting a fire when a vehicle is left running over grass due to its significant heat output. This component is designed to reduce harmful emissions by facilitating chemical reactions that convert toxic gases from the engine's exhaust into less harmful substances. However, during operation, the catalytic converter reaches extremely high temperatures, often exceeding 1,200 degrees Fahrenheit.

When a vehicle is idling over dry grass, the intense heat from the catalytic converter can ignite the grass below. This risk is especially pronounced if the grass is dry and flammable, creating a dangerous situation. Other components, such as the engine block and fuel tank, while vital to the vehicle's operation, do not produce the same levels of heat in proximity to the ground that can lead to ignition. The exhaust pipe, which carries exhaust gases away from the engine, can also become hot, but it is the catalytic converter that poses the highest risk of causing a fire when a vehicle is running over combustible materials like grass.