Which action is crucial when you first notice an engine overheating?

Turning on the heater is a crucial action when you first notice an engine overheating because it helps dissipate some of the heat from the engine. When you activate the heater, it utilizes the engine's coolant and circulates it through the heater core. This process draws heat away from the engine and can help reduce the overall temperature. This is a temporary measure that can buy you some time until you can safely pull over and address the underlying issue causing the overheating.

Increasing engine speed would likely exacerbate the problem by putting more strain on the engine, potentially leading to more severe overheating. Pulling into traffic also poses safety risks and can increase the danger of an accident or further engine damage. Checking the oil level is important for general engine maintenance but does not directly address an overheating issue in the immediate moment. Thus, activating the heater is the most effective and relevant action to take when first noticing excessive engine temperature.