Preheat Engine -

In diesel vehicles, a pigtail-shaped light indicates that the are heating the cylinder heads before starting, usually lasting 30 seconds or less.

I can help clarify what "Preheat Engine" means for your specific situation. Should I preheat my Tiffin Bus 450 DP engine? - Facebook PREHEAT ENGINE

Prevents oil congealing, reduces internal engine wear from cold-start oil starvation, and ensures easier starting. In diesel vehicles, a pigtail-shaped light indicates that

Are you asking about a or an external heater (like a block heater)? - Facebook Prevents oil congealing, reduces internal engine

For electric heaters, it is often suggested to run them for 1–5 hours, though some keep them on overnight, say.

"PREHEAT ENGINE" refers to the process of warming an engine's internal components and fluids (oil and/or coolant) before starting it in cold weather, usually at or below 20°F (-7°C), to reduce wear and ensure a successful start.