Why should you never turn the power on to the heater until the engine is full of coolant?

Turning the power on to the heater before the engine is filled with coolant can lead to overheating and potential damage to the heater core, which is part of the engine's cooling system. If the coolant is low or absent, the heater will not have the necessary heat exchange medium to regulate temperature effectively. This absence can cause the heater's internal components to become excessively hot and ultimately lead to the heater burning up. The heater operates by transferring heat from the engine coolant; without the coolant present, it cannot perform this function safely, risking failure or catastrophic damage to the heater system.

Other options suggest severe issues, but they do not accurately address the direct consequences related to the operation of the heater without coolant. For example, while overheating can cause an engine to fail, the specific concern with not having coolant in relation to the heater is focused on the risk to the heater itself.