What happens if a thermostat is stuck open?

When a thermostat is stuck open, it continuously allows coolant to flow through the engine's cooling system, which prevents the engine from reaching its optimal operating temperature. As a result, the engine may run too cold. Operating at excessively low temperatures can lead to inefficient fuel combustion and reduced engine performance, impacting overall vehicle efficiency.

The other options are not applicable because a stuck-open thermostat does not prevent the engine from starting and does not directly cause overheating or incorrect heater function; rather, it leads to a situation where the engine does not achieve the necessary heat levels for proper operation.