What condition may result in poor fuel mileage and poor heater performance in a vehicle?

The condition of a stuck open thermostat can lead to poor fuel mileage and poor heater performance for several reasons. The thermostat's primary function is to regulate the engine's operating temperature by controlling coolant flow. When a thermostat is stuck in the open position, the engine may not reach its optimal operating temperature.

When the engine runs cooler than intended, it can lead to less efficient combustion of fuel. Engines are designed to function most efficiently at a specified temperature range, and if they operate below this range, it can cause incomplete combustion and result in poor fuel mileage.

Additionally, a stuck open thermostat can adversely affect heater performance. The heater in a vehicle relies on the engine's coolant being heated before it is circulated into the passenger compartment. If the engine does not warm up sufficiently, the coolant will also remain cooler, which means the heater will blow cold or lukewarm air instead of providing the necessary warmth.

This combined effect of reduced engine efficiency and inadequate heater output solidly links a stuck open thermostat to the symptoms of poor fuel mileage and poor heater performance in a vehicle.