If the engine coolant temperature rises toward the upper limit while the thermostat is functioning, what is a likely explanation about the thermostat?

• A. The thermostat is stuck open.
• B. The thermostat does not affect temperature.
• C. The thermostat is not needed.
• D. The thermostat is stuck closed.

Answer: (D)

The thermostat controls when coolant moves to the radiator based on temperature. When the engine warms up, the thermostat should open to allow heat to escape through the radiator; when it’s cold, it stays closed to help the engine reach operating temperature efficiently. If the temperature climbs toward the upper limit even though the thermostat is described as functioning, the most likely explanation is that it’s stuck closed. A closed thermostat blocks coolant flow to the radiator, causing heat to build up in the engine and temperatures to rise. If it were stuck open, the engine would tend to run cooler because coolant would continuously circulate and dissipate heat. The other options don’t fit because the thermostat does affect temperature and is needed to regulate it.