If the engine warms up slowly and stays cold, the most likely cause is a faulty thermostat.

• A. Leaking radiator hose
• B. Faulty thermostat
• C. Air in cooling system
• D. Water pump failure

Answer: (B)

The cooling system relies on a thermostat to control when coolant flows between the engine and radiator. When the engine is cold, the thermostat stays closed to let the engine warm up quickly. Once the engine reaches operating temperature, it opens to let coolant circulate to the radiator and maintain that temperature. If the thermostat is faulty and stuck open, coolant keeps circulating all the time, so the engine never reaches its normal operating temperature. That’s why it warms up slowly and stays cold.

Other problems would produce different signs: a leaking radiator hose can lead to coolant loss and potential overheating or low coolant levels; air in the cooling system can cause hotspots and erratic temperatures; a failing water pump typically reduces coolant flow and also leads to overheating rather than a persistent cold engine.