Testing your car's thermostat is a straightforward diagnostic task you can perform with basic tools. The most reliable method involves checking the engine's temperature directly at the thermostat housing using an infrared thermometer or an OBD-II scanner while the engine warms up. A stuck-open thermostat will cause the engine to warm up very slowly, while a stuck-closed thermostat will lead to rapid overheating.
Start with a cold engine. Locate the upper radiator hose, which connects to the thermostat housing. As the engine runs, feel this hose. It should remain cool for the first 5-10 minutes. When the engine reaches its operating temperature (typically between 195°F and 220°F or 90°C-105°C), the thermostat opens, and the hose should quickly become hot as coolant flows through it. If the hose gets hot immediately, the thermostat is likely stuck open. If it never gets hot and the engine temperature gauge climbs into the red, the thermostat is probably stuck closed.
For a more precise test, use an infrared thermometer to measure the temperature at the thermostat housing and the upper and lower radiator hoses. You can also use an OBD-II scanner to read the live data for the Engine Coolant Temperature (ECT) sensor. This gives you a digital, accurate reading.
| Symptom | Upper Radiator Hose Feel | Engine Temp Gauge Reading | Likely Thermostat Status |
|---|
| Normal Operation | Stays cool, then gets hot suddenly | Rises to midpoint and stays | Functioning correctly |
| Stuck Open | Gets warm gradually, very quickly | Rises slowly, may not reach normal temp | Stuck Open |
| Stuck Closed | Remains cool even when engine is hot | Climbs into the red (overheating) | Stuck Closed |
| Weak Spring | Gets warm slowly, not a distinct hot/cold | Fluctuates below normal temp | Stuck Partially Open |
Always use caution when working with a hot engine and cooling system to avoid burns. If you confirm the thermostat is faulty, replacement is the only solution.
