The thermostat, also known as the temperature regulator, is a core component of a vehicle's cooling system. Its primary function is to intelligently adjust the coolant circulation path based on engine temperature, helping the engine quickly reach and maintain its optimal operating temperature range. A malfunctioning thermostat can easily cause engine overheating or prolonged low-temperature operation, both of which can cause irreversible damage to the engine. Below are detailed methods to check the thermostat's working condition.

I. Preliminary Symptom Observation

Before performing a disassembly inspection, you can preliminarily determine whether the thermostat has issues by observing the following abnormal symptoms:

1. Engine Overheating: If the thermostat is stuck in the closed position, the coolant circulation path is blocked, preventing effective heat dissipation and causing the engine temperature to rise rapidly.

2. Slow Engine Warm-Up: When the thermostat is stuck in the open position, the coolant remains in the large circulation mode, rapidly carrying away a large amount of heat, causing the engine to warm up much slower than normal.

3. Abnormal Temperature Gauge Fluctuations: If the temperature gauge shows erratic readings (suddenly high or low), this may indicate an unstable thermostat that is frequently opening and closing, disrupting normal coolant circulation.

4. Weak Cabin Heating: In cold weather, if the cabin heating system fails to produce sufficient warmth, it is likely due to a faulty thermostat causing the engine to operate at too low a temperature, unable to provide enough heat for the heating system.

II. Practical Inspection Methods

Method 1: Cold Start Observation

1. Ensure the engine is completely cooled (best checked after leaving the vehicle overnight).

2. With the engine cold, open the radiator cap (Important: Never open the radiator cap when the engine is hot, as high-temperature coolant may spray out and cause severe burns).

3. Observe the coolant flow:

• Under normal conditions, there should be no noticeable coolant flow in the first few minutes after a cold start, as the thermostat remains closed, allowing only small-loop circulation.

• As the engine warms up, when the temperature reaches the thermostat’s opening threshold (typically 82–95°C, varying by vehicle model), the coolant should begin flowing.

• If coolant flows immediately after startup, the thermostat may be stuck open.

• If the engine temperature rises significantly but coolant still does not flow, the thermostat may be stuck closed.

Method 2: Temperature Measurement

1. Use an infrared thermometer or contact temperature probe. 

2. After a cold start, measure the temperatures of the upper and lower radiator hoses:

• Initially, the upper hose should gradually warm up, while the lower hose remains relatively cool.

• Once the engine reaches normal operating temperature, both hoses should stabilize at similar temperatures.

• If both hoses heat up simultaneously, the thermostat may be stuck open, forcing continuous large-loop circulation.

• If the upper hose becomes very hot while the lower hose stays cold, the thermostat may be failing to open, preventing proper coolant circulation. 

Note: This method has limitations, as other cooling system issues can mimic these symptoms. Combine with other tests for accuracy.

Method 3: Bench Test (Most Accurate)

1. Carefully remove the thermostat following the vehicle’s service manual.

2. Prepare a container of cold water and a heat source.

3. Submerge the thermostat in cold water and verify it is fully closed.

4. Gradually heat the water while monitoring the temperature with a thermometer.

5. Observe:

• The temperature at which the thermostat begins to open and when it is fully open. Compare these values to the rated opening temperature stamped on the thermostat.  

• Check the valve’s travel distance (typically 8–10mm when fully open).
  

6. Stop heating and allow the water to cool. Verify the thermostat reseals completely.

III. Precautions

1. Prioritize Safety: Never open the radiator cap on a hot engine. High-pressure coolant can cause severe burns.

2. Specifications Vary: Thermostat opening temperatures differ by vehicle model. Consult the service manual or a professional for exact specifications.

3. Inspect Related Components: If the thermostat is faulty, also check coolant condition (level, freeze point, pH) and the entire cooling system for leaks or blockages.

4. Replacement Tips: Always replace the thermostat gasket and ensure correct installation orientation to prevent leaks.

5. Bleed the System: After replacement, purge air from the cooling system to avoid airlocks and check for leaks.

IV. Professional Recommendation

For owners unfamiliar with automotive repair, we recommend visiting a certified mechanic if a thermostat issue is suspected. Though small, the thermostat critically impacts engine performance and longevity. Include cooling system inspections in regular maintenance and adhere to the manufacturer’s recommended replacement intervals for coolant and thermostats to prevent failures.

Summary

By following these methods, you can preliminarily assess the thermostat’s condition. If a fault is confirmed, replace it promptly to avoid further engine damage.