What are the symptoms of a faulty thermostat in a Jetta?

[JamesonRose]

Jetta thermostat failure symptoms: 1. Coolant temperature exceeds 110 degrees. After stopping and turning off the engine, open the hood and feel the upper and lower coolant hoses in the radiator. If there is a significant temperature difference between the two hoses, it indicates a thermostat malfunction; 2. The coolant temperature fails to reach normal operating temperature for an extended period. Normally, within about five minutes after a cold start, the coolant temperature should reach 85 to 105 degrees. If it takes much longer to reach this temperature, it indicates a thermostat issue. The thermostat automatically adjusts the amount of water entering the radiator based on the coolant temperature, altering the water circulation range to regulate the cooling system's heat dissipation capacity, ensuring the engine operates within the appropriate temperature range.