What are the symptoms of a faulty car thermostat?

[LondonLynn]

Faulty car thermostat symptoms: It may cause abnormal increase or decrease in engine coolant temperature. Here are methods to determine thermostat failure: 1. Inspection after engine start - Open the coolant reservoir filler cap to check if there's water flow in the coolant reservoir. If not, it indicates the thermostat is damaged or foreign objects are stuck between the main valve switches. 2. Temperature check by touching upper and lower radiator hoses - Start the engine and feel the hoses after 3 minutes. With a properly functioning thermostat, the upper and lower hoses should have different temperatures, typically with the upper hose being hot and the lower hose being cool.