It is a spring and a wax pellet, and it can overheat a $10,000 engine or keep it forever cold.
Stuck closed cooks; stuck open confuses
The thermostat decides when coolant flows. Stuck closed, the engine overheats in minutes — the classic tow call. Stuck open is sneakier: the engine never fully warms, the heater is lukewarm, mileage quietly drops, and the computer eventually sets a code. Both versions are the same small part, and replacing it on an aging cooling system is routine driveway work.
Quick tips
- Temp gauge normally settles mid-range and stays — watch for drift either way
- Replace the thermostat during any big cooling repair while the system is open
- After any coolant work, air pockets can mimic thermostat failure — burping matters
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