The most reliable way to fix a car thermostat is to replace it. A thermostat is a simple, relatively inexpensive component, and diagnosing a faulty one is often more time-consuming than the replacement itself. If your car is overheating, especially in city driving, or is slow to reach normal operating temperature on the highway, a stuck thermostat is a likely culprit. Before starting, ensure the engine is completely cool to avoid burns from hot coolant.
Diagnosis is Key
First, confirm the thermostat is the problem. After the engine is cool, start the car and feel the upper radiator hose. As the engine warms up, the hose should remain cool until the engine reaches its operating temperature (typically around 195°F or 90°C), at which point the thermostat opens and the hose should quickly become hot. If the hose gets hot immediately, the thermostat is stuck open. If it never gets hot, the thermostat is stuck closed, causing overheating.
Replacement Process
You'll need a new thermostat, a new gasket or O-ring, and fresh coolant.
- Drain Coolant: Place a drain pan under the radiator and open the drain valve, or loosen the lower radiator hose to drain a portion of the coolant.
- Locate and Remove Housing: The thermostat housing is usually where the upper radiator hose connects to the engine. Remove the bolts securing it.
- Swap Components: Carefully remove the old thermostat, noting its orientation. Clean the mating surface on the engine and install the new thermostat with the new gasket. The spring side typically faces the engine.
- Reassemble and Refill: Reinstall the housing, refill the cooling system with a 50/50 mix of coolant and distilled water, and bleed any air from the system by running the engine with the radiator cap off until the thermostat opens and coolant circulates.
| Common Thermostat Failure Symptoms & Data | |
|---|
| Symptom | Indicates Thermostat is... |
| Engine overheats quickly in traffic | Stuck Closed |
| Heater blows cold air, temp gauge reads low | Stuck Open |
| Erratic temperature gauge fluctuations | Sticking Intermittently |
| Coolant leaks from thermostat housing | Failed Gasket/Seal |
| Poor fuel economy | Stuck Open (engine runs too cool) |
| Typical Replacement Cost (Part) | $20 - $80 |
| Typical Replacement Cost (Professional Labor) | $100 - $300 |
