Cooling
Thermostat
A small valve that keeps the engine at operating temperature by controlling coolant flow to the radiator.
Free guide
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Cooling system flow
Interactive — tap a partTap a part to see how it fits in the system.
What it does
- Blocks coolant flow until the engine warms up
- Opens around 82–92 °C to allow cooling
- Keeps the engine at the ideal temperature
When it fails
- Engine takes forever to warm up
- Overheating after warm-up
- P0128 code (thermostat below temperature)
- Weak cabin heat in winter
How to diagnose
- Feel both radiator hoses after warm-up: both hot means it opens
- Measure coolant temperature live with a scanner
- Test the thermostat in hot water
Maintenance
- Replace the thermostat with the coolant service
- Use the correct temperature rating
Use the diagnosis order. When a part fails, confirm each hypothesis with measurements before replacing anything — codes and symptoms overlap.