How to know if a Subaru thermostat is bad?
A Subaru thermostat is bad if you notice overheating, running unusually cold, weak cabin heat, or a fluctuating temperature reading. Leaks around the thermostat housing or abnormal pressure in the cooling system are also red flags.
What a Subaru thermostat does
The thermostat sits between the engine and radiator and opens and closes to regulate coolant flow. When the engine is cold, it stays closed to help the engine reach operating temperature quickly. Once the engine reaches its normal temperature, the thermostat opens to let coolant circulate to the radiator. In many Subaru engines, the opening temperature is around 180–195°F (82–88°C). A faulty thermostat can fail in two common ways: it may stay closed, causing rapid overheating, or it may stick open, preventing the engine from reaching proper temperature and reducing heater efficiency. A bad thermostat can also affect fuel economy and emissions because the engine runs outside its designed temperature window.
Common signs a Subaru thermostat may be bad
These symptoms are commonly associated with thermostat issues, though other cooling-system problems can produce similar results. Consider them as the leading indicators to test further.
Two common failure modes
Thermostat stuck closed: the engine overheats quickly, especially in traffic or when idle. Thermostat stuck open: the engine runs cooler than normal and the heater may not produce sufficient warmth. In some cases the valve can partially open, causing erratic temperature behavior.
- Engine overheats or runs hotter than normal, particularly at idle or in slow traffic.
- Temperature gauge climbs to high or fluctuates instead of staying steady.
- Engine takes a long time to warm up, and the heater produces weak or no warm air.
- Coolant leaks around the thermostat housing or hoses, or a sweet coolant smell is present.
- Check Engine Light appears with coolant-temperature-related codes or indicators.
- Steam or visible coolant boiling from under the hood when the engine is hot.
These signs can also be caused by other cooling-system problems such as a faulty radiator cap, a failing water pump, a clogged radiator, or air pockets in the system. A thorough check is needed to confirm the thermostat as the root cause.
How to diagnose a Subaru thermostat at home
Before you begin, ensure the engine is cool and the cooling system is depressurized. Do not open a hot radiator cap, and follow the model-specific service guidelines. The steps below help you determine whether the thermostat is the likely fault.
- Inspect the cooling system for leaks, damaged hoses, and a swollen or cracked thermostat housing.
- Run the engine with the radiator cap off (only when cool) and monitor the temperature gauge as the engine warms up. If the gauge rises rapidly and then stabilizes at a high temperature, the thermostat may be sticking.
- Feel the upper radiator hose after the engine reaches operating temperature. It should become hot as coolant circulates; if it remains cool while the gauge shows normal operating temperature, the thermostat may not be opening properly.
- Check cabin heater performance. If the heater never gets warm or takes too long to heat, the thermostat may be stuck open, preventing the engine from reaching temperature.
- Do a bench test of the thermostat: remove it from the engine and place it in hot water that simulates operating temperature (around 180–195°F / 82–88°C). It should start to open at the rated temperature and progressively open as the water heats. If it does not open or opens too slowly, replace it.
- Consider a professional coolant system pressure test or diagnostic scan to rule out other causes such as a faulty coolant-temperature sensor, water pump, or trapped air pockets in the cooling system.
Note: Subaru engines may require a careful bleed procedure to remove air pockets from the cooling system. Refer to the specific model-year service manual for guidance; improper bleeding can produce misleading temperature readings and misdiagnosis.
When to replace the thermostat and what to expect
If tests indicate a faulty thermostat, replacement is the reliable fix. Replace the thermostat with a new unit and gasket, and inspect the housing for wear. Many Subaru thermostats are straightforward to replace with basic hand tools. It’s wise to replace the coolant at the same time and to dry-bleed the system after installation to avoid air pockets.
Costs vary by model and region, but thermostat replacement is a mid-range repair. Labor and parts depend on whether you do it yourself or have a shop perform the work.
Summary
A bad Subaru thermostat typically presents as overheating, running excessively cold, poor heater output, or a fluctuating temperature gauge, often with coolant leakage around the thermostat housing. Start with symptom review, then perform basic at-home checks (hose temperature, gauge behavior, heater performance), and consider a bench-test of the thermostat. If confirmed, replacing the thermostat (and coolant) provides a reliable fix. When in doubt, seek professional assistance to ensure proper bleeding and cooling-system function.
