A Practical Guide to the Engine Coolant Thermostat for the 2016-2023 Subaru Impreza
Understand the signs of a failing thermostat and learn how to choose the right replacement for your Impreza's engine.
- A failing thermostat most often causes overheating (stuck closed) or a P0128 check engine light (stuck open).
- For reliability, choose a genuine Subaru thermostat or an aftermarket one from an OEM supplier like Aisin.
- Always use a thermostat with the factory-specified temperature rating to avoid performance issues and check engine lights.
- When replacing the thermostat, it's best practice to also drain and refill the coolant.
Is Your Impreza's Thermostat Failing?
A thermostat is a simple part, but when it fails, it can cause noticeable problems. It can fail in two ways: stuck open or stuck closed. Here are the common signs to watch for.
Stuck Closed: Engine Overheats
This is the most serious type of failure. If the thermostat is stuck closed, it traps coolant in the engine, preventing it from reaching the radiator. This will cause your engine to overheat quickly. You'll see the temperature gauge climb into the red, and you may get a warning light. If this happens, pull over safely and turn off the engine to prevent serious damage.
Stuck Open: Engine Runs Too Cool
If the thermostat is stuck open, it allows coolant to flow to the radiator all the time. This makes it very difficult for your engine to warm up to the correct operating temperature, especially in cold weather or at highway speeds. The most common symptom is a Check Engine Light with code P0128 (Coolant Temperature Below Thermostat Regulating Temperature). You may also notice your cabin heater isn't blowing hot air and your fuel mileage gets worse.
Known Issues for 2016-2023 Subaru Impreza
While there are no widespread recalls for the thermostat itself on these models, there are a couple of related items to be aware of:
- 2017 Impreza ECU Recall (WTP-75): Some 2017 models were recalled for an engine control unit (ECU) software update. The issue was related to the radiator fan's operation with winter-blend fuels, which could cause stalling. This was not a physical thermostat failure, but an important cooling system-related update for that model year.
- P0128 Check Engine Light: This code is very common on Subarus when the thermostat fails in the open position. If you see this code, the thermostat is the most likely culprit.
Choosing the Right Aftermarket Thermostat
For a critical part like a thermostat, quality matters. Using a part with the wrong temperature rating or one that fails quickly can cause engine problems. Here’s a breakdown of available aftermarket brands.
OEM vs. Aftermarket Quality
Many Subaru specialists recommend using a genuine OEM thermostat because they are built to exact specifications. However, some aftermarket brands are OEM suppliers, offering the same quality without the dealership price tag.
- Tier 1: Aisin
Aisin is a well-respected Japanese company that is an OEM supplier for Subaru and many other carmakers. Choosing an Aisin thermostat is the closest you can get to buying a genuine Subaru part. They are known for their reliability and precise fit, making them a top recommendation. - Tier 2: Gates
Gates is a major name in aftermarket parts, especially for belts and cooling components. They are generally considered a reliable choice and a good balance between cost and quality for Subaru owners. - Tier 3: Motorad
Motorad is a budget-friendly option. While they meet basic standards, some professional mechanics and users have reported inconsistent quality and a shorter lifespan compared to OEM or Aisin parts. It can be a suitable choice if budget is the primary concern, but be aware of the mixed reviews.
Replacement Cost for a 2016-2023 Impreza Thermostat
The cost to replace the thermostat is reasonable, but labor can vary because the part is located at the bottom of the engine.
| Part/Service | Estimated Cost |
|---|---|
| New Aftermarket Thermostat | $20 - $57 |
| New OEM Thermostat | $23 - $35 |
| Shop Labor | $150 - $300 |
| Total Professional Job | $175 - $350 |
Costs are estimates and can vary by location and repair shop. It is highly recommended to replace the engine coolant at the same time.
Frequently Asked Questions
What temperature thermostat does my Impreza need?
You must use a thermostat with the same temperature rating as the original factory part. For the 2016-2023 Impreza, this is typically around 189-195°F. Using a lower temperature thermostat can trigger a P0128 check engine light and hurt fuel economy. Always confirm the spec for your specific vehicle.
Can I replace just the thermostat, or do I need the housing?
For this generation of Impreza, the thermostat is a separate part from the housing. In most cases, you only need to replace the thermostat and its gasket. The housing only needs replacement if it is cracked, warped, or heavily corroded.
What other parts should I replace at the same time?
It is a perfect time to do a coolant flush and fill with new Subaru Super Coolant. You should also inspect the radiator hoses connected to the thermostat housing. If they feel brittle, soft, or look swollen, replace them now to prevent future leaks.
Is an OEM thermostat really better than aftermarket for my Impreza?
While genuine Subaru parts are always a safe choice, a high-quality aftermarket part from an OEM supplier like Aisin will provide the same performance and reliability. Given that the price for a genuine Subaru thermostat is very competitive, it is often worth getting the OEM part to guarantee a perfect match.
Technical Specifications
OEM Part Numbers: 21210AA191 21210AA190 21210AA260
Always verify the correct part number and temperature rating for your vehicle's VIN. The thermostat is located on the lower radiator hose connection at the engine block.
We Have This Part in Stock
The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
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