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If you drive a Subaru Outback, a correctly functioning thermostat matters for engine performance and cabin comfort. A thermostat that opens or closes at the wrong temperature can cause overheating, poor heater output, or inefficient fuel use. This guide highlights the top shortlisted thermostats selected for fit and reliable temperature regulation, and it explains the practical criteria used to evaluate them. Read on to understand what to look for and which characteristics tend to matter most when choosing a replacement thermostat.
Looking for other parts? See all Subaru Outback parts & accessories.
Top Picks at a Glance
| Best overall The best all-around choice for most situations | Subaru Engine Coolant Thermostat 21200AA072 with Gasket 21236AA010 |  |
| Best quality Built to a higher standard if you want something more solid | Subaru Engine Coolant Thermostat and Gasket Set (21200AA072 & 21236AA010) |  |
| Best Value The sweet spot between price and what you get | Subaru Engine Coolant Thermostat (21200AA072) – Genuine Subaru |  |
| Best budget The cheapest option that still gets the job done reliably | A-Premium Engine Coolant Thermostat with Seal for Subaru Outback |  |
| Highest Rated A top-rated pick with lots of positive feedback | Subaru Thermostat Assembly 21200AA170 (Genuine) |  |
| Most popular A crowd favorite that many people choose with confidence | Subaru Thermostat and Gasket Kit (21210AA080) |  |
| Best Seller A high-volume bestseller chosen again and again | AISIN THF-003 Engine Coolant Thermostat with Gasket |  |
1. Subaru Engine Coolant Thermostat 21200AA072 with Gasket 21236AA010
| Brand | Subaru |
|---|---|
| Manufacturer Part Number | 21200AA072S |
| Includes | Thermostat and gasket |
| Applicable models | Multiple Subaru models listed in description |
This is the factory-quality Subaru engine coolant thermostat sold with its mating gasket as a direct replacement for compatible Subaru models. It installs in place of the original thermostat to return the cooling system to the manufacturer's intended operating temperature, which helps regulate fan operation, reduce overheating risk, and prevent prolonged high-temperature exposure that can damage seals and sensors. Buyers report it fits as described and brings temperatures back to the expected range, improving drivability and engine longevity compared with some aftermarket units. The package is simple and focused on function rather than extras, and because it is a genuine Subaru part you get the factory fit and temperature calibration. Limitations are that you must confirm the correct thermostat rating for your specific engine and year before installing, and it does not include additional hardware beyond the gasket so other seals or hoses should be inspected and replaced separately if worn.
Best for: Balanced buyers who prioritize a reliable, factory-quality repair and want to restore correct cooling behavior without experimenting with aftermarket alternatives.
Less Ideal for: Drivers looking for an upgraded or performance thermostat with different temperature ratings or users who prefer a full repair kit that includes extra hoses or hardware.
- Genuine Subaru part with factory fit and calibration
- Includes the matching gasket for a proper seal during installation
- Direct-fit replacement for many Subaru models listed in the description
- Helps restore correct engine operating temperature and reduce cooling-related issues
- Does not specify thermostat opening temperature on the product page so cross-checking with the vehicle is necessary
- Contains only the thermostat and gasket so additional maintenance parts may be needed during service
Verdict: A factory Subaru thermostat and gasket that gives a dependable, direct-fit replacement to restore correct cooling system temperature control.
2. Subaru Engine Coolant Thermostat and Gasket Set (21200AA072 & 21236AA010)
| Item model number | 21200AA072 & 21236AA010 |
|---|---|
| Fits models | Subaru Outback 3.6L-H6 (listed fitments include multiple Subaru models) |
| Part type | Thermostat & gasket set |
This genuine Subaru thermostat and gasket set is a factory-quality replacement that restores correct coolant regulation and sealing when the original thermostat fails. It installs in the vehicle's coolant housing to control engine temperature and comes with the matching gasket for a proper seal, avoiding leaks that can accompany reused gaskets. Buyers report it returns operating temperature to the expected range, which helps maintain engine efficiency and prevents overheating. Being a Subaru part means fitment and function match factory expectations, which simplifies installation for DIYers and technicians familiar with Subaru cooling systems. The main limitation is that this is a direct replacement part rather than an upgraded performance thermostat with a different opening temperature or heavy-duty materials, so it will not change thermostat behavior or cooling thresholds beyond factory design. If you need a different spring curve or a thermostat tuned for increased cooling on track vehicles, choose an aftermarket performance unit instead.
Best for: Performance-oriented Subaru owners focused on reliable, factory-spec cooling performance who want parts that fit and behave like the original on road and spirited driving.
Less Ideal for: Drivers looking to lower or raise thermostat opening temperature for racing or heavy-duty cooling upgrades should consider a purpose-built aftermarket performance thermostat instead.
- Genuine Subaru part for factory-fit compatibility and reliable operation
- Includes matching gasket to ensure a proper seal and reduce leak risk
- Restores correct engine operating temperature for normal driving and engine efficiency
- Not a performance or modified-temperature thermostat so it does not alter cooling characteristics
Verdict: A factory Subaru thermostat and gasket set that restores original cooling behavior and fitment but does not provide performance tuning.
3. Subaru Engine Coolant Thermostat (21200AA072) – Genuine Subaru
| Manufacturer Part Number | 21200AA072 |
|---|---|
| Manufacturer | Subaru |
| Model Name | WRX 2002-2014 / STI 2004-2020 |
| OEM Part Number | 21200 AA072 |
This is the factory-quality Subaru engine coolant thermostat (part number 21200AA072) designed to replace a failed or aging thermostat in compatible Subaru models. It regulates coolant flow by opening at the specified temperature to allow normal engine warm-up and then maintain operating temperature, which helps prevent overheating and restore proper heater performance. As a genuine Subaru part it matches factory fit and function, so installation is straightforward and it integrates with the original thermostat housing. Practical benefits include reliable temperature control, long-term durability expected from an OEM part, and widespread fitment across many Subaru engines listed in the product details. The main limitation is that it ships without a replacement gasket, so you should source a gasket or replace the housing gasket during the job to ensure a proper seal. Also, if you prefer an aftermarket kit that includes hardware and gasket, this OEM-only option may require an extra purchase.
Best for: Practical optimizers who value a factory-quality, no-surprises replacement that restores proper engine temperature control without experimenting with aftermarket brands.
Less Ideal for: Buyers looking for an all-in-one repair kit with gasket and extras or those seeking lower-cost aftermarket bundles that include installation parts.
- Genuine Subaru part built to factory specifications for reliable fit and function.
- Broad listed compatibility with many Subaru models and engine variants.
- Proven durability and consistent temperature regulation compared with unknown aftermarket units.
- Does not include a thermostat gasket or installation hardware.
- Limited to the specific Subaru part number so no additional accessories are provided.
Verdict: Choose this genuine Subaru thermostat if you want a factory-fit, reliable replacement for Outback cooling systems but plan to buy a gasket separately.
4. A-Premium Engine Coolant Thermostat with Seal for Subaru Outback
| Start to Open Temperature | 82°C |
|---|---|
| Gasket or Seal Included | Yes |
| Material | Metal and Rubber |
This A-Premium thermostat is a direct replacement designed to open at 82°C and includes a gasket, so it installs as a single-unit swap for compatible Subaru Outback engines. It regulates coolant flow to help the engine reach and maintain operating temperature, which aids warm-up, heater performance, and prevents overheating. The unit combines metal and rubber components and ships in a boxed package with one thermostat and seal. For budget-focused repairs it offers a simple way to restore temperature control without buying an OEM part. Expect typical aftermarket fitment work such as checking housing surfaces and using the included seal to avoid leaks. The main limitations are that it is an aftermarket part rather than factory-branded and longevity depends on installation quality and cooling-system condition, so replacing associated hoses or coolant for older Outbacks is wise when doing the job.
Best for: Budget-conscious owners looking for an affordable replacement thermostat to restore proper engine temperature control on a Subaru Outback and finish the repair quickly without sourcing additional seals.
Less Ideal for: Drivers seeking a factory-identical or premium long-life thermostat should consider higher-end or OEM options instead.
- Includes gasket/seal so no separate seal purchase is required
- Start-to-open temperature specified at 82°C for predictable thermostat behavior
- Boxed single-unit packaging simplifies replacement jobs
- Metal and rubber construction follows common thermostat build for durability
- Aftermarket part rather than factory-branded unit which may vary from OEM tolerances
- Limited long-term durability data compared with established name-brand thermostats
Verdict: A cost-effective, boxed replacement thermostat with an 82°C opening and included seal that gets an Outback's cooling system working again without a big expense.
5. Subaru Thermostat Assembly 21200AA170 (Genuine)
| Manufacturer | Subaru |
|---|---|
| Model | Thermostat |
| Item model number | 21200AA170 |
| Manufacturer Part Number | 21200AA170 |
This genuine Subaru thermostat (part number 21200AA170) replaces the factory unit to regulate engine coolant flow and help maintain proper operating temperature. Installed in the cooling circuit, it opens and closes at the designed temperature to prevent overheating and aid efficient warm-up. As a factory replacement, the unit matches OEM fit and function so it installs like the original and restores correct temperature control without modification. Practical benefits include predictable performance, compatibility with Subaru service procedures, and long-term reliability when paired with correct seals and installation practices. Note that some listings do not include the sealing gasket so you may need to purchase the gasket (21236AA010) separately and follow standard coolant system bleeding steps after fitting. This part is focused on like-for-like replacement rather than performance tuning or modified cooling setups.
Best for: Reliability-focused buyers who prioritize factory-quality replacements to maintain a Subaru Outback's original cooling performance and minimize future cooling-related problems.
Less Ideal for: Owners seeking performance thermostats, adjusted opening temperatures, or parts for heavily modified cooling systems should consider aftermarket specialized units instead.
- Genuine Subaru part for direct factory-fit replacement
- Matches original fit and function for straightforward installation
- Consistent temperature regulation to prevent overheating and support engine longevity
- Durable machined exterior consistent with factory manufacturing quality
- Gasket may be sold separately requiring an additional purchase
- Not intended for performance or modified cooling systems
Verdict: If you want a factory-quality, direct-replacement thermostat for a Subaru Outback, this genuine Subaru unit delivers reliable, OEM-fit temperature control.
6. Subaru Thermostat and Gasket Kit (21210AA080)
| Item model number | 21210AA080 |
|---|---|
| Manufacturer | Subaru |
| OEM Part Number | 21210 AA080 |
This is a factory-quality thermostat and gasket kit made to match Subaru specifications for fit and operating temperature. It replaces a failing thermostat to restore correct coolant flow and stable engine operating temperature, and is typically installed during cooling-system repairs or when diagnosing overheating and erratic gauge readings. The kit uses Subaru parts so the valve setpoint and housing fit the original location without modification, which simplifies installation and reduces the chance of leaks when paired with proper gasket installation. Practical benefits include predictable temperature control that keeps the engine in its designed operating range and the convenience of an exact-fit replacement so you avoid trimming or adaptation. Limitations are that it is a direct replacement only for compatible Subaru engines and does not include other cooling parts like hoses or a new water pump, so a wider cooling-system overhaul will require additional purchases.
Best for: Buyers who prioritize convenience and reliability and want a hassle-free, exact-fit replacement that restores factory cooling behavior with minimal troubleshooting.
Less Ideal for: Those rebuilding or upgrading an entire cooling system or seeking aftermarket performance thermostats with altered temperature setpoints should consider other options.
- Genuine Subaru part for factory-fit and correct valve setpoint
- Includes gasket to reduce risk of leaks when installed properly
- Simple direct replacement speeds installation and avoids modification
- Only replaces the thermostat and gasket so additional cooling components may still be needed
Verdict: A convenient, factory-spec thermostat kit for Subaru owners who want an exact-fit replacement to restore proper engine temperature control.
7. AISIN THF-003 Engine Coolant Thermostat with Gasket
| Manufacturer Part Number | THF-003 |
|---|---|
| OEM Part Number | 21210-AA080 |
| Material | Stainless Steel |
| Included Components | Thermostat and O-Ring |
The AISIN THF-003 thermostat replaces the vehicle's temperature-regulating valve and comes with a gasket for a complete swap. Designed to match factory operating temperature, it installs as a direct replacement in compatible Subaru models and is intended for routine cooling-system maintenance or repairs that follow timing-belt or head-gasket service. The part is machined stainless steel and marketed as OEM-match, which helps ensure correct fit, consistent opening temperature, and reliable coolant flow when the engine reaches operating temperature. Buyers report straightforward installation and stable cabin heat and engine temperature after replacement. Some users noted occasional quality-control issues or fitment errors when the wrong engine variant was selected, so confirm exact engine fitment before ordering. This thermostat is best for drivers who want a like-for-like replacement that restores original cooling behavior without changing the engine's thermal settings.
Best for: Risk-averse mainstream buyers who prioritize factory-style reliability and want a straightforward, like-for-like replacement to restore normal cooling performance.
Less Ideal for: Those looking to change engine warm-up characteristics or fit a universal performance thermostat should consider performance-oriented alternatives or consult a specialist.
- Factory-matched operating temperature for consistent engine warm-up and cooling
- Includes gasket for a more complete replacement job
- Machined stainless-steel construction for durability
- Direct-fit replacement that simplifies installation compared with aftermarket universal units
- Fitment depends on correct engine selection and some buyers reported receiving an incorrect or damaged unit
Verdict: Aisin THF-003 delivers a factory-matched, direct-replacement thermostat with gasket that restores original cooling behavior for compatible Subaru models.
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Select your Subaru Outback configuration to see the most relevant Thermostats recommendations
Choosing the Right Subaru Outback Thermostat: Key Factors to Consider
Correct Fitment and Mounting
Accurate physical fit is the first requirement because the thermostat must seat properly in the Subaru Outback housing to create a reliable seal and function. If the flange dimensions, bolt pattern, or overall height differ, you can see leaks or poor thermostat operation.
Check for components that list vehicle year ranges and include necessary gaskets or O-rings. Proper fit reduces installation time and lowers the risk of coolant loss from an improperly sealed housing, which can lead to overheating or air pockets in the cooling circuit.
Opening Temperature and Tolerance
A thermostat opens at a specified temperature to allow coolant flow; the target opening temperature and how tightly the part holds that spec affect engine warm-up and operating temperature. Parts that open too early can cause the engine to run cold and reduce heater performance, while those that open late can increase the risk of overheating.
Look for thermostats with tight calibration tolerances and documentation of their activation range. Consistent opening behavior helps maintain stable engine temperature under different driving conditions and during rapid changes in load or ambient temperature.
Seal and Gasket Quality
The seal between the thermostat and housing prevents coolant leakage and maintains system pressure. Poor-quality gaskets or seals can degrade quickly, especially where dissimilar metals or age-related warping are present, leading to weeping or larger leaks at the thermostat housing.
Choose components that include robust gaskets or recommend compatible sealants. Consider the material resilience to coolant chemistry and thermal cycling so the seal maintains integrity over repeated heat-up and cool-down cycles.
Material Durability and Corrosion Resistance
Thermostats operate in a harsh environment with hot coolant, contaminants, and corrosion-promoting conditions. The valve, housing plate, and fasteners should resist corrosion and wear to avoid sticking, erosion, or catastrophic failure over time.
Metals and coatings that withstand electrochemical degradation and scale formation help preserve movement and responsiveness. Durable materials reduce the likelihood of premature replacement and contribute to consistent temperature control across the part's service life.
Valve Responsiveness and Movement
How quickly and smoothly the thermostat valve opens and closes affects transient engine temperatures and heater output. Sticky or sluggish valves can cause temporary overheating or delayed warm-up of the cabin, particularly during stop-and-go traffic or repeated short trips.
Look for designs that demonstrate low friction movement and reliable spring tension. Proper responsiveness helps maintain thermal stability during sudden load changes, improving drivability and protecting engine components from thermal stress.
Ease of Installation and Serviceability
A thermostat that is straightforward to install reduces labor time and the chance of installation errors that could cause leaks or misalignment. Consider whether the part includes necessary gaskets, seals, and clear instructions, and whether it fits within the typical access available on the Subaru Outback engine bay.
Service-friendly features such as standard fastener sizes and clear orientation marks help ensure correct installation. Easy replacement also means routine maintenance can be completed with lower effort, minimizing vehicle downtime and complexity for technicians or DIYers.
How We Chose the Best Subaru Outback Thermostat
We evaluated thermostats for Subaru Outback using criteria that reflect real-world service and longevity. Key filters included correct fitment and OEM-style mounting dimensions, consistent opening temperature tolerance, housing and seal quality to reduce the risk of coolant leaks, and material durability against corrosion and scale. We also considered valve responsiveness under varying engine loads and cold-start conditions, as well as compatibility with the vehicle cooling system and thermostat housing design.We excluded units that lacked clear fitment documentation or that used nonstandard mounting hardware requiring modifications. We also set aside very short-lifespan designs and parts without explicit temperature calibration data. The resulting shortlist prioritizes components that balance accurate temperature control, long-term seal integrity, and ease of installation for typical service scenarios.
Our full evaluation process is outlined in our review methodology.
FAQ
What are common signs of a bad thermostat in my Subaru Outback?
Common signs include engine overheating, inconsistent cabin heat, or the engine taking too long to reach operating temperature. You may also notice temperature gauge fluctuations or coolant leaks at the thermostat housing. These symptoms often prompt inspection or testing of the thermostat.
How can I test a thermostat without removing it?
You can monitor engine warm-up behavior and observe the upper radiator hose for temperature change as the engine reaches operating temperature. Another approach is to use an infrared thermometer to check when coolant begins circulating. These noninvasive checks give an indication but are less definitive than bench testing a removed unit.
Can a thermostat cause my engine to overheat?
Yes, a thermostat that is stuck closed or opens at too high a temperature can prevent coolant flow and contribute to overheating. Overheating often develops progressively, so addressing early signs helps avoid more serious engine damage. Other cooling system faults can cause similar symptoms, so comprehensive diagnosis is useful.
Will replacing the thermostat fix low cabin heat?
Often it will because the thermostat controls engine warm-up rate and coolant flow to the heater core. If the thermostat opens too early or sticks open, the engine may run too cool and the heater output will be weak. However, low cabin heat can also stem from air in the system or issues with the heater core or blower.
What should I look for when checking the thermostat housing for leaks?
Inspect the housing flange and gasket area for coolant residue, corrosion, or visible gaps. Pay attention to softened or compressed gaskets and any signs of pitting on mating surfaces. A tight, evenly seated gasket and clean mating surfaces reduce the chance of a housing seal leak.
How often should the thermostat be replaced on a Subaru Outback?
There is no fixed interval for all vehicles because lifespan depends on driving patterns and coolant condition. Thermostats are commonly replaced during major coolant service or when symptoms appear. Regular cooling system maintenance and monitoring can help determine the right timing for replacement.
Conclusion
Prioritize correct fitment, reliable opening temperature, and good seal quality when choosing a Subaru Outback thermostat. Those factors most directly affect temperature stability, cabin comfort, and the risk of coolant leaks. Balance material durability and valve responsiveness against ease of installation to match your service needs.Ultimately, selecting a thermostat that fits precisely and holds its calibration will minimize follow-up service and help maintain consistent engine operating temperature. Consider the tradeoffs between longevity and serviceability based on how you use the vehicle and how you prefer to handle maintenance.
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