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A correctly functioning thermostat matters for engine longevity and cabin comfort in the Subaru Forester XT. The right Subaru Forester XT thermostat helps the engine reach and hold optimal temperature quickly, reduces wear from extended cold operation, and prevents the high-temperature conditions that can stress head gaskets and turbo components.This guide shortlists the top thermostat options for the Forester XT and explains the practical tradeoffs you should consider. We evaluated fitment, build quality, and thermal performance so you can pick a thermostat that suits your driving style and maintenance preferences. Read on for the key evaluation points and next steps for installation or diagnosis.
Looking for other parts? See all Subaru Forester parts & accessories.
Top Picks at a Glance
| Best Overall The best all-around choice for most situations | Subaru Thermostat and Gasket 21210AA221 (Genuine) |  |
| Best Quality Built to a higher standard if you want something more solid | Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010) |  |
| Best Value The sweet spot between price and what you get | Allmotorparts Engine Thermostat with Gasket (21200AA072) |  |
| Best Budget The cheapest option that still gets the job done reliably | FOXSTAR Engine Thermostat (21200AA170) for Subaru Cooling System |  |
| Highest Rated A top-rated pick with lots of positive feedback | Mishimoto Racing Thermostat for Subaru WRX/Forester XT (MMTS-WRX-15) |  |
| Most Popular A crowd favorite that many people choose with confidence | AISIN THF-003 Engine Coolant Thermostat with Gasket |  |
| Best Seller A high-volume bestseller chosen again and again | Anleecap Thermostat THF-003 (82°C / 180°F) for Subaru Forester and Select Models |  |
1. Subaru Thermostat and Gasket 21210AA221 (Genuine)
| Manufacturer | Subaru |
|---|---|
| Item model number | 21210AA221 |
| Manufacturer Part Number | 21210AA221 |
| OEM Part Number | 21210-AA221 |
This genuine Subaru thermostat and gasket (part number 21210AA221) is a factory replacement intended to restore proper coolant temperature control on compatible turbocharged Forester models and select Subaru vehicles. It replaces the thermostat housing component and includes the gasket needed for a correct seal when installing by the water pump on the listed applications. In normal use it regulates engine warm-up and prevents overcooling or overheating, which helps maintain heater performance and engine efficiency. As a factory part it matches original fit and function so installation is straightforward for experienced DIYers or technicians and it removes uncertainty about fitment that can come with aftermarket units. The main limitations are narrow model fitment and that this listing does not include any additional cooling system parts, so expect to check hoses, coolant, and the water pump separately when servicing the system.
Best for: Buyers who want a direct factory-quality replacement to restore proper coolant control on compatible turbo Forester models and prefer the confidence of a genuine OEM part.
Less Ideal for: If you need a universal thermostat, a performance-upgraded unit, or are working on an unsupported vehicle, consider aftermarket or model-specific alternatives.
- Factory genuine Subaru part for reliable fit and compatibility
- Includes gasket for correct sealing during installation
- Intended for turbocharged Forester and other listed Subaru applications so fitment is specific
- Limited to the Subaru models listed and not a universal replacement
Verdict: Choose this genuine Subaru thermostat and gasket when you want a factory-fit replacement to reliably restore coolant temperature control on the specified turbo Forester and compatible Subaru models.
2. Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010)
| Manufacturer Part Number | 21200AA072S |
|---|---|
| Includes | Thermostat and gasket |
| Compatible Models (excerpt) | Forester, Impreza, Legacy, Outback, Baja, SVX, WRX STI (see fitment years) |
This genuine Subaru thermostat and matching gasket replace the factory unit to restore proper coolant flow and temperature control. It installs in the engine cooling housing and is used when a failing thermostat is causing overheating, poor warm-up, or erratic cabin heat. As an OEM part it matches factory tolerances and sealing surfaces, which reduces fitment issues and the need for modification compared with aftermarket units. The package includes the correct gasket so you get a complete seal at installation. The main limitations are that it is a direct replacement with standard temperature calibration rather than a performance or race thermostat, so it will not change operating temperature for tuned engines, and you must confirm the exact model year fitment before ordering. For routine maintenance or repair on turbocharged Subaru engines this part offers reliable, factory-spec operation and straightforward installation for a mechanic or experienced DIYer.
Best for: Buyers who want a reliable, factory-quality replacement to restore correct coolant control on compatible Subaru engines and prefer a direct-fit OEM part over aftermarket alternatives.
Less Ideal for: Drivers seeking a lower- or higher-temperature performance thermostat for tuning or racing applications should consider dedicated performance thermostats instead.
- Factory genuine Subaru part for precise fit and compatibility
- Includes matching gasket for a complete seal at installation
- Direct-fit replacement reduces installation time and need for modifications
- Appropriate for repair and maintenance on turbocharged Subaru engines listed in the fitment
- Supplied thermostat has standard factory calibration and is not designed to alter operating temperature for performance tuning
Verdict: Choose this genuine Subaru thermostat and gasket when you need a direct-fit, factory-quality replacement to restore proper cooling performance on compatible Subaru engines.
3. Allmotorparts Engine Thermostat with Gasket (21200AA072)
| Brand | Allmotorparts |
|---|---|
| Model Name | 21200AA072 |
| Material | Stainless Steel |
| Item model number | 21200AA072 |
This Allmotorparts thermostat replaces Subaru part numbers 21200AA072, 21236AA010, and 21210AA080 and is intended to restore proper coolant flow and temperature control when a factory thermostat fails. Installation is typical of inline engine thermostats: remove the upper radiator hose, swap the unit and gasket, and reassemble while taking care to position any jiggle pin at 12 o'clock for vertical installations. The unit uses a stainless steel flange and a channel-constructed spring frame for durability and claims OE-style fit, form, and function. For day-to-day driving it returns normal warm-up times, prevents overheating once installed correctly, and helps maintain consistent cabin heat. Buyers should note this is an aftermarket part rather than a factory Subaru component and correct fit depends on confirming exact vehicle fitment; some users may need to address corroded housing studs during installation. Overall it is a pragmatic replacement that balances durability and value for routine Subaru cooling-system repairs.
Best for: Buyers who want a practical, cost-conscious replacement to restore proper engine temperature control on compatible Subaru models and prefer a durable, ready-to-install unit.
Less Ideal for: If you prioritize only genuine factory Subaru parts or need dealer-sourced components for warranty reasons, choose a manufacturer-supplied thermostat instead.
- Direct replacement for multiple Subaru part numbers to simplify ordering
- Stainless steel mounting flange and reinforced spring frame for improved durability
- Includes gasket for a single-step thermostat housing service
- Designed to match factory-style fit and operation for compatible engines
- Aftermarket component rather than a genuine factory Subaru part
Verdict: A durable, budget-focused aftermarket thermostat that restores proper coolant control for compatible Subaru engines and includes a gasket for straightforward replacement.
4. FOXSTAR Engine Thermostat (21200AA170) for Subaru Cooling System
| Manufacturer | FOXSTAR |
|---|---|
| Model | Engine Thermostat |
| Item model number | 21200AA170 |
| Exterior | Machined |
This FOXSTAR thermostat is a direct-replacement style unit made to match Subaru part 21200AA170 and several related OEM numbers, intended to restore correct coolant flow and engine operating temperature. It is constructed from stainless steel and copper with a machined housing and sealed components, so it resists corrosion and handles repeated heat cycles. Typical use is straightforward: remove the old thermostat at the top radiator hose, swap in this unit, and reinstall the housing to regain proper warm-up and prevent overheating or overcooling. The main benefits are its compatibility with many Subaru models and its simple plug-and-play fitment for someone doing a DIY coolant-system repair. Limitations are that fitment requires verifying the OEM number before purchase and installation still needs basic tools and coolant drain/refill; there is no vehicle-specific calibration or electronic components to improve temperature control beyond factory behavior.
Best for: Buyers looking for an affordable, straightforward replacement thermostat to restore normal engine temperature control on older Subaru Forester XT cars and similarly aged Subaru engines.
Less Ideal for: People seeking performance thermostats with different opening temperatures or owners who want a dealer-installed part with manufacturer-backed warranty and support.
- Matches multiple Subaru OEM part numbers for broad compatibility with older Subaru models
- Stainless steel and copper construction offers corrosion and heat resistance
- Machined exterior and sealed housing reduce leak risk when installed correctly
- Requires confirming the OEM part number to ensure correct fit before buying
Verdict: A budget-friendly, factory-style replacement that restores proper coolant flow for Subaru engines matching part 21200AA170.
5. Mishimoto Racing Thermostat for Subaru WRX/Forester XT (MMTS-WRX-15)
| Compatible vehicles | 2015+ Subaru WRX, 2014+ Forester XT |
|---|---|
| Activation temperature | 165.2°F (74°C) |
| Exterior | Machined |
| Manufacturer | Mishimoto |
The Mishimoto racing thermostat replaces the factory unit to open at a lower temperature so coolant circulates earlier and the FA20DIT stays cooler under street and track conditions. It is a direct-fit replacement for compatible Subaru WRX and Forester XT models and installs like a stock thermostat, making it suitable for routine maintenance or cooling upgrades. Key features include a machined exterior, a lower activation temperature (165.2°F versus the factory 190.4°F), and Mishimoto’s lifetime warranty. In practice this means reduced peak engine temperatures during spirited driving or towing, which can lower thermal stress and help preserve engine components. The unit is focused on thermal performance rather than altering heater output or flow characteristics beyond earlier opening. Buyers should expect a straightforward swap with the benefit of more consistent operating temperatures; those seeking aggressive track-only temperature control or detailed flow tuning may need a full cooling-system upgrade in addition to this thermostat.
Best for: Buyers who prioritize engine longevity and consistent operating temperatures for a Subaru Forester XT and want a simple, reliable upgrade that installs like a factory replacement.
Less Ideal for: Drivers focused solely on maximum track-only cooling or who plan extensive cooling-system modifications should consider more comprehensive radiator or flow upgrades instead.
- Lower opening temperature reduces peak engine temps during spirited driving
- Direct-fit replacement for 2015+ WRX and 2014+ Forester XT for simple installation
- Machined exterior construction for durable, performance-oriented build
- Backed by Mishimoto lifetime warranty for long-term reliability
- Lower activation temperature may slightly delay cabin heat until the engine warms to the new threshold
Verdict: Choose this thermostat to lower peak engine temperatures with a simple, reliable direct-fit upgrade that protects the FA20DIT in Subaru Forester XT use.
6. 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 gasket |
The AISIN THF-003 is a direct-fit replacement thermostat designed to restore proper coolant flow and temperature control in compatible Subaru applications. It installs in place of the factory unit and is supplied with a matching gasket to simplify reassembly. AISIN states the unit is manufactured to OEM-matched specifications and built from durable materials to deliver the correct opening temperature and consistent operation over time. For routine repairs or cooling-system rebuilds this thermostat removes guesswork about temperature calibration and reduces the chance of overheating or poor cabin heat. The part is straightforward to install for anyone comfortable with basic cooling system service, though access and bleeding the cooling system still require standard shop procedures. This is a single-purpose replacement thermostat with no adjustable temperature or performance tuning features, so it is intended strictly as a factory-style fix rather than an upgrade for modified engines.
Best for: Buyers who want a reliable, factory-style replacement that restores correct coolant regulation in a Subaru and value proven brand manufacturing and straightforward installation.
Less Ideal for: Drivers seeking performance thermostats with lower or higher opening temperatures or those who need cross-vehicle universal fitment should look at performance or adjustable options instead.
- OEM-matched design for accurate opening temperature and fit
- Includes gasket for a complete thermostat-to-housing seal
- Manufactured from durable materials for long-term service
- Direct replacement that simplifies routine cooling-system repairs
- Not adjustable or performance-oriented; no alternative temperature options
- Fitment limited to specific Subaru and Saab applications so verification is required
Verdict: Choose the AISIN THF-003 when you want a factory-quality, direct-fit thermostat that restores correct engine temperature control on compatible Subaru models.
7. Anleecap Thermostat THF-003 (82°C / 180°F) for Subaru Forester and Select Models
| Part numbers | THF-003, 21200-AA170, 21210-AA080 |
|---|---|
| Opening temperature | 82°C / 180°F |
| Fitment note | Forester 2004-2016 compatible |
| Includes | Thermostat and seal |
This Anleecap thermostat is a direct-fit replacement built to match the factory 82°C/180°F opening temperature used in many Subaru engines. It arrives as a complete kit with the thermostat and seal so installation is a straightforward swap during a coolant service. For everyday driving the thermostat restores correct engine warm-up and maintains stable operating temperature, which helps cabin heat performance and consistent engine efficiency. The product listing cites specific Subaru part numbers (THF-003, 21200‑AA170, 21210‑AA080) and a wide fitment range that includes Forester model years 2004–2016, which simplifies parts matching for compatible cars. Expect standard construction and a factory-spec temperature rating rather than performance or high-flow modifications, so it is best for owners wanting a reliable, OEM-style replacement. If you need premium materials or altered temperature behavior for tuning or motorsport use, this part does not provide those features.
Best for: A cautious Subaru owner who wants a direct, factory-spec replacement thermostat that restores normal temperature control without modifications.
Less Ideal for: Drivers seeking a performance thermostat with a different opening temperature or enhanced cooling for high-load track use should look elsewhere.
- Designed to match factory 82°C/180°F opening temperature for consistent engine warm-up
- Sold as a complete kit including seal for straightforward replacement
- Lists matching Subaru part numbers to simplify correct fitment
- Covers a broad range of Subaru Forester model years (2004–2016) for compatibility
- Not engineered for increased flow or altered temperature for performance applications
- Limited to the specified temperature rating and factory-style materials
Verdict: Choose this thermostat if you want a straightforward, factory-spec replacement that restores correct temperature control on compatible Subaru Forester models.
Choosing the Right Subaru Forester XT Thermostat: Key Factors to Consider
Correct Temperature Rating
Thermostat temperature rating determines at what coolant temperature the valve opens. For a turbocharged Subaru Forester XT, an appropriate opening temperature ensures the engine warms up quickly to reduce internal wear while still allowing the cooling system to prevent sustained high temperatures under boost. An incorrect rating can cause the engine to run too cool, reducing efficiency and increasing oil contamination, or run too hot, risking gasket or component damage.
Temperature calibration consistency is also important. Some thermostats open progressively over a range rather than snapping fully open at a single temperature. Understanding the thermostat's target opening temperature and its behavior across that range helps you pick a part that matches your driving conditions and climate.
Fitment and Part Number Compatibility
Accurate fitment to the Forester XT's housing and coolant passages is essential to avoid leaks and ensure reliable operation. Verify that the thermostat's dimensions, flange shape, and mounting hole pattern match the vehicle's housing. Confirming the correct part number fitment reduces risk of purchasing a unit with the wrong flange thickness or valve orientation, which can impede flow or prevent the thermostat from sealing properly.
Pay attention to whether the thermostat includes the correct gasket or O-ring, or if a separate housing seal is required. Using the proper seal materials and ensuring a flat mating surface will prevent coolant leaks and premature failures.
Housing Seal and Gasket Quality
The thermostat housing seal is a common source of coolant leaks if it is brittle, improperly sized, or absent. For the Forester XT, durable seal materials that tolerate antifreeze and temperature cycling are critical. A low-quality gasket can compress unevenly and allow coolant to seep at the thermostat flange or along the housing seam.
Inspect whether the thermostat kit supplies a new seal and whether the seal design matches the vehicle's housing style. Consider replacing the housing gasket and cleaning mating surfaces during installation to reduce the chance of a thermostat housing seal leak in the near term.
Valve Action and Build Durability
Thermostat valve action—how smoothly and reliably the valve opens and closes—affects temperature control. A sticky or poorly machined valve can stick open or closed intermittently, causing engine running cold thermostat stuck open symptoms or overheating. Look for thermostats with robust springs, corrosion-resistant components, and tight manufacturing tolerances.
Durability matters in turbocharged applications where coolant temperatures and thermal cycling are more aggressive. Materials like stainless steel bodies and high-quality wax or waxless actuators typically last longer and resist corrosion from modern antifreeze formulations.
Installation Ease and Serviceability
Some thermostat kits include the necessary seals and fasteners while others require reuse of old components. Choosing a kit that offers complete installation parts reduces labor time and the chance of leaks. Easy-to-follow fitment information and standard fastener sizes help make the replacement process straightforward for DIYers and technicians alike.
Consider whether replacing the thermostat requires partial cooling-system draining and if the vehicle needs specific torque values for housing bolts. Simpler installations lower the risk of installation errors that could lead to thermostat replacement cost increases or repeated repairs.
Thermal Performance Under Load
In a Forester XT, thermal performance under sustained load and boost is important. The thermostat should maintain steady engine operating temperature during extended highway runs and spirited driving. A thermostat that modulates smoothly will help the engine control systems manage boost and ignition timing without sudden temperature swings.
Assess whether the thermostat design supports adequate coolant flow when fully open and whether it allows consistent circulation through the radiator and heater core. Good thermal performance reduces the likelihood of thermostat symptoms overheating during heavy use.
Common Thermostat Symptoms and Early Diagnosis
Recognizing thermostat issues early can prevent engine damage. Common signs include a slow or erratic temperature gauge, heater core that blows cool air when the engine is warm, or frequent temperature spikes under load. Overheating under boost, sudden temperature jumps, or long warm-up times are all diagnostic clues.
A systematic approach helps isolate thermostat problems from other cooling-system faults. Check coolant level and condition, inspect the thermostat housing and seals for leaks, verify heater core performance, and monitor temperature changes during a controlled drive. Documenting when symptoms occur and under what conditions makes diagnosis more reliable.
- Slow warm-up: could indicate thermostat stuck open.
- Engine overheating under load: may point to thermostat stuck closed or poor thermal regulation.
- Coolant leaks at the housing: inspect for gasket failure or improper seal.
- Intermittent gauge movement: check for air pockets or a sticking valve.
How To Test A Thermostat On The Forester XT
Testing the thermostat involves observing its behavior as the engine passes through normal operating temperature. Start with a cold engine and feel upper and lower radiator hoses as the car warms up. If the thermostat is working, the upper hose will stay cool until the engine reaches the opening temperature, then both hoses will warm as coolant begins to circulate.
For a more direct test, remove the thermostat housing after draining enough coolant to avoid spills and inspect the thermostat for physical sticking or corrosion. You can bench-test thermostats by heating them in water and monitoring when the valve starts to open, but only use this method if you are comfortable with basic shop safety and have an appropriate thermometer. Reinstall with a new gasket and ensure the housing bolts are torqued to specification.
- Warm-up hose test to detect stuck-open condition.
- Controlled drive to observe temperature stability under load.
- Bench testing in heated water for valve opening verification.
Preventing Thermostat Housing Seal Leaks
To reduce the chance of a thermostat housing seal leak, always replace the gasket or O-ring when changing the thermostat. Clean both mating surfaces thoroughly and inspect the housing for warping or corrosion that can prevent a proper seal. Use the recommended seal material for the Forester XT and avoid over-tightening bolts which can crush soft gaskets.
Consider using a replacement kit that includes a new housing seal and any required fasteners. After installation, pressure-test the cooling system or run the engine and inspect for any seepage around the housing while the system is cold and again when warm.
- Replace gasket or O-ring each time the thermostat is changed.
- Clean and inspect mating surfaces before installation.
- Torque bolts to manufacturer specifications to prevent distortion.
When To Replace The Thermostat Instead Of Other Cooling Parts
If you experience temperature control issues but coolant condition, hoses, radiator, and water pump all appear sound, the thermostat is a common and relatively low-cost component to replace. Prioritize replacement when you see persistent warm-up problems, frequent overheating episodes tied to valve operation, or physical evidence of thermostat failure during inspection.
However, if coolant is heavily contaminated, the water pump shows signs of failure, or the radiator is clogged, replacing the thermostat alone may not resolve the problem. In those cases, address the root cause first and then install a new thermostat to ensure proper system function.
- Replace thermostat for chronic warm-up or intermittent overheating symptoms.
- Inspect coolant condition and water pump before assuming thermostat is sole issue.
- Pair thermostat replacement with a coolant flush if contamination is present.
DIY Replacement Tips For Forester XT Owners
Prepare by gathering the correct replacement thermostat, new gasket or O-ring, coolant, and basic hand tools. Park on level ground, allow the engine to cool, and drain enough coolant to avoid spills when removing the housing. Keep a clean workspace and note bolt locations to ensure correct reassembly.
After installing the new thermostat and seal, refill with fresh coolant and properly bleed air from the system. Monitor the vehicle during the first short drives to confirm stable temperature behavior and check for any signs of leaks. If you are unsure at any step, consult a repair manual or a qualified technician.
- Always replace the housing seal with the thermostat.
- Bleed the cooling system to remove air pockets after installation.
- Test drive under varied load to confirm stable operation.
Still looking for the perfect fit?
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How We Chose the Best Subaru Forester XT Thermostat
We prioritized items that match the Forester XT's cooling system requirements and have designs proven to manage turbocharged engine thermal loads. Our evaluation focused on concrete, vehicle-relevant attributes: correct temperature rating and calibration, housing and seal compatibility, valve actuation quality, corrosion-resistant materials, and ease of installation including gasket and seal provisions.We also considered thermal response behavior under load and during cold starts, since the XT variant uses a turbocharged engine where rapid warm-up and stable operating temperature matter. Durability factors such as metal type, spring quality, and resistance to coolant corrosion were weighted heavily. Fitment clarity and availability of explicit part number fitment information were required for inclusion.Excluded from consideration were items lacking clear Forester XT fitment data, units with unverified or inconsistent temperature ratings, and thermostats intended for unrelated engine variants. We did not include products without proper housing or seal specifications that would increase installation risk.
Our full evaluation process is outlined in our review methodology.
FAQ
What are the most common signs of a bad thermostat in a Forester XT?
Common signs include long warm-up times, the heater blowing cool air even after the engine warms, sudden temperature spikes under load, and visible coolant leaks at the thermostat housing. Each symptom often points to sticky valve action, incorrect opening, or a failed housing seal.
How much does thermostat replacement typically cost for a Forester XT?
Thermostat replacement cost depends on parts and labor. The thermostat itself varies in design, and additional costs may arise if a new housing seal, coolant, or gasket is needed. Labor time can vary based on service access and whether the cooling system requires draining.
Can I test the thermostat without removing it?
Yes. A simple test is to monitor upper and lower radiator hose temperatures during a cold start and warm-up. If the upper hose remains cool and then quickly warms when the thermostat opens, it is likely functioning. For a definitive check, a bench test or visual inspection after removal provides clearer results.
Will a leaking thermostat housing cause overheating?
A leaking housing seal can contribute to low coolant levels and air entering the system, which in turn can cause overheating symptoms. Small leaks may not immediately overheat the engine but can lead to performance issues and should be addressed promptly.
Should I change the thermostat when replacing the coolant?
You do not always need to replace the thermostat when changing coolant, but it is a good opportunity to inspect the thermostat and housing seal. If the existing thermostat shows age, corrosion, or inconsistent operation, replacing it during a coolant service can prevent future issues.
How do I know the correct thermostat part number fitment for my Forester XT?
Confirm fitment by checking the vehicle's year, engine configuration, and the part number compatibility listed by suppliers or the vehicle's service literature. Ensure the chosen thermostat matches the Forester XT's housing dimensions and includes the appropriate seal type for reliable installation.
Final Verdict
Prioritize a thermostat that matches the Forester XT's specified temperature rating and offers reliable valve action and durable seals. Fitment accuracy and gasket quality reduce the risk of leaks and repeat repairs, while thermal performance under load preserves engine health and turbocharger reliability.Balance ease of installation and part completeness with durability when choosing a replacement. If you address coolant condition and confirm no other cooling-system failures, installing a correctly rated thermostat is a cost-effective way to restore stable engine temperature and consistent cabin heating.
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