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If your Subaru Legacy is taking too long to warm up or the engine temperature fluctuates, the thermostat could be the culprit. Choosing the right thermostat matters because the wrong unit can cause poor fuel economy, excessive wear, or overheating. This guide covers the top shortlisted thermostats specifically for Subaru Legacy models, focusing on fitment, opening temperature, and build quality. We assessed each option against the core factors that affect everyday drivability and long-term engine health. Read on to see which features to prioritize when selecting a replacement.
Looking for other parts? See all Subaru Legacy parts & accessories.
Top Picks at a Glance
| Best Reviewed A top-rated pick with lots of positive feedback | Subaru Thermostat and Gasket (21210AA211) |  |
| Most Popular A crowd favorite that many people choose with confidence | Subaru Engine Coolant Thermostat (Part 21200AA072) |  |
| Best Quality Built to a higher standard if you want something more solid | Subaru Engine Coolant Thermostat and Gasket Set (21200AA072 & 21236AA010) |  |
| Best Budget The cheapest option that still gets the job done reliably | DAZOM Engine Coolant Thermostat (replaces 21200AA072) |  |
| Most Sold A high-volume bestseller chosen again and again | AISIN Engine Coolant Thermostat with Gasket |  |
| Best Value for Money The sweet spot between price and what you get | Subaru Genuine Thermostat Assembly 21200AA170 |  |
| Best Overall The best all-around choice for most situations | Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010) |  |
1. Subaru Thermostat and Gasket (21210AA211)
| Manufacturer | Subaru |
|---|---|
| Model | Thermostat |
| Manufacturer Part Number | 21210AA211 |
| OEM Part Number | 21210 AA211 |
This Subaru-branded thermostat is a direct factory-style replacement intended for 2012–2016 Crosstrek, 2012–2014 Impreza 2.0L, and 2013–2014 Legacy and Outback 2.5L non-turbo engines. It regulates coolant flow to maintain correct engine operating temperature and installs in place of the original thermostat during routine cooling system service or when a failed thermostat causes overheating or poor warm-up. The part is machined and carries Subaru part numbers, so fitment and sealing are straightforward for anyone doing thermostat replacement with basic hand tools. Using a factory-quality unit reduces compatibility guesswork and helps preserve factory cooling behavior. Limitations are that this is a vehicle-specific, single-purpose part so you must confirm engine and model year fitment before ordering, and it does not include auxiliary components beyond the gasket shown, so additional hoses, clamps, or coolant may be needed during a full service.
Best for: Reliability-focused buyers who prioritize factory-quality replacements and want a straightforward, no-surprises thermostat swap to maintain cooling system reliability.
Less Ideal for: Buyers looking for performance or modified cooling components, or those who need a universal or multi-vehicle solution should consider other aftermarket options.
- Factory-brand replacement designed to match original fit and function
- Machined exterior for consistent sealing and installation
- Includes matching gasket for a single-step thermostat swap
- Fits specific Subaru 2.0L and 2.5L non-turbo engines commonly found in Legacy and Crosstrek
- Only compatible with specified Subaru non-turbo models and years, so it is not universal
- Supplies are limited to the thermostat and gasket only; additional service parts may be required
Verdict A factory-style Subaru thermostat and gasket that offers reliable, exact-fit replacement for specified Legacy and related models to maintain stock cooling performance.
2. Subaru Engine Coolant Thermostat (Part 21200AA072)
| Manufacturer Part Number | 21200AA072 |
|---|---|
| Manufacturer | Subaru |
| Model Name | WRX 2002-2014 / STI 2004-2020 |
| OEM Part Number | 21200 AA072 |
This is the genuine Subaru engine coolant thermostat offered as a direct factory-quality replacement for compatible Subaru models. It installs in the thermostat housing to regulate coolant flow and maintain correct engine operating temperature, restoring proper warm-up and preventing overheating once fitted. Buyers get a factory-spec part manufactured to Subaru standards, which simplifies fitment and reduces the risk of compatibility headaches compared with aftermarket units. The listing notes broad model coverage for Legacy, Outback, Impreza, Forester, WRX and STI years commonly serviced, and multiple customer reports confirm correct fit and resolution of overheating issues. Practical limitations are that the part is the thermostat alone and does not include a gasket, so plan to buy or reuse the housing gasket when replacing the unit. Also, choosing a genuine part means you will not get bundled accessories some aftermarket kits include, such as a new gasket or mounting hardware.
Best for: Convenience-driven owners who want a straightforward, factory-quality swap that minimizes fitment risk and lets them get the job done quickly with confidence.
Less Ideal for: Buyers who prefer repair kits with gaskets and hardware included or who want lower-cost aftermarket options with bundled parts should consider alternatives.
- Genuine Subaru factory-quality replacement manufactured to OEM standards
- Wide model coverage for many Subaru Legacy, Outback, Impreza, Forester, WRX and STI applications
- Proven fit and function based on customer reports resolving overheating and fitment issues
- Does not include the thermostat housing gasket or additional installation hardware
- No bundled accessories or extras that some aftermarket kits provide
Verdict Choose this genuine Subaru thermostat when you want a reliable, factory-spec replacement that fits without guesswork, but buy a gasket separately.
3. Subaru Engine Coolant Thermostat and Gasket Set (21200AA072 & 21236AA010)
| ASIN | B01MQ5ND43 |
|---|---|
| Item model number | 21200AA072 & 21236AA010 |
This genuine Subaru thermostat and gasket set replaces the factory thermostat and comes with the matching gasket for a proper seal. It is used during coolant system service or when the engine is running hotter than normal to restore correct coolant flow and operating temperature. As a factory-quality part, fitment is exact and installation avoids the trial-and-error issues common with aftermarket units. Buyers will appreciate that the set addresses both the thermostat and sealing surface so leaks are less likely after installation. The main limitation is that it is a direct replacement part only for specific Subaru model ranges, so you must confirm compatibility with your Legacy model year and engine; it does not include additional hardware or performance upgrades beyond factory spec. For performance-minded owners who want reliable, OEM-temperature control, this part returns the cooling system to factory behavior without modifying thermostat calibration.
Best for: Performance-oriented owners who prioritize factory-correct cooling behavior and reliable, OEM replacement parts to maintain consistent engine temperature during spirited driving.
Less Ideal for: Buyers looking to change thermostat calibration for higher operating temperature or aftermarket performance thermostats should consider specialized performance parts instead.
- Genuine Subaru part numbers ensure factory-quality fit and function
- Includes gasket for a correct seal and reduced risk of coolant leaks after installation
- Direct-fit for multiple Subaru models so no modification is required
- Restores factory thermostat behavior for accurate engine temperature control
- Only restores factory temperature control and does not provide higher or lower operating set points for performance tuning
- Compatibility limited to specified Subaru model years and engine variations
Verdict Choose this genuine Subaru thermostat and gasket set when you want factory-fit temperature control and a reliable seal rather than aftermarket performance changes.
4. DAZOM Engine Coolant Thermostat (replaces 21200AA072)
| Replaces part numbers | 21200AA072, 21236AA010, 21210AA080 |
|---|---|
| Fitment type | Direct Replacement |
| Compatible vehicle (selection) | Subaru Legacy (1990-2019) |
This DAZOM engine coolant thermostat is a straightforward direct-replacement part intended to restore proper coolant flow and temperature regulation in compatible Subaru engines. It installs in place of the factory thermostat and is used when a failed thermostat causes overheating or poor warm-up. The unit claims a stainless steel mounting flange and spring frame for durability and is shipped as a single thermostat ready for installation. For a budget-focused replacement it provides the key function: opening and closing at the designed temperature to protect the engine. Practical benefits include an advertised fit for Subaru Legacy model years covered in the compatibility list and a simple drop-in installation for mechanically inclined owners. Limitations are typical of low-cost aftermarket parts: long-term longevity is uncertain compared with higher-end or factory-quality units, and customers should verify the jiggle pin orientation or bleed features match the vehicle’s vertical installation requirement before fitting. Expect a no-frills, cost-effective fix rather than an upgrade in durability or performance.
Best for: Budget-conscious owners looking for an inexpensive, ready-to-install thermostatic replacement to get a Subaru Legacy back to normal operating temperature without paying for premium parts.
Less Ideal for: Owners who prioritize long-term durability or want a warranty-backed factory-quality replacement should consider higher-end or OEM options instead.
- Direct replacement fit for Subaru Legacy model years listed, simplifying installation.
- Stainless steel flange and spring frame advertised for improved strength over basic stamped parts.
- Single-piece supply makes it easy to replace a failed thermostat without additional components.
- Manufacturer longevity claims are unverified so long-term reliability may vary.
- May lack specific bleed/jiggle pin arrangement used on some factory installations, requiring attention during install.
Verdict A low-cost, direct-replacement thermostat that restores function for Subaru Legacy owners who need an affordable repair but may not match factory longevity.
5. AISIN Engine Coolant Thermostat with Gasket
| Manufacturer | AISIN |
|---|---|
| OEM Part Number | 21200-AA072 |
| Position | Center |
This AISIN thermostat is a direct replacement unit intended to restore proper engine cooling control on compatible Subarus such as the Legacy. It installs in place of the factory part and comes with a gasket, so it is suitable for routine timing-belt or cooling-system service. AISIN markets the part as engineered to match factory-specified opening temperature and fit, and buyers report consistent opening/closing behavior when bench-tested. The machined exterior and inclusion of a seal simplify installation and reduce the chance of leaks when the correct gasket is supplied. A few users noted fitment or vent-centering differences on non-legacy applications and one report of an incorrect temperature limit, so confirming the exact engine fitment for your Legacy is important before installing. For a risk-averse owner wanting a like-for-like replacement from a recognized parts supplier, this thermostat offers a conservative, low-surprise option, but double-check engine code fitment to avoid mismatched parts.
Best for: Risk-averse Subaru Legacy owners who prioritize a factory-style replacement from a reputable supplier and want a straightforward part for scheduled cooling-system or timing-belt maintenance.
Less Ideal for: Buyers seeking performance thermostats with altered opening temperatures or custom cooling behavior should consider performance-focused alternatives instead.
- Designed to match factory-specified opening temperature for consistent cooling control
- Includes gasket to simplify replacement and reduce leak risk when the correct seal is provided
- Machined exterior and OEM-matched construction for dependable long-term use
- Made by a recognized automotive supplier with a history of OEM-quality components
- Some reported fitment or vent-centering issues on certain installations
- At least one user reported temperature-limit mismatch triggering a cooldown warning
Verdict A conservative, OEM-matched replacement for Subaru Legacy owners who want a reliable, factory-spec thermostat but should verify exact engine fitment before buying.
6. Subaru Genuine Thermostat Assembly 21200AA170
| Manufacturer | Subaru |
|---|---|
| Model | Thermostat |
| Item model number | 21200AA170 |
| Exterior | Machined |
This is the genuine Subaru thermostat assembly (part number 21200AA170) intended as a direct replacement for factory units. Installed in the coolant housing, it regulates engine temperature by opening and closing as coolant reaches its designed operating range. As a factory-quality part it matches original fit and function, which simplifies installation and reduces the chance of fitment issues on vehicles that use this number. Typical use is a routine replacement during overheating diagnosis or cooling system service. Buyers get the reassurance of correct valve calibration and durable construction versus cheaper aftermarket alternatives. One practical limitation is the listing does not always include the sealing gasket, which some buyers must purchase separately to complete the job. Also this is a direct replacement part rather than a performance thermostat, so it will not change operating temperature for tuning purposes.
Best for: Practical optimizers who prioritize reliability and correct fit when repairing a Subaru Legacy and want a straightforward, factory-quality replacement without experimenting with aftermarket options.
Less Ideal for: Drivers seeking altered thermostat behavior for performance tuning or those who prefer bundled kits that always include replacement gaskets and seals.
- Genuine Subaru part so fit and valve calibration match factory specifications
- Simple drop-in replacement that minimizes fitment headaches
- Manufactured from durable components intended for long service life
- Well documented part number makes ordering the correct replacement straightforward
- Gasket may not be included and must sometimes be purchased separately
- Not a performance thermostat so it does not alter operating temperature for tuning
Verdict Choose this genuine Subaru thermostat when you want a no-nonsense, factory-matching replacement for a Subaru Legacy rather than a performance or aftermarket variant.
7. Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010)
| Brand | Subaru |
|---|---|
| Manufacturer Part Number | 21200AA072S |
| Included parts | Thermostat and gasket |
| Fits models (range) | Legacy 1990-2009, 2015-2019 (see vehicle fitment) |
This is the factory-quality Subaru thermostat and matching gasket intended as a direct replacement for compatible Subaru engines. It installs in place of the original thermostat to regulate coolant flow and maintain the intended operating temperature, which helps keep engine performance, emissions and component life stable. The kit includes the thermostat and gasket so you replace the common wear items in one service. Buyers report the part brings temperatures back to the factory range and prevents erratic fan cycling that can come from a stuck or incorrect-temperature thermostat. The main limitation is fitment: you must confirm the part number and temperature rating match the Legacy engine variant you have because different years and engines use different thermostats; installing a different opening temperature can trigger faults or change warm-up behavior. For someone who wants a direct, factory-spec replacement, this is the straightforward option; those seeking an altered temperature or performance thermostat should look elsewhere.
Best for: Balanced all-round buyers who prioritize reliability and maintaining factory performance and want a simple, direct replacement to keep a Legacy running as intended.
Less Ideal for: Drivers who want an aftermarket thermostat tuned for different operating temperatures or those installing on heavily modified engines seeking altered cooling behavior.
- Factory-quality Subaru part designed to match original fit and function.
- Includes gasket so both sealing surfaces are replaced during installation.
- Restores correct operating temperature and reduces unnecessary fan cycling when fitted correctly.
- Requires verifying the exact part number and opening temperature to ensure correct fit and engine behavior.
- Not intended for modified engines or drivers who want a different thermostat opening temperature.
Verdict A genuine Subaru thermostat and gasket combo that restores factory temperature control for Legacy owners who want a reliable, direct replacement.
Choosing the Right Subaru Legacy Thermostat: Key Factors to Consider
Correct Fitment and Part Number
Matching the thermostat to your Subaru Legacys specific engine and year is the baseline requirement. An exact fit ensures the thermostat seats properly in the housing, avoids flow restrictions, and prevents leaks that can arise from poor alignment.
Incorrect part-number fitment can change flange geometry or flange thickness, which affects seal compression and coolant routing. Always confirm the vehicle-specific part number and verify that the unit is intended for your engine configuration to avoid installation issues and ensure reliable operation.
Rated Opening Temperature and Tolerance
The thermostats opening temperature determines when coolant begins circulating through the radiator and directly influences warmup time and operating temperature. A thermostat that opens too early can leave the engine running cold, while one that opens too late can contribute to overheating symptoms.
Check both the nominal opening temperature and the acceptable tolerance range. Tight tolerances yield more predictable warmup behavior and maintain consistent engine temperature under varying loads and ambient conditions.
Seal Design and Housing Interface
A reliable seal at the thermostat housing prevents coolant loss and maintains pressure in the cooling system. Look for thermostats with robust gasket or O-ring designs and quality mating surfaces that match the Subaru Legacy housing contours.
Poor seal design or incompatible flange dimensions increase the risk of a thermostat housing seal leak and can lead to persistent drips or pressure loss. Consider how the seal compresses during installation and whether a separate housing gasket or sealant is required.
Build Materials and Corrosion Resistance
Thermostats are exposed to constant thermal cycling and coolant chemistry, so material quality matters. Components made from corrosion-resistant metals and durable polymers last longer and resist sticking or warping from repeated heat exposure.
Inferior materials can accelerate wear on the valve, hinge, or spring, increasing the chance of symptoms like erratic temperature readings or a thermostat stuck open. Durable finishes and compatible materials with common coolant types help ensure consistent performance over time.
Flow Characteristics and Valve Movement
When the thermostat opens, it should provide unobstructed coolant flow appropriate for the engines cooling needs. Valve design affects how quickly flow ramps up and how evenly coolant circulates through the block and radiator.
Smooth, predictable valve movement reduces temperature swings during throttle changes and city-to-highway transitions. Consider whether the valve action is progressive and if the design supports consistent flow at partial openings to avoid localized hot spots in the engine.
Ease of Installation and Serviceability
A thermostat that installs cleanly reduces labor time and the risk of installation errors. Features such as clear alignment marks, integrated seals, and standard flange dimensions simplify replacement on Subaru Legacy models.
Also consider whether the design makes future servicing straightforward, for example if the unit can be removed without disturbing other components or if the housing gasket is reusable. Ease of installation lowers the chance of incorrect torque, seal damage, or other issues that can affect long-term reliability.
FAQ
What are common signs of a bad thermostat in a Subaru Legacy?
Common signs often include slow warmup, erratic temperature gauge behavior, or the engine running cold if the thermostat is stuck open. Overheating or frequent coolant loss can indicate a thermostat stuck closed or a related seal leak. Observing these symptoms alongside coolant-level checks can help narrow the cause.
How can I test a thermostat without removing it?
You can monitor temperature rise after a cold start and watch the upper radiator hose for a sudden increase in heat as the thermostat opens. Another noninvasive method is using a scan tool to track coolant temperature and compare it to expected warmup curves. These methods are indicative but not definitive compared with bench testing.
Will replacing the thermostat fix overheating issues?
Replacing the thermostat can resolve overheating when the thermostat is sticking closed or opening at the wrong temperature. However, overheating often depends on multiple components, such as coolant flow, radiator condition, and the water pump. Diagnose the entire cooling system to identify all contributing causes.
How important is the thermostats opening temperature?
Opening temperature is important because it sets the engines normal operating range and affects warmup efficiency. An appropriate opening point helps the engine reach its operating temperature quickly and maintain it under load. Choosing a thermostat with the correct rated temperature reduces stress on emission controls and improves cabin heat availability.
Can a thermostat housing seal leak mimic thermostat failure?
Yes. A housing seal leak can produce coolant loss and localized cooling issues that resemble thermostat problems. Leaks may also cause air ingress and erratic temperature readings. Inspect the housing seal area when diagnosing thermostat symptoms to distinguish between a failed thermostat and a seal issue.
How often should a thermostat be replaced in routine maintenance?
There is no fixed interval, but thermostats are commonly replaced when performing major coolant service or when symptoms appear. Routine inspections during coolant changes help identify early wear. Replacing the thermostat during head-gasket or water-pump service is often recommended to avoid redundant labor later.
How We Chose the Best Subaru Legacy Thermostat
We shortlisted thermostats based on criteria that directly affect cooling-system performance and long-term reliability for Subaru Legacy vehicles. Our evaluation emphasized fitment accuracy to the vehicles housing, the thermostats rated opening temperature and tolerance, and the materials used for the valve and seal surfaces. We also considered flow characteristics when open, durability under repeated thermal cycling, and the integrity of the housing seal interface to reduce leak risk. Units that lacked clear fitment data, unspecified opening temperature tolerances, or substandard seal materials were excluded. We did not include products without verifiable specifications or those intended for unrelated vehicle platforms. The resulting list focuses on items that meet practical repair needs: correct part-number fitment, predictable warmup behavior, robust sealing at the housing, and endurance against repeated heat exposure.
Our full evaluation process is outlined in our review methodology.
Conclusion
When choosing a Subaru Legacy thermostat, prioritize correct part-number fitment and the rated opening temperature to ensure predictable warmup and stable engine temps. Pay close attention to seal design, material quality, and flow characteristics, since these affect leak resistance and coolant circulation. Balance ease of installation and long-term durability with the specific operating needs of your vehicle. Addressing the thermostat and the surrounding cooling-system components together gives the best chance of resolving temperature-related issues reliably.
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- Check out all Subaru Legacy parts & accessories
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