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If your Subaru Forester is taking too long to warm up or is running too hot, choosing the right thermostat matters for daily driving and long trips. The right replacement restores proper engine temperature, improves heater performance, and helps avoid overheating or excessive wear. This guide covers seven shortlisted thermostats chosen for the Forester based on fitment accuracy, temperature response, and build quality. Read on to understand the criteria we used and what to look for when selecting a replacement for your vehicle.
Looking for other parts? See all Subaru Forester parts & accessories.
Top Picks at a Glance
| Best Reviewed A top-rated pick with lots of positive feedback | Subaru Genuine Thermostat Assembly 21200AA170 |  |
| Most Popular A crowd favorite that many people choose with confidence | Subaru Genuine Thermostat and Gasket Kit (21210AA080) |  |
| 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 | A-Premium Engine Coolant Thermostat with Seal |  |
| Most Sold A high-volume bestseller chosen again and again | MotoRad 7416-203 203°F Fail‑Safe Thermostat |  |
| Best Value for Money The sweet spot between price and what you get | Subaru Engine Coolant Thermostat (Part 21200AA072) — Genuine Subaru |  |
| Best Overall The best all-around choice for most situations | Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010) |  |
1. Subaru Genuine Thermostat Assembly 21200AA170
| Manufacturer | Subaru |
|---|---|
| Model | Thermostat |
| Item model number | 21200AA170 |
| Exterior | Machined |
This is a genuine Subaru thermostat assembly intended as a direct replacement for the Forester cooling system. It installs in place of the original thermostat to regulate coolant flow and maintain proper operating temperature during normal driving and highway use. As a factory part, it matches the original fit and function, which simplifies installation and reduces the chance of compatibility issues that can arise with aftermarket units. Buyers should note the seal is sold separately in some listings so plan to source the correct gasket when replacing the part. The machined exterior and OEM part number make it a dependable choice for routine maintenance or when addressing overheating or erratic temperature behavior. The main limitations are that it is a single component replacement and will not include ancillary items some installers expect, such as the gasket, and availability timelines may vary depending on seller stock.
Best for: Reliability-focused owners who prioritize factory-quality replacements to maintain a Forester's cooling system and minimize the risk of fitment or performance issues.
Less Ideal for: Buyers who want a full service kit or aftermarket performance thermostats with modified temperature settings should consider alternative options.
- Genuine Subaru part that matches factory fit and function
- Machined exterior construction for consistent thermal operation
- Direct replacement simplifies installation and reduces fitment guesswork
- Well-reviewed by owners for reliable temperature control in service
- Gasket or seal may be sold separately requiring an extra purchase
- Sold as a single replacement part without additional installation hardware
Verdict Choose this genuine Subaru thermostat when you want a factory-matching, reliable replacement for a Forester rather than an aftermarket alternative.
2. Subaru Genuine Thermostat and Gasket Kit (21210AA080)
| Brand | Subaru |
|---|---|
| Item model number | 21210AA080 |
| Manufacturer Part Number | 21210AA080 |
| OEM Part Number | 21210 AA080 |
This is a genuine Subaru thermostat and gasket kit intended as a direct factory-quality replacement to restore proper engine cooling control. It typically replaces a failing thermostat that sticks open or closed and is used during routine cooling-system repairs or when diagnosing overheating or poor warm-up behavior. The kit includes the thermostat set to the correct operating temperature and a matching gasket for a proper seal, which simplifies installation and reduces the chance of leaks. Buyers report it fits precisely and returns engine temperature to the normal operating band, making it a straightforward repair for owners who prefer factory-spec parts. Limitations are that it is a single thermostat and gasket only, so additional hoses, coolant, or a housing may be required depending on vehicle condition, and you must confirm the exact Forester engine and year fitment before ordering. This part favors a straightforward replacement rather than performance tuning or custom cooling upgrades.
Best for: Convenience-driven buyers who want a straightforward, factory-quality replacement that fits and functions like the original without aftermarket guessing; ideal for owners seeking a quick, dependable repair.
Less Ideal for: Drivers looking to modify cooling behavior, install a performance thermostat, or those needing a complete cooling-system overhaul with multiple replacement components should consider other options.
- Genuine Subaru factory-quality part for accurate fit and function
- Includes gasket to help ensure a proper seal during installation
- Restores correct operating temperature when replacing a sticking thermostat
- Simple one-for-one replacement that avoids modification or adaptation
- Only includes thermostat and gasket so related items like coolant or housing are not supplied
- Fitment must be verified for the specific Forester engine and year before purchase
Verdict A direct factory replacement thermostat and gasket kit for owners who want the convenience and reliability of a Subaru-original repair.
3. Subaru Engine Coolant Thermostat and Gasket Set (21200AA072 & 21236AA010)
| ASIN | B01MQ5ND43 |
|---|---|
| Item model number | 21200AA072 & 21236AA010 |
| Package Dimensions | 4 x 3 x 3 inches |
This thermostat and gasket set replaces the engine coolant thermostat with factory-quality Subaru parts (21200AA072 and 21236AA010). It is used when a failing thermostat causes overheating, high idle temperatures, or poor warm-up performance; install it as a direct-fit replacement during routine cooling system repairs. The set includes the thermostat and the matching gasket so you get correct sealing and proper valve seating without modifying the housing. Because it is a genuine Subaru part, fitment and tolerance match the factory cooling system which simplifies installation and reduces the chance of leaks or temperature variance. The main limitation is that this is a direct OEM replacement rather than an upgraded performance thermostat, so it restores factory temperature control but will not change thermostat opening temperature or cooling behavior for performance tuning. Expect a straightforward swap for maintenance-minded drivers who want reliable, factory-spec operation.
Best for: Performance-oriented Subaru owners who prioritize factory-spec reliability and precise fit when maintaining cooling system performance on their Forester.
Less Ideal for: Drivers seeking a lower-cost generic replacement or an altered-temperature performance thermostat for modified engines should consider alternatives.
- Genuine Subaru part numbers ensure factory-fit compatibility with supported models
- Includes both thermostat and gasket for a single replacement job and proper sealing
- Matches factory tolerances to reduce installation fitment issues
- Simplifies repairs by avoiding aftermarket fitment adjustments
- Not a performance or altered-temperature thermostat so it will not change cooling characteristics
- Limited to exact fitment models and does not serve as a universal replacement
Verdict Buy this when you want a genuine Subaru thermostat and gasket that restores factory cooling behavior; skip it if you need a performance-tuned thermostat.
4. A-Premium Engine Coolant Thermostat with Seal
| Start to Open Temperature (°C) | 82 |
|---|---|
| Gasket or Seal Included | Yes |
| Material | Metal and Rubber |
This A-Premium thermostat replaces the factory thermostat function by opening at a specified temperature to control coolant flow and help the engine maintain proper operating temperature. It is packaged with a seal and uses a metal-and-rubber construction intended for routine cooling-system repairs. Typical use is a direct swap when the original thermostat sticks or during a coolant system service; installation follows standard thermostat replacement steps and fits vehicles that require the referenced part numbers. The unit lists an 82°C start-to-open temperature, which means it behaves like many factory-spec thermostats and should restore normal warm-up and cabin heating compared with a failed unit. Advantages for budget repairs are straightforward: simple design, included seal, and compatibility with common Subaru part numbers. Limitations are that this is an aftermarket part from a third-party brand so fit and long-term durability can vary versus a factory-quality unit, and it does not include extensive mounting hardware or a multi-piece housing assembly that some replacement kits provide.
Best for: Budget-conscious Subaru owners seeking a low-cost, functional replacement thermostat to restore normal engine warm-up and heating performance without paying for dealer parts.
Less Ideal for: Drivers who want an exact factory-brand part, extended-life premium materials, or a complete housing kit should consider other options.
- Includes a gasket/seal for a single direct replacement
- Start-to-open temperature rated at 82°C matching common factory behavior
- Metal and rubber construction for standard thermostat operation
- Compact, single-piece unit simplifies installation
- Aftermarket part so long-term durability may differ from factory-quality components
- No additional mounting hardware or multi-piece housing included
Verdict A low-cost, direct-replacement thermostat that restores correct operating temperature for compatible Subarus but may not match factory-brand longevity.
5. MotoRad 7416-203 203°F Fail‑Safe Thermostat
| Temperature opening point | 203 °F |
|---|---|
| Voltage | 230 Volts |
| Power source | Battery Powered |
| Material | Plastic, Metal, Abs |
The MotoRad 7416-203 is a fail-safe replacement thermostat built to prevent overheating by opening at 203°F; it installs like a factory replacement and is intended for vehicle-specific fitments. It uses a mechanical thermostatic element with a lockable design to resist unintended movement and the listing says each unit is tested and calibrated before shipping. Typical use is direct replacement during a cooling system repair or thermostat failure, where the thermostat restores proper coolant flow once the engine reaches operating temperature. Practical benefits include a familiar, factory-style fit and a tested setpoint that helps maintain stable engine temperatures. However, some reviewers report intermittent failures where units arrived stuck closed or failed to open during bench testing, so pre-install testing is advisable. The product is simple to use but does not add remote diagnostics or active control features, and any thermostat failure risks engine damage if not caught early.
Best for: Risk‑averse mainstream buyers who prioritize a straightforward, factory‑style replacement to restore proper cooling without experimental upgrades; suitable for owners wanting a tested, plug‑and‑play part.
Less Ideal for: Drivers who need advanced monitoring or want a smart/programmable thermostat alternative or those unwilling to bench‑test components before installation.
- Fail‑safe thermostatic design with a 203°F opening point for consistent temperature control
- Vehicle‑specific fitment intended to match factory installation and mounting
- Lockable mechanism reduces the chance of unintended adjustment during service
- Manufacturer states each unit is tested and calibrated before shipping
- Customer reports show occasional units arriving stuck closed or failing in bench tests
- No electronic monitoring or diagnostics to warn of impending failure
Verdict A straightforward, tested 203°F fail‑safe replacement for restoring cooling system function, but test units before install due to some reports of stuck units.
6. Subaru Engine Coolant Thermostat (Part 21200AA072) — Genuine Subaru
| Manufacturer | Subaru |
|---|---|
| Item model number | 21200AA072 |
| OEM Part Number | 21200 AA072 |
| Compatible models | Forester (listed among compatible Subaru models) |
This genuine Subaru thermostat (part 21200AA072) replaces a failed unit to restore proper coolant circulation and engine temperature regulation on compatible Forester models. It is a factory-quality replacement used when a thermostat sticks closed or causes overheating, and is typically installed during a coolant system repair or when doing a water pump or timing belt service. As an OEM part it matches factory fit and function, so installation is straightforward and it eliminates fitment guesswork. The listing notes it does not include a gasket, so plan to buy a separate gasket or replace the water pump assembly that supplies one. Buyers will appreciate the reassurance of factory specifications and wide range of compatible Subaru models, but you give up any cost savings of aftermarket bundles that include gaskets or new housing hardware. For Forester owners who want a reliable, no-surprises swap that mirrors the original component, this is a practical, value-oriented choice.
Best for: Practical optimizers who prioritize reliable, factory-quality repairs and want a direct, no-surprises replacement for Forester cooling-system maintenance.
Less Ideal for: Buyers seeking the lowest upfront cost or a complete install kit with gasket and hardware should consider aftermarket options that bundle those items.
- Genuine Subaru part manufactured to factory specifications for reliable fit and function
- Restores proper coolant flow and temperature control, addressing overheating or thermostat-stuck issues
- Listed compatibility with many Subaru model years reduces fitment uncertainty for Forester repairs
- Simple direct-replacement design keeps installation straightforward for home mechanics or shops
- Does not include a thermostat gasket, requiring a separate purchase during installation
- Higher cost than some aftermarket thermostats that include extra hardware or warranties
Verdict A dependable factory Subaru thermostat for Forester owners who want an exact OEM replacement, but remember it does not include a gasket.
7. Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010)
| Brand | Subaru |
|---|---|
| Manufacturer Part Number | 21200AA072S |
| Compatible Models | Listed Subaru models including Forester, Impreza, Outback, Legacy, WRX STI, Baja, SVX |
This is the factory-quality Subaru engine coolant thermostat sold with its matching gasket and intended as a direct replacement on supported Subaru models. It regulates coolant flow to hold engine temperature in the proper range, preventing overheating and helping the heater and emissions systems operate as designed. Installation is typical for a thermostat change and is straightforward for a DIYer with basic tools or a mechanic. Because it is a genuine Subaru part, fitment and temperature calibration match factory expectations, which helps avoid odd fan behavior, fault codes, or drivability issues that can result from incorrect aftermarket set points. The main limitation is the need to confirm you have the correct thermostat temperature rating before installing, since some Subaru variants use different opening temperatures and installing the wrong spec can trigger a low-temperature fault. Aside from that, this part offers a reliable, long-term replacement when you want factory-spec performance.
Best for: Balanced buyers who prioritize reliable, factory-spec replacements and want to preserve correct engine temperature and long-term dependability without experimenting with aftermarket variants.
Less Ideal for: Owners seeking a lower- or higher-temperature performance thermostat for tuning or racing applications should consider purpose-built aftermarket options instead.
- Factory-quality Subaru part for correct fit and calibration
- Includes gasket for a complete thermostat replacement
- Direct-fit for a wide range of Subaru models listed in the product description
- Requires verifying correct temperature rating for specific vehicle to avoid faults or drivability issues
- Not a universal or performance thermostat option for modified engines
Verdict If you want a dependable, factory-spec thermostat for Subaru Forester and related models, this genuine Subaru unit is the safest, long-term choice.
Choosing the Right Subaru Forester Thermostat: Key Factors to Consider
Correct Fitment and Mounting
Thermostat fitment matters because incorrect dimensions or mounting patterns can cause improper sealing, coolant leaks, or installation difficulties. Confirm the flange dimensions, bolt pattern, and whether the thermostat uses an integrated housing or sits inside the factory housing. If the part does not match the Forester's specifications exactly, you may need gaskets or modifications that compromise reliability. Proper fitment reduces the chance of coolant loss and ensures the thermostat senses the correct coolant flow path for accurate temperature control.
Opening Temperature and Tolerance
The thermostat's specified opening temperature determines when coolant is allowed to flow to the radiator and directly affects engine warm-up and operating temperature. Choose a unit whose rated opening temperature matches the vehicle specification to maintain correct engine efficiency and emissions behavior. Also consider tolerance ranges and how tightly the valve responds to temperature changes. A thermostat with a wide tolerance may cause fluctuating temperatures, while a tight tolerance helps the engine reach and hold its intended operating point.
Seal and Housing Integrity
Seals and housing quality are critical to preventing coolant leaks and maintaining pressure in the cooling system. Look for thermostats that use durable gasket materials and well-machined housing interfaces to avoid common failure points such as silicone degradation or warped mating surfaces. A robust seal reduces the likelihood of a thermostat housing seal leak and helps maintain consistent coolant flow. Where applicable, check whether the replacement requires a new gasket or O-ring and whether those parts are included.
Material Durability and Corrosion Resistance
Thermostat components are exposed to coolant and temperature cycles that can accelerate corrosion and material fatigue. Choosing thermostats made from corrosion-resistant metals and high-temperature polymers extends service life and reduces the chance of valve sticking or housing failure. Long-lasting materials are especially important if you drive in climates with wide temperature swings or use different coolant formulations. Durable construction also helps prevent signs of bad thermostat behavior over time.
Valve Mechanism and Spring Quality
The internal valve and spring determine how smoothly and reliably the thermostat opens and closes as coolant temperature changes. A well-designed valve mechanism minimizes sticking and ensures a consistent flow rate once the opening temperature is reached. High-quality springs maintain correct preload over many cycles, preventing issues such as the engine running cold if the thermostat is stuck open or overheating if it sticks closed. Inspecting specification data on valve travel and spring ratings can inform expectations about responsiveness.
Installation Requirements and Serviceability
Consider how easy the thermostat is to install and whether routine service items like gaskets are included. Some replacements require special tools, sealants, or repositioning of adjacent components, which affects DIY feasibility and labor time for professional installation. Serviceability also covers whether the thermostat is part of an integrated housing that may need full replacement, versus a standalone unit. Choosing an option with clear instructions and common mounting features simplifies maintenance and reduces the chance of installation-related leaks.
FAQ
How do I know if my Subaru Forester thermostat is failing?
Common signs include slow warm-up, fluctuating engine temperature, and coolant temperature indicating overheating under load. You might also notice reduced heater output or persistent high or low temperature readings. These symptoms often point to thermostat issues but can also stem from other cooling system components, so diagnosis should consider the whole system.
Can I test a thermostat without removing it?
You can get initial clues by monitoring temperature behavior during warm-up and by checking when coolant flows through the upper radiator hose. For a definitive test, removing the thermostat and using a controlled hot-water bath to observe its opening temperature is most reliable. Portable temperature-check methods give useful information but are not always conclusive.
Should I replace the gasket or seal when changing the thermostat?
Yes, replacing the gasket or O-ring during thermostat service is standard practice because old seals often become brittle or compressed. A new seal helps prevent leaks and ensures proper pressure in the cooling system. If the replacement part does not include a new gasket, source the correct seal before installation.
What causes a thermostat to stick open or closed?
Thermostats can stick due to corrosion, debris buildup, degraded wax elements, or weakened springs. Exposure to contaminated or incorrect coolant accelerates these failure modes. Regular coolant maintenance and using proper coolant formulations can reduce the risk of thermostat sticking.
Is it necessary to match the exact opening temperature when replacing a thermostat?
Matching the vehicle-specified opening temperature is generally recommended because it maintains intended engine warm-up behavior and operating temperature. Small variations may be acceptable, but larger mismatches can affect heater performance, emissions, and engine efficiency. Consult service specifications to confirm the correct temperature.
How We Chose the Best Subaru Forester Thermostat
We focused on thermostats that offer correct fitment for Subaru Forester models and predictable temperature control under normal driving conditions. Our filters prioritized OEM-equivalent dimensions and mounting patterns, reliable valve and spring designs for accurate opening temperatures, and durable materials for corrosion resistance. We evaluated published opening temperatures and tolerance ranges, the integrity of seals and housings, and the presence of quality assurance documentation such as manufacturer temperature specs. We excluded options with ambiguous fitment, incomplete specification data, or known reports of early failure. We also left out universal thermostats that require modification to install. The resulting list reflects products that meet core mechanical fit and thermal performance expectations for Subaru Forester owners, while omitting items with unclear compatibility or short expected service life.
Our full evaluation process is outlined in our review methodology.
Conclusion
When choosing a Subaru Forester thermostat, prioritize correct fitment, the specified opening temperature, and robust seal integrity. Those factors have the biggest influence on warm-up time, heater performance, and avoiding leaks. Balance material durability and valve mechanism quality against installation complexity. Replacing the gasket and following proper installation procedures will help ensure long-term reliability and consistent engine temperature control.
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