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Keeping your 2009-2013 Subaru Forester at the right operating temperature matters for performance, fuel economy, and long-term engine health. A thermostat that opens and seals correctly prevents overheating and avoids prolonged cold running that can increase wear. In this guide we shortlisted the top thermostats that fit the 2009–2013 Forester and focused on the factors that actually affect durability and temperature control.Below you will find the top thermostat options for this Forester range along with clear evaluation points like opening temperature, fitment accuracy, and durability so you can pick the part that matches how you use the vehicle.
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 Engine Coolant Thermostat (Part 21200AA072) |  |
| Best Value The sweet spot between price and what you get | Techie Engine Thermostat TQTS4125 (21200AA072) for Subaru 2.5L and 3.6L Engines |  |
| Best Budget The cheapest option that still gets the job done reliably | FOXSTAR Engine Thermostat (21200AA170) for Subaru |  |
| Highest Rated A top-rated pick with lots of positive feedback | AISIN THF-003 Engine Coolant Thermostat with Gasket |  |
| Most Popular A crowd favorite that many people choose with confidence | Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010) |  |
| Best Seller A high-volume bestseller chosen again and again | Anleecap Engine Coolant Thermostat (172°F) for Subaru 1998-2013 Forester and Select Models |  |
1. 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 a factory Subaru replacement thermostat (part 21200AA072) that replaces the vehicle's temperature-regulating valve to maintain proper engine operating temperature. It installs in the coolant housing and is used when the stock thermostat is stuck open or closed, causing poor warm-up, overheating, or erratic heater performance. As a genuine Subaru part it matches factory fit and specifications so installation is straightforward for DIYers with basic tools or for professional shops following Subaru service procedures. The OEM build helps ensure predictable warm-up times and consistent thermostat opening temperature, which supports engine efficiency and cabin heat when needed. Limitations are typical of single-piece thermostats: it does not include gaskets or housing components in the listing and compatibility is limited to Subaru models and years specified by Subaru for this part, so confirm fitment before purchase. Overall this is a simple, reliable factory replacement that prioritizes correct temperature control over aftermarket styling or performance tweaks.
Best for: Buyers who want a straightforward, factory-quality replacement thermostat to restore correct engine temperature control on their Subaru and prefer direct-fit OEM parts over aftermarket options.
Less Ideal for: Those seeking performance thermostats with altered opening temperatures or a kit that includes housing, gaskets, and extra hardware should consider aftermarket or complete kits instead.
- Genuine Subaru part engineered to factory specifications for reliable fit and function
- Direct replacement for the specified Subaru models so no modification is required
- Restores correct engine warm-up and temperature regulation, improving heater performance and engine efficiency
- Listing does not explicitly include gasket or additional installation hardware
- Compatibility limited to Subaru models listed for this part so fitment must be verified
Verdict: If you want a dependable, factory-spec replacement to restore proper cooling and heater function on a matching Subaru, this genuine Subaru thermostat is the straightforward choice.
2. Techie Engine Thermostat TQTS4125 (21200AA072) for Subaru 2.5L and 3.6L Engines
| Opening Temperature | 76.67°C (170°F) |
|---|---|
| Reference Part Numbers | 21200AA072, 21236AA010, 21210AA080 |
| Compatible Models | Subaru Forester 1998-2013 and select Impreza, Legacy, Outback, Tribeca, WRX STI |
| Brand | Techie |
This Techie replacement thermostat is intended as a straightforward factory-style swap for Subaru 2.5L and 3.6L cooling systems and is listed with a 170°F (76.67°C) opening temperature. Installation is the same as a standard thermostat: drain coolant, remove the housing, replace the unit and gasket, and refill and bleed the cooling system. The product description highlights quality control and stable temperature regulation, which matters for preventing overcooling or overheating in daily driving and light towing. Because it matches common Subaru reference part numbers, it is useful when you need a like-for-like replacement rather than a performance or modified-temperature unit. Limitations are that fitment must be verified against your VIN or factory part number and there is no detailed material spec in the listing, so long-term corrosion resistance and exact housing finish are not specified. For a practical DIY or shop replacement on the specified Subaru models, it gives a simple, purpose-built option that restores normal thermal control.
Best for: Buyers who want a reliable, like-for-like replacement to restore stock cooling behavior on their Forester without changing thermostat calibration and who prioritize a practical, no-frills fix.
Less Ideal for: Drivers seeking a thermostats tuned for higher or lower operating temperatures for performance or economy gains should consider purpose-built performance units instead.
- Factory-style 170°F (76.67°C) opening temperature suitable for stock engine tuning
- Lists multiple Subaru reference part numbers for straightforward OEM-equivalent replacement
- Simple direct-fit design that supports standard DIY installation steps
- Positive customer feedback with a high average rating indicating consistent performance
- Listing does not specify construction materials or corrosion resistance
- Fitment requires verification against your vehicle VIN or existing part number
Verdict: A practical, direct-fit replacement that restores stock thermostat behavior for 2009–2013 Forester owners who want a dependable, like-for-like repair.
3. FOXSTAR Engine Thermostat (21200AA170) for Subaru
| Replaces part numbers | 21200-AA170, 21210-AA080, 21200-AA070 |
|---|---|
| Materials | Stainless steel and copper |
| Exterior | Machined |
| Item model number | 21200AA170 |
This FOXSTAR thermostat is a direct-fit replacement designed to restore proper coolant flow and engine temperature control on compatible Subaru vehicles. It replaces common Subaru part numbers including 21200-AA170 and 21210-AA080, and is meant to be installed where the upper radiator hose meets the engine. Made from stainless steel and copper with machined exterior treatment, the unit resists corrosion and is intended to provide a reliable seal. In use it performs the basic thermostat function: opening at the correct temperature to allow coolant circulation and closing when the engine cools, which helps prevent overheating and improves warm-up efficiency. Installation is straightforward for anyone comfortable with basic cooling-system service and typically involves removing the thermostat housing and swapping units. The main limitations are that fitment must be verified against the OEM number and that aftermarket materials and long-term durability may vary compared with factory parts, so expect routine inspection during coolant service intervals.
Best for: Buyers seeking an affordable, drop-in replacement thermostat to restore proper engine temperature control on a 2009–2013 Subaru Forester and who prioritize cost-effectiveness over OEM branding.
Less Ideal for: Drivers who require guaranteed factory-spec calibration or who prefer dealer-sourced parts with official manufacturer warranty should consider OEM options instead.
- Direct replacement for common Subaru thermostat part numbers for easier compatibility checking
- Stainless steel and copper construction offers corrosion and heat resistance
- Machined exterior and treated housing intended to improve sealing and fit
- Simple, straightforward installation matching factory location and function
- Aftermarket longevity and exact temperature calibration may differ from factory-original parts
Verdict: A budget-friendly, direct-fit replacement for restoring correct coolant temperature control on 2009–2013 Subaru Forester models.
4. AISIN THF-003 Engine Coolant Thermostat with Gasket
| Manufacturer Part Number | THF-003 |
|---|---|
| OEM Part Number | 21210-AA080 |
| Included Components | Thermostat and O-ring |
| Material | Stainless Steel |
This AISIN THF-003 thermostat replaces the factory unit and is intended to regulate engine coolant flow at the correct OE-specified opening temperature. It ships with a gasket and is manufactured to match factory form, fit, and function so installation is straightforward for a routine cooling system service. In typical use it sits between the engine and radiator hose to open at the designed temperature, helping the engine warm quickly and then maintain stable operating temperature during normal and highway driving. Notable features include OE-matched engineering, stainless steel construction, and inclusion of the O-ring/gasket. The practical benefits are predictable temperature control, long service life from quality materials, and minimal fuss during replacement. Limitations are that fitment is vehicle- and engine-specific, so you must confirm it matches the 2009–2013 Forester engine variant before buying, and the part listing does not include detailed temperature rating in the product specs on the page.
Best for: Buyers who prioritize dependable, factory-quality replacements to restore reliable cooling system temperature control on their Forester and prefer parts engineered to match factory performance.
Less Ideal for: People seeking a performance-modified thermostat with a different opening temperature or those who need a universal fit part for multiple vehicle types.
- OE-matched design for correct operating temperature and fit
- Includes gasket/O-ring for a complete thermostat replacement
- Stainless steel construction aimed at durable, long-lasting service
- Manufactured by a reputable OEM parts supplier with industry experience
- Fitment varies by engine and model year so verification is required before purchase
Verdict: Choose the AISIN THF-003 when you want a dependable, factory-matched thermostat replacement that restores correct cooling system temperature control.
5. Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010)
| Manufacturer Part Number | 21200AA072S |
|---|---|
| Includes | Thermostat and gasket |
| Compatible Models | Multiple Subaru models and years listed in product description |
This genuine Subaru thermostat assembly replaces the factory thermostat and includes the matching gasket for a complete install. It is designed as a direct fit for multiple Subaru models in the specified model years, so installation is straightforward for a mechanic or competent DIYer using factory mounting points. Using a factory-quality thermostat preserves the engine's designed warm-up and cooling behavior, which helps maintain fuel economy, emissions, and heater performance. The included gasket reduces the chance of leaks that can occur when reusing an old seal. Limitations are that this is an exact factory replacement with no performance upgrades, so drivers wanting higher or lower open temperatures for racing or heavy towing should look at performance aftermarket options. Also the fitment must match the vehicle year and engine variant listed to avoid compatibility issues.
Best for: Buyers who prioritize correct, factory-quality replacement parts and want reassurance from a genuine Subaru component with documented vehicle fitment and many positive reviews.
Less Ideal for: Drivers seeking aftermarket performance thermostats or custom cooling behavior should consider upgraded options tailored for racing or heavy-load applications.
- Genuine Subaru part designed to match factory fit and function
- Includes matching gasket to reduce leak risk during installation
- Direct-fit replacement simplifies installation and reduces need for modification
- Maintains factory engine warm-up and cooling characteristics
- Not a performance thermostat so it does not change operating temperature for motorsport or heavy-duty use
Verdict: Choose this genuine Subaru thermostat assembly when you want a factory-fit replacement that restores proper cooling system function with minimal fuss.
6. Anleecap Engine Coolant Thermostat (172°F) for Subaru 1998-2013 Forester and Select Models
| Opening temperature | 172°F |
|---|---|
| Manufacturer | Anleecap |
| Manufacturer part numbers | THF-002, 21200-AA072, 34012 |
| Fitment note | Fits 1998–2013 Subaru Forester (see full list) |
This Anleecap thermostat is a direct replacement-style unit that opens at 172°F to regulate coolant flow and help the engine reach and hold its normal operating temperature. It is intended for routine service or emergency swap-outs on compatible Subaru models and comes with a seal for installation. In use the thermostat promotes faster warm-up after cold starts and stable temperature control under normal driving, which reduces the risk of overheating and supports consistent combustion. Construction claims corrosion and heat resistance, so it should be durable for typical highway and city use, though longevity depends on coolant condition and installation quality. The listing includes multiple manufacturer part numbers so you can confirm fitment before buying, but make sure to verify the part number against your vehicle to avoid a mismatch. This part is a straightforward, low-complexity replacement that suits standard maintenance or preventative replacement schedules but does not include advanced features like variable opening or calibrated testing tools.
Best for: Buyers who want a straightforward, factory-style replacement that restores normal engine warm-up and cooling behavior and who prioritize a conservative, low-risk repair approach.
Less Ideal for: Drivers looking for performance or modified-engine thermostats with nonstandard opening temperatures or tunable cooling behavior should look at specialty options instead.
- 172°F opening temperature suitable for restoring correct operating range
- Includes seal for a complete gasketed installation
- Direct-replacement fitment list covers 1998–2013 Forester models
- Claims of corrosion and heat-resistant materials for improved durability
- Does not include installation hardware beyond the seal or any testing/calibration tools
- Fitment requires confirming supplied part numbers to avoid ordering the wrong variant
Verdict: A simple, factory-style 172°F replacement thermostat that restores proper engine temperature control for conservative Forester owners seeking a low-risk service part.
Choosing the Right 2009-2013 Subaru Forester Thermostat: Key Factors to Consider
Opening Temperature and Tolerance
The thermostat's specified opening temperature determines when coolant starts circulating through the radiator. For the 2009–2013 Forester, choosing a thermostat with the correct nominal opening temp and a tight tolerance keeps the engine in its optimal range more consistently. A thermostat that opens too late can cause higher idle and transient overheating, while one that opens too early can leave the engine running cold, increasing fuel consumption and emissions.
Tolerance matters because real-world parts vary. A unit with a plus-or-minus tolerance of a few degrees can still maintain normal operating temperature, but wider variance increases the chance of driveability issues. When selecting a replacement, prioritize thermostats that publish an accurate opening temperature and a small tolerance band.
Fitment and Seal Integrity
Correct physical fitment to the Forester thermostat housing is essential to prevent leaks and ensure reliable operation. Even a thermostat with the right opening temperature can fail to perform if its flange, gasket surface, or O-ring does not mate precisely with the housing.
Pay attention to the seal material and whether the package includes the appropriate gasket or O-ring. Poor sealing can lead to a thermostat housing seal leak that shows up as coolant loss or localized corrosion. Choose parts that match the OEM interface dimensions to avoid additional machining, sealants, or repeated service.
Material Quality and Corrosion Resistance
Thermostats operate in a corrosive environment of coolant and heat cycles, so construction materials and finishes matter. Valves, frames, and springs made from corrosion-resistant metals or treated finishes last longer and maintain predictable thermal behavior.
Plastic components or cheaper metals may deform or pit over time, changing the opening characteristics or causing binding. For a Forester that sees cold-weather operation or extended service intervals, prioritize units with proven corrosion resistance to reduce the risk of premature failure.
Valve Design and Flow Characteristics
Beyond the opening temperature, the valve design determines how quickly and evenly coolant flows once the thermostat opens. Some designs provide progressive opening for smooth warm-up and steady cooling, while others open abruptly, which can cause sudden temperature swings.
Consider how you use your Forester. If you tow, drive in hilly terrain, or frequently run sustained highway speeds, a thermostat that provides strong, steady flow once open helps control temperature under load. For primarily city driving, a more gradual opening can improve cabin warm-up and reduce cold-run symptoms.
Durability Under Thermal Cycling
Every thermostat endures repeated cycles between cold start and operating temperature. Components that fatigue under thermal cycling will change their behavior, such as shifting the effective opening temperature or sticking in a partially open position.
Look for thermostats with robust springs and durable valve seats that are designed to withstand many cycles. Longevity reduces the chance you will face issues like engine running cold thermostat stuck open or premature replacement intervals.
Installation Ease and Service Considerations
Thermostat replacement on the 2009–2013 Forester is a routine maintenance task for many owners, so ease of installation is an important practical factor. Parts that include the proper gasket, require no additional machining, and fit directly into the existing housing save time and reduce the risk of installation errors.
Also account for service items such as whether the part requires new O-rings, if the housing needs to be replaced when resealing, and whether special tools are necessary. Choosing a unit with straightforward instructions and common hardware will reduce labor time and help ensure a reliable repair.
Common Thermostat Symptoms on the 2009–2013 Forester
Recognizing thermostat-related symptoms early helps you address issues before they damage the engine. Overheating under light load or slow warm-up after startup are two classic signs. Overheating often appears as rising temperature under idling or during uphill runs, while slow warm-up shows as a long period before the cabin heater becomes effective.
Other symptoms include fluctuating temperature gauge readings, coolant leaks around the thermostat housing, and poor fuel economy from prolonged cold operation. These indicators can also stem from related cooling system problems, so confirm the thermostat's condition as part of a broader diagnostic.
- High gauge reading during idle or low-speed climbing
- Extended warm-up times and weak cabin heat
- Visible coolant at the thermostat housing or hose connections
- Temperature gauge that rapidly swings between hot and normal
How To Test a Thermostat Without Special Equipment
You can perform basic checks on a Forester thermostat using simple observation and a few household tools. After the engine is cold, start and let it idle while watching the upper radiator hose. The hose should remain relatively cool until the thermostat opens. As the engine reaches opening temperature, the hose will suddenly warm as coolant begins flowing to the radiator.
Another basic test is to monitor the temperature gauge and time how long it takes to reach normal operating temperature under similar ambient conditions. For a more definitive bench test, remove the thermostat and place it in water heated slowly while measuring the temperature at which the valve begins to open. This confirms whether the thermostat matches its specified opening point.
- Observe upper radiator hose temperature change during warm-up
- Time warm-up to operating temp under consistent conditions
- Bench-test removed thermostat in heated water to verify opening point
Thermostat Housing and Coolant Seal Maintenance
The interface between the thermostat and its housing is a frequent source of leaks on the Forester. When replacing the thermostat, inspect the housing face for corrosion, nicks, or unevenness that can prevent a good seal. Clean mating surfaces thoroughly and use the correct gasket or O-ring specified for the replacement part.
If you notice a thermostat housing seal leak, do not rely solely on sealant to fix major surface damage. In many cases a deteriorated housing should be resurfaced or replaced to ensure a reliable seal. Regular coolant changes and proper coolant concentration also reduce galvanic corrosion that can compromise sealing surfaces over time.
- Inspect housing face for pitting before installing a new thermostat
- Always use the correct gasket or O-ring supplied or specified
- Replace or resurface housing if mating surface is damaged
When a Thermostat Might Not Be the Cause
Several cooling system issues mimic thermostat failure. A failing water pump, clogged radiator passages, or a stuck radiator cap can cause overheating or inconsistent temperature readings. Low coolant level or air trapped in the system can also produce similar symptoms.
Before committing to a thermostat replacement, confirm the rest of the cooling system is functioning. Bleed trapped air, verify coolant condition, and inspect hoses and the radiator for blockage or collapse. Addressing these items first reduces unnecessary parts replacement and focuses the repair where it will be most effective.
- Check water pump and radiator condition when diagnosing overheating
- Verify correct coolant level and bleed air from the system
- Inspect radiator cap and hoses for proper pressure and flow
Routine Maintenance Intervals and When To Replace
Thermostats are often replaced as a preventive item during major coolant service or when performing head gasket or water pump work. For the 2009–2013 Forester, consider replacing the thermostat when the cooling system is being drained for other major repairs to avoid future labor duplication.
If you observe signs such as overheating events, slow warm-up, or leaks at the housing, replace the thermostat promptly. Regular inspection during coolant service and attention to symptoms will keep repair costs down and prevent consequential engine damage from uncontrolled temperatures.
- Replace during major coolant or engine front-end service
- Change immediately if overheating, poor warm-up, or leakage is present
- Inspect during routine coolant flushes and maintenance checks
Still looking for the perfect fit?
Select your Subaru Forester configuration to see the most relevant Thermostats recommendations
How We Chose the Best 2009-2013 Subaru Forester Thermostat
We evaluated candidate thermostats for the 2009–2013 Subaru Forester using a focused set of technical and practical criteria designed to reflect how owners maintain and drive these vehicles. Key evaluation criteria included: accurate opening temperature and tolerance, material and construction quality of the valve and gasket interface, compatibility with the Forester coolant system and OEM housing, resistance to corrosion and scale, and long-term sealing performance under typical service intervals. We also considered ease of installation for owners performing engine thermostat replacement and whether the unit required any additional gaskets or O-rings.Models were excluded if their published specifications did not match the temperature opening range recommended for the Forester or if fitment details were ambiguous. We filtered out thermostats that rely on nonstandard housings or require substantial modification. The final selection balances correct thermal performance, installation simplicity, and durability—factors that most directly affect symptoms like overheating, engine running cold, and coolant leaks at the thermostat housing.
Our full evaluation process is outlined in our review methodology.
FAQ
What temperature should the thermostat open in a 2009–2013 Forester?
Thermostats specify a nominal opening temperature and a tolerance. For the 2009–2013 Forester, owners typically choose units that match the OEM opening temperature to maintain proper operating range. Check published specs for the exact opening point and tolerance before replacing.
How can I tell if the thermostat is causing overheating?
Common signs include the engine reaching high temperature quickly, overheating under light load, or the upper radiator hose remaining cool while the engine gets hot. Confirm by monitoring hose temperature during warm-up and checking for coolant flow once the engine is warm.
Is a stuck-open thermostat common, and what does it cause?
A thermostat stuck open does occur and typically causes the engine to run colder than normal, leading to longer warm-up times and reduced fuel economy. It is less likely to cause immediate overheating but can affect performance and emissions.
Should I replace the gasket or O-ring when installing a new thermostat?
Yes. Replacing the gasket or O-ring is recommended because the old seal may be compressed or degraded. New seals help prevent thermostat housing seal leaks and ensure a reliable installation.
Can I test the thermostat without removing it?
Basic checks such as observing radiator hose temperature changes and timing warm-up can indicate thermostat behavior. For definitive verification you can remove the thermostat and bench-test it in heated water to observe the opening temperature.
How often should I inspect the cooling system on my Forester?
Inspect the cooling system at least annually and during any scheduled coolant service. Regular checks for leaks, coolant condition, and hose softness help detect issues early and prevent thermostat-related failures.
Final Verdict
For the 2009–2013 Subaru Forester, prioritize a thermostat that matches the recommended opening temperature, provides reliable sealing at the housing, and uses durable materials that withstand thermal cycling. Consider ease of installation and whether the part includes the correct gasket or O-ring to avoid repeat service.Balancing accurate temperature control with proper fitment and corrosion resistance will minimize overheating and cold-run symptoms. Use the evaluation factors above to pick the thermostat that best aligns with your driving habit and maintenance plans.
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