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If you drive a 2002-2007 Subaru Impreza, a healthy thermostat matters for daily reliability, fuel economy, and cabin heat. A thermostat that sticks open can let the engine run cold and reduce performance, while one that sticks closed risks overheating and costly damage. You need a part that fits correctly, responds reliably to temperature changes, and seals to prevent coolant loss.This guide shortlists the top thermostat options compatible with the 2002-2007 Impreza. We focus on fitment, thermal control, and durability to help you pick the right replacement for routine maintenance or troubleshooting. Below are the shortlisted thermostats and the evaluation criteria used to vet them.
Looking for other parts? See all Subaru Impreza parts & accessories.
Top Picks at a Glance
| Best Overall The best all-around choice for most situations | Subaru Genuine Thermostat and Gasket Kit (21210AA080) |  |
| Best Quality Built to a higher standard if you want something more solid | AISIN Engine Coolant Thermostat with Gasket (THF-002) |  |
| Best Value The sweet spot between price and what you get | Allmotorparts 21200AA072 Replacement Engine Thermostat with Gasket |  |
| Best Budget The cheapest option that still gets the job done reliably | FOXSTAR Engine Thermostat for Subaru (21200AA170 / 21210AA080 Compatible) |  |
| Highest Rated A top-rated pick with lots of positive feedback | Subaru Genuine Engine Coolant Thermostat (21200AA072) |  |
| 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 | Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010) |  |
1. Subaru Genuine Thermostat and Gasket Kit (21210AA080)
| Manufacturer | Subaru |
|---|---|
| Item model number | 21210AA080 |
| Manufacturer Part Number | 21210AA080 |
This Subaru genuine thermostat and gasket kit is a factory replacement designed to restore proper engine cooling control on compatible Subaru models. It includes the thermostatic unit and sealing gasket so you can replace the failed component and reseal the housing in one job. Typical use is straightforward: remove the old thermostat, fit the new unit with the gasket, and refill and bleed the cooling system. Being a genuine Subaru part it matches factory fit, tolerances, and materials which reduces the chance of leakage and fitment headaches compared with generic aftermarket units. The kit’s main practical benefit is confidence in compatibility and longevity when maintaining the cooling system. Limitations are that it is a single-purpose replacement part with no performance upgrades and you must follow proper coolant and bleeding procedures during installation; if you need higher-temperature operation or a performance thermostat, this factory-spec unit will not provide that.
Best for: Buyers who want a reliable, factory-spec replacement to restore proper coolant control without modifying engine behavior, especially when prioritizing correct fit and long-term dependability.
Less Ideal for: Drivers seeking a performance thermostat, altered temperature calibration, or aftermarket styling and tuning upgrades should choose a different part.
- Factory-quality fit and materials designed to match Subaru specifications
- Includes gasket for a complete thermostat housing reseal
- Direct replacement simplifies installation and reduces fitment uncertainty
- Reduces risk of coolant leaks compared with mismatched aftermarket parts
- Not a performance or high-temperature thermostat upgrade
Verdict: Choose this genuine Subaru thermostat and gasket kit when you want a no-fuss factory-match replacement to reliably restore cooling system function.
2. AISIN Engine Coolant Thermostat with Gasket (THF-002)
| Manufacturer Part Number | THF-002 |
|---|---|
| OEM Part Number | 21200-AA072 |
| Position | Center |
| Includes | Thermostat and O-ring gasket |
This AISIN thermostat is a direct replacement designed to restore correct coolant flow and temperature control in vehicles that use this part. It installs in the center position and ships with the gasket needed for a single thermostat swap. AISIN emphasizes OE-matched form, fit, and function and specifies the correct operating temperature so the cooling system returns to factory behavior. The machined exterior and OEM part number match make it a straightforward choice for routine maintenance or when a failing thermostat causes overheating or poor warm-up. For performance-focused drivers on 2002–2007 Impreza models, it maintains factory temperature calibration which helps preserve engine tuning and boost control consistency. Limitations are that this is a direct replacement, not a modified or high-flow performance thermostat, so it will not change thermostat opening temperature or significantly alter coolant flow characteristics beyond restoring stock behavior.
Best for: Drivers who want a reliable, factory-spec replacement to restore correct engine temperature control and maintain existing performance tuning on a 2002–2007 Impreza.
Less Ideal for: Drivers seeking lower or higher opening temperatures or increased coolant flow for modified engines should consider purpose-built performance thermostats instead.
- OE-matched design for reliable fit and correct operating temperature
- Includes gasket for a complete single-unit replacement
- Machined exterior and OEM part number simplify identification and installation
- Manufactured by a reputable supplier known for factory-quality components
- Not a performance or high-flow thermostat so it will not change operating temperature or coolant flow characteristics
Verdict: Choose this AISIN thermostat when you need a dependable, factory-matched replacement to restore correct temperature control on a 2002–2007 Subaru Impreza.
3. Allmotorparts 21200AA072 Replacement Engine Thermostat with Gasket
| Compatible models | Multiple Subaru models listed including Impreza 1993-2014 and WRX/STI variants |
|---|---|
| Replaces part numbers | 21200AA072, 21236AA010, 21210AA080 |
| Material | Stainless Steel |
| Includes | Thermostat with gasket |
This Allmotorparts thermostat replaces factory-style units and controls coolant flow to help the engine reach and maintain proper operating temperature. It installs in the outlet casting using a stainless steel mounting flange and a channel-constructed spring frame for strength, and it ships with a gasket for a complete housing seal. In daily use the thermostat will stay closed until the engine warms, then open to send coolant to the radiator, which prevents both overcooling and overheating. The construction aims to match factory fit, so it is suitable for routine tuneups or when a failing thermostat shows overheating, erratic temperature, or coolant leaks. Limitations are typical of aftermarket parts: this is not an OEM part and long-term durability may vary by driving conditions, and installation can expose studs or frozen nuts that require care. For a practical, budget-conscious maintenance replacement on a compatible Subaru, this unit offers a straightforward swap and ready-to-fit components.
Best for: Buyers who prioritize reliable, cost-effective maintenance for a Subaru and want a direct-fit replacement to restore proper engine temperature control without extra modification.
Less Ideal for: Those seeking an exact factory-manufactured thermostat or racers wanting performance-tuned temperature control should consider OEM or performance-specific options instead.
- Designed to match factory fit, simplifying installation
- Includes gasket for a complete housing seal
- Stainless steel mounting flange and reinforced spring frame for improved strength
- Replaces multiple Subaru part numbers for easier compatibility checks
- Aftermarket part rather than genuine factory component so long-term durability can vary
Verdict: A practical, direct-fit aftermarket thermostat that restores proper cooling system function on compatible Subaru Impreza models with minimal fuss.
4. FOXSTAR Engine Thermostat for Subaru (21200AA170 / 21210AA080 Compatible)
| Manufacturer | FOXSTAR |
|---|---|
| Model | Engine Thermostat |
| Item model number | 21200AA170 |
| Exterior | Machined |
This FOXSTAR replacement thermostat is a budget-friendly option that replaces Subaru part numbers like 21200-AA170 and 21210-AA080 for Impreza-era engines. It is made from stainless steel and copper components with a machined housing and is intended to control coolant flow to keep the engine in its optimal temperature band. Installation is straightforward for anyone familiar with basic cooling-system work since it fits the thermostat housing at the end of the upper radiator hose. In use it restores proper warm-up and helps avoid the fuel-economy and drivability issues that come from a stuck-open or stuck-closed thermostat. The practical benefits are reliability of temperature control and corrosion resistance from the materials used. Limitations are typical for aftermarket budget parts: long-term durability is not documented and fit relies on matching OEM part numbers exactly, so confirm compatibility before purchase.
Best for: Buyers seeking an affordable, direct-fit replacement thermostat for a 2002–2007 Subaru Impreza who prioritize getting the cooling system back to proper operating temperature without paying for OE branded parts.
Less Ideal for: Owners who want documented long-term warranty coverage or vintage-restoration-grade components should consider higher-end or dealer-sourced options.
- Direct replacement for several Subaru thermostat part numbers listed in the description
- Stainless steel and copper construction offers corrosion and high-temperature resistance
- Machined exterior and treated housing intended to improve sealing and fit
- Simple install that uses the factory thermostat location and requires basic hand tools
- No manufacturer-backed long-term durability data or warranty statement provided
Verdict: A cost-conscious direct-replacement thermostat that restores proper engine warm-up and coolant control for 2002–2007 Subaru Impreza applications when OEM part numbers match.
5. Subaru Genuine Engine Coolant Thermostat (21200AA072)
| Manufacturer Part Number | 21200AA072 |
|---|---|
| Manufacturer | Subaru |
| Model Name | WRX 2002-2014 / STI 2004-2020 |
| OEM Part Number | 21200 AA072 |
This is a genuine Subaru replacement thermostat made to factory specifications and intended to restore correct coolant flow and temperature control in compatible Subaru engines. It installs in place of the original unit and is typically used during routine cooling-system maintenance or when a failing thermostat causes overheating or poor warm-up performance. The part is manufactured to Subaru standards, so buyers can expect the same temperature rating and fitment the factory intended, which reduces the risk of fitment hassles or premature failure compared with generic aftermarket units. Because it is a direct-fit genuine part, it simplifies installation for DIYers who already have basic tools and knowledge of the cooling system. The main limitation is that it is a direct replacement only for the listed Subaru applications, so confirm model and engine fit before purchasing, and it does not include a gasket or housing unless specified by the seller.
Best for: Buyers prioritizing long-term dependability and exact factory fit for Subaru cooling systems who want a direct replacement that matches factory performance.
Less Ideal for: Drivers wanting an upgraded high-performance thermostat or universal aftermarket options with different temperature calibrations should consider performance-oriented alternatives.
- Genuine Subaru part manufactured to factory specifications for reliable temperature control
- Direct-fit replacement reduces fitment issues and simplifies installation
- Covers a wide range of Subaru models including Impreza and WRX, ensuring correct application for listed vehicles
- Sold as the thermostat unit only and may require separate gasket or housing parts for complete installation
Verdict: Choose this genuine Subaru thermostat when you want a factory-match replacement that restores original cooling performance and reliability for the specified Subaru models.
6. AISIN THF-003 Engine Coolant Thermostat with Gasket
| Manufacturer Part Number | THF-003 |
|---|---|
| OEM Part Number | 21210-AA080 |
| Included Components | Thermostat and O-ring gasket |
| Material | Stainless Steel |
The AISIN THF-003 is a direct-fit replacement thermostat designed to restore correct engine warm-up and cooling behavior in compatible Subaru and Saab models. It installs in place of the factory unit and comes with an O-ring style gasket, so it is typically used during routine cooling-system repairs or when a failing thermostat is causing overheating or poor heater performance. AISIN emphasizes OE-matched operating temperature and engineered form, fit, and function, which helps ensure consistent temperature control and easier installation for DIYers and independent shops. The stainless steel construction and factory-specified calibration aim to provide dependable long-term operation. Limitations are that fitment is vehicle and engine-specific, so you must confirm compatibility for 2002-2007 Impreza variants before buying, and the listing does not include detailed installation hardware beyond the gasket.
Best for: Buyers who prioritize a proven, factory-quality replacement to restore correct engine temperature control and who value parts from established manufacturers and real-world reviews.
Less Ideal for: If you need a universal or performance thermostat with altered opening temperature, or if you are unsure about exact engine fitment, choose a different option or consult a parts specialist.
- OE-matched operating temperature for predictable cooling system behavior
- Includes gasket for a ready replacement during thermostat service
- Stainless steel construction for corrosion resistance and durability
- Made by a large, experienced automotive parts manufacturer known for factory-quality components
- Compatibility varies by engine and year so fitment must be verified before purchase
Verdict: Choose the AISIN THF-003 when you want a factory-matched, dependable replacement thermostat that includes a gasket and restores correct cooling performance for compatible Impreza engines.
7. Subaru Engine Coolant Thermostat with Gasket (21200AA072 & 21236AA010)
| Manufacturer Part Number | 21200AA072S |
|---|---|
| Brand | Subaru |
| Included Parts | Thermostat and gasket |
| ASIN | B01DWG0XXM |
This is a factory-quality Subaru replacement thermostat sold with the matching gasket for a straightforward coolant system repair. It replaces the original thermostat housing component and is intended for direct-fit installation on compatible Subaru models, restoring proper engine warm-up and cooling behavior. Because it is a genuine Subaru part, expect OEM fit, correct sealing with the included gasket, and reliable temperature control compared with aftermarket units. Typical use is as a like-for-like service part during routine cooling system maintenance or when the thermostat fails and causes overheating or poor heater performance. The main limitations are that it only fits the listed Subaru models and part number, so confirm compatibility before ordering, and it does not include additional hardware beyond the gasket. For buyers who prefer factory-spec replacements and minimal fuss during installation, this covers the core need without extras or performance tuning options.
Best for: Buyers who want a risk-averse, factory-spec replacement for routine cooling system repairs on supported Subaru vehicles and value correct fit and proven reliability.
Less Ideal for: Shoppers seeking performance thermostats with altered temperature ratings or those needing a universal or custom-fit part for nonlisted vehicles should look elsewhere.
- Genuine Subaru part for factory-quality fit and function
- Includes matching gasket to ensure proper sealing at installation
- Direct-fit design simplifies replacement with no modification required
- Widely used across Subaru model years, making it familiar to mechanics
- Only covers vehicles listed by Subaru so it is not a universal thermostat
- Does not include extra mounting hardware or hoses
Verdict: Choose this genuine Subaru thermostat and gasket when you want a direct-fit, factory-quality replacement to restore correct engine cooling on compatible Subaru models.
Choosing the Right 2002-2007 Subaru Impreza Thermostat: Key Factors to Consider
Fitment and Compatibility
Correct fitment is the most important factor for a 2002-2007 Subaru Impreza thermostat. A direct-fit unit that matches the Impreza thermostat housing and bolt pattern reduces installation time and minimizes the risk of coolant leaks.
Compatibility also covers whether the thermostat uses the same gasket or O-ring type the vehicle requires. Mismatched housings or incorrect flange thickness can change coolant flow and cause sealing problems. Ensuring the replacement lists explicit fitment for the 2002 through 2007 Impreza model years prevents surprises and the need for additional adapters or machining.
Opening Temperature and Thermal Accuracy
Thermostats are rated to open at a specific temperature, typically close to the engine's optimal operating point. Choosing a thermostat with the proper opening temperature keeps the engine within its designed thermal band for efficient combustion and emissions control.
Thermal accuracy matters because a thermostat that opens too late can cause overheating and one that opens too early can let the engine run cold. For the Impreza, prioritize units with stamped or specified opening temperatures and minimal variance between the rated and actual opening points to avoid drivability issues.
Material Quality and Corrosion Resistance
Coolant is chemically active and components in contact with it must resist corrosion and galvanic reactions. Thermostats constructed with stainless steel, brass, or high-grade alloys for the valve and housing reduce the chance of premature failure.
Seals and gaskets should be made of coolant-compatible elastomers to prevent swelling or degradation. Better materials extend service life and lower the likelihood of a thermostat housing seal leak that can lead to coolant loss and overheating.
Valve Mechanism and Stick Resistance
A thermostat's valve must move smoothly across temperature cycles. Sticky valves are a common failure mode that cause either overheating or engine running cold if the valve becomes stuck closed or open respectively.
Look for thermostats with proven plunger designs, proper spring tension, and corrosion-resistant internals. These design choices reduce the risk of the valve jamming after years of exposure to coolant and heat cycles.
Gasket and Housing Interface
The gasket or seal between the thermostat and housing is a frequent source of leaks. A thermostat that includes a correct, well-shaped gasket or seals cleanly against the Impreza housing reduces the chance of a persistent drip or pressure loss.
Consider whether the replacement requires a separate gasket and whether the housing surface needs resurfacing. Proper sealing also affects coolant flow paths and pressure, so a good interface preserves cooling system integrity.
Ease of Installation and Serviceability
Thermostat replacement on the Impreza is often done during routine coolant service, so a unit that is straightforward to install saves time and reduces labor complexity.
Features that matter include clear orientation markings, included gasket or O-ring, and standard fastener access. Also consider whether the design allows for easy testing and inspection without removing large engine components. Service-friendly choices make future maintenance simpler.
Common Thermostat Symptoms on a 2002-2007 Impreza
Recognizing thermostat-related symptoms helps target the repair quickly. Overheating while idling, a rapidly climbing temperature gauge, or boiling coolant smell often indicate a thermostat stuck closed or restricted coolant flow. Conversely, if the engine takes unusually long to reach operating temperature or the heater provides little warmth, the thermostat may be stuck open.
Other signs include coolant leaks around the thermostat housing seam, fluctuating temperature readings, or unusual thermostat housing corrosion. Addressing symptoms early helps avoid head gasket damage or warped components that result from prolonged overheating.
- High temp gauge at idle or under light load
- Engine running cold and poor heater performance
- Coolant pooling or seepage at the thermostat housing
- Rapid temperature swings while driving
How to Test a Thermostat on This Subaru
A simple test begins with a cold engine. Remove the thermostat and visually inspect the valve and gasket. If removal is not possible immediately, monitor the coolant temperature behavior during a warm-up; a thermostat that fails to begin closing or opening at expected temperatures is suspect.
You can also test by watching the upper radiator hose as the engine warms. The hose should remain cool until the thermostat opens, then get hot as coolant flows. For a removed unit, a pot of water and a thermometer let you observe the opening temperature directly, but ensure you note the specified opening point for the Impreza when interpreting results.
- Watch upper radiator hose temperature during warm-up
- Inspect housing area for coolant leaks or corrosion
- Bench-test removed thermostat in hot water with thermometer
Dealing with Thermostat Housing Seal Leaks
Seal leaks at the thermostat housing are a common reason for coolant loss on older Impreza models. When replacing the thermostat, always clean mating surfaces and inspect the housing for pitting, distortion, or damaged bolt bosses. A new gasket or O-ring that matches the housing profile is essential.
If the housing is corroded or warped, simply swapping the thermostat may not stop leaks. In that case, consider resurfacing the sealing face or replacing the housing. Proper torqueing of fasteners in a crisscross pattern and following torque specs prevents deformation that leads to leaks.
- Clean mating surfaces before installing new gasket
- Check housing for corrosion or warping
- Use correct torque sequence and specifications
Routine Maintenance Tips to Prevent Thermostat Failure
Regular coolant changes using the correct coolant type and concentration reduce corrosion and deposit buildup that can impair thermostat operation. Follow the Impreza maintenance schedule for coolant service intervals and use a recommended coolant formulation to protect metal components and seals.
During routine service, inspect the thermostat area, hoses, and clamps for age-related wear. Keeping the cooling system free of scale, oil contamination, and air pockets will help the thermostat operate reliably over time.
- Replace coolant at manufacturer-recommended intervals
- Inspect hoses and clamps during coolant service
- Bleed air from the cooling system after replacement
Installation Notes Specific to the 2002-2007 Impreza
When installing a replacement thermostat, orient it as specified by the vehicle manual and confirm the sealing surface is clean. Use the correct gasket type and replace any crush washers or O-rings that come with the housing bolts. Tighten bolts evenly to avoid distorting the housing.
After installation, refill and bleed the cooling system to remove trapped air. Monitor temperature behavior across a few warm-up and short drive cycles to confirm proper operation before returning the vehicle to regular use.
- Confirm thermostat orientation before sealing
- Replace gasket or O-ring and any related hardware
- Bleed the cooling system and verify temperature stability
Still looking for the perfect fit?
Select your Subaru Impreza configuration to see the most relevant Thermostats recommendations
How We Chose the Best 2002-2007 Subaru Impreza Thermostat
We evaluated candidates using criteria tailored to the 2002-2007 Subaru Impreza cooling system and common failure modes. Primary considerations included exact fitment and OEM-equivalent housings to avoid leaks at the thermostat gasket and housing interface. Thermal response and opening temperature tolerance were assessed to ensure the thermostat maintains the Impreza's designed operating range without causing prolonged cold running or premature overheating. Build quality and materials were examined, specifically corrosion-resistant components and robust valve mechanisms that withstand typical coolant chemistry and seasonal thermal cycling. Ease of installation was factored in, including whether the unit pairs with a reusable or replaceable gasket and whether the housing alignment simplifies bolt torquing. Long-term reliability signals such as stamped temperature ratings, reinforced seals, and resistance to sticking were also prioritized. Exclusions: we excluded universal thermostats that require major housing modifications, novelty units without clear temperature specs, and items lacking fitting information for the 2002-2007 Impreza. The result is a focused list that balances correct fitment, consistent thermal performance, and installation practicality.
Our full evaluation process is outlined in our review methodology.
FAQ
How long does a thermostat typically last on a 2002-2007 Impreza?
Thermostat life varies with coolant quality and driving conditions, but they often last many years. Regular coolant maintenance extends life, while corrosion or scale buildup can shorten service intervals.
Can a bad thermostat cause my Impreza to overheat only at idle?
Yes. A thermostat that sticks closed or opens inconsistently can allow overheating at idle when engine load is low. It is one of several possible causes, so check coolant flow and fan operation too.
Is it okay to drive a short distance with a thermostat-related leak?
Driving with a coolant leak is risky because it can lead to rapid coolant loss and overheating. If the leak is minor and you must move the car, monitor temperature closely and top off coolant, then address the leak promptly.
What symptoms indicate the thermostat is stuck open on the Impreza?
Common signs of a stuck-open thermostat include prolonged warm-up times, poor heater performance, and the engine running below normal operating temperature. These issues often show up soon after startup in cool ambient conditions.
Do I need to replace the thermostat gasket when doing a thermostat swap?
Yes. Replacing the gasket or O-ring is recommended whenever you service the thermostat. Reusing an old seal increases the risk of leaks and may lead to additional repairs.
Will testing the thermostat on the car give accurate results?
On-vehicle observations like hose temperature changes provide useful clues but are not definitive. A bench test in hot water is more accurate for confirming the thermostat opens at the specified temperature.
Final Verdict
Choose a thermostat that balances accurate opening temperature, correct fitment, and durable materials for the 2002-2007 Subaru Impreza. Prioritize units that specify fitment for these model years, include the proper gasket, and use corrosion-resistant internals to reduce the risk of sticking or leaks. Consider ease of installation and whether the housing requires any resurfacing when selecting a replacement.Addressing symptoms promptly and following proper installation and bleeding procedures will restore reliable engine temperature control and prevent the larger cooling system damage that may result from a faulty thermostat.
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