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If your Subaru Impreza is running hot or taking too long to reach operating temperature, swapping the thermostat can make a big difference. Choosing the right Subaru Impreza thermostat matters because the correct opening temperature and reliable valve action affect warm-up times, heater performance, and overall engine cooling balance. This guide covers seven shortlisted thermostats chosen for accurate fitment and dependable flow control. Read on to understand the evaluation criteria used and what features will matter most for your Impreza before you pick a replacement.
Looking for other parts? See all Subaru Impreza parts & accessories.
Top Picks at a Glance
| Best Reviewed A top-rated pick with lots of positive feedback | Subaru Thermostat Assembly 21200AA170 (Genuine OEM) |  |
| Most Popular A crowd favorite that many people choose with confidence | Subaru Thermostat and Gasket Kit 21210AA080 (Genuine Subaru) |  |
| 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 THF-003 Engine Coolant Thermostat with Gasket |  |
| Best Value for Money The sweet spot between price and what you get | Subaru Engine Coolant Thermostat (21200AA072) – Genuine Subaru OEM |  |
| Best Overall The best all-around choice for most situations | Subaru Engine Coolant Thermostat and Gasket (21200AA072 & 21236AA010) |  |
1. Subaru Thermostat Assembly 21200AA170 (Genuine OEM)
| Brand | Subaru |
|---|---|
| Model | Thermostat |
| Manufacturer Part Number | 21200AA170 |
This genuine Subaru thermostat assembly replaces the factory unit and restores proper coolant flow and temperature regulation on compatible Subaru models. It installs in place of the original part and is typically used during a cooling-system repair or as part of scheduled maintenance when a stuck thermostat causes overheating or poor cabin heat. The product is a direct manufacturer part so it matches factory fit and function, which reduces fitment guesswork and compatibility concerns. Buyers will appreciate the machined finish and consistent performance reported by users, and the part is suitable for a DIY swap or professional installation. Be aware the thermostat may not include the sealing gasket, which many owners need to buy separately, so plan for that extra item during replacement. If you expect any performance tuning or altered thermostat calibration, this is the stock-temperature unit rather than an aftermarket high- or low-temperature alternative.
Best for: Reliability-focused owners who prioritize factory-quality replacements and want a straightforward, dependable fix for cooling-system issues on their Subaru Impreza.
Less Ideal for: Drivers seeking performance-tuned or alternative-temperature thermostats should consider aftermarket options designed for altered cooling behavior.
- Genuine Subaru part number 21200AA170 for factory-fit replacement
- Matches factory fit and function to simplify installation
- Machined exterior finish indicating consistent manufacturing quality
- Widely reported to restore proper coolant temperatures after replacement
- Gasket may be sold separately requiring an additional purchase
- Not a performance or modified-temperature thermostat option
Verdict Choose this genuine Subaru thermostat when you want a factory-quality, drop-in replacement to restore reliable cooling on your Impreza.
2. Subaru Thermostat and Gasket Kit 21210AA080 (Genuine Subaru)
| Item model number | 21210AA080 |
|---|---|
| Manufacturer | Subaru |
| Manufacturer Part Number | 21210AA080 |
| OEM Part Number | 21210 AA080 |
This is a factory-quality thermostat and gasket kit made to the same specifications Subaru uses and sold under Subaru branding. It replaces a failing thermostat to restore proper coolant flow and normal operating temperature, which prevents overheating and erratic temperature gauge behavior. Installation is typical for an engine cooling job: drain some coolant, remove the housing, swap the old thermostat and gasket, and reassemble. Buyers will appreciate the exact fit and consistent opening temperature that preserve engine warm-up characteristics and cooling system calibration. The kit is straightforward for DIYers with basic tools and is the safer choice when you want a part that matches factory fit, clearances, and sealing surfaces. The main limitation is that it is a direct replacement rather than an upgraded performance thermostat, so it does not change thermostat temperature setpoint or flow characteristics beyond factory intent.
Best for: Convenience-driven buyers who prioritize a direct factory-quality replacement that fits without modification and restores normal cooling system behavior with minimal fuss.
Less Ideal for: Enthusiasts seeking a modified thermostat for altered warm-up or cooling behavior should consider aftermarket performance options instead.
- Factory-quality fit and finish for reliable sealing and alignment
- Matches Subaru's specified opening temperature for predictable engine warm-up
- Includes gasket to simplify replacement and reduce leak risk
- Well suited for DIY installation with standard hand tools
- Does not offer a different temperature setpoint or performance upgrade
Verdict Choose this genuine Subaru thermostat kit when you want a hassle-free, factory-spec replacement for your Impreza that restores correct engine temperature control.
3. Subaru Engine Coolant Thermostat and Gasket Set (21200AA072 & 21236AA010)
| ASIN | B01MQ5ND43 |
|---|---|
| Item model number | 21200AA072 & 21236AA010 |
This is a factory-quality thermostat and gasket set that replaces the original parts on compatible Subaru engines. Installed in the thermostat housing, the unit regulates coolant flow and helps maintain normal operating temperature for consistent engine performance. Because it is a genuine Subaru part, fitment is direct and the gasket is matched to the thermostat for a reliable seal during installation. For a performance-focused owner this part restores correct temperature control that can reduce overheating, maintain stable engine timing and combustion, and support consistent turbocharger or engine performance in WRX/STI models. The main limitation is that it is a single-purpose replacement part with no performance upgrades over factory specification, so users looking for a higher or adjustable temperature thermostat will need an aftermarket performance unit instead. Installation requires basic mechanical skill and coolant system draining, so factor labor time if not installing yourself.
Best for: Performance-oriented Subaru owners who prioritize factory-fit parts to restore reliable temperature control and consistent engine performance in the Impreza lineup.
Less Ideal for: Drivers seeking a performance-tuned or adjustable thermostat to alter operating temperature should consider aftermarket options instead.
- Factory-quality Subaru part for exact fit and compatibility with specified Subaru models
- Includes matching gasket to ensure a proper seal and reduce leak risk
- Direct-fit replacement simplifies installation and avoids modification
- Restores correct engine temperature control for consistent performance
- No performance or adjustable thermostat options beyond factory specification
- Requires coolant draining and basic mechanical work for installation
Verdict Choose this genuine Subaru thermostat and gasket set when you want a direct factory-quality replacement to restore correct temperature control in an Impreza, but pick an aftermarket unit if you need a performance-tuned thermostat.
4. DAZOM Engine Coolant Thermostat (replaces 21200AA072)
| Replaces part numbers | 21200AA072, 21236AA010, 21210AA080 |
|---|---|
| Fitment type | Direct Replacement |
| Brand | DAZOM |
| Item model number | T003 |
This DAZOM engine coolant thermostat is a low-cost, direct-replacement unit designed to fit Subaru applications that use part number 21200AA072. It installs like a factory replacement and is typical for routine cooling-system repairs where the stock thermostat has failed. The design uses a stainless-steel mounting flange and a channel-constructed spring frame intended to improve seating and durability compared with basic stamped units. For everyday driving the thermostat regulates coolant flow and engine warm-up in the same way a factory thermostat does, and the seller notes a jiggle-pin orientation for correct bleeding in vertical installations. Practical benefits include simple bolt-in fitment for compatible Subaru models and a single-piece package ready for a DIY swap. Limitations are the lack of detailed materials testing or long-term durability data and no additional gasket or sensor hardware in the package, so expect to reuse or replace associated seals during installation.
Best for: Budget-conscious Subaru owners focused on restoring proper cooling function without paying for a premium OEM part, especially those comfortable doing a DIY thermostat swap.
Less Ideal for: Owners seeking a long-term performance upgrade or those who want an all-inclusive kit with new seals and hardware should consider higher-end or OEM options instead.
- Direct-replacement fit for Subaru applications using part number 21200AA072
- Stainless-steel mounting flange and reinforced spring frame for improved seating and strength
- Simple single-piece unit that is straightforward to install for DIY repairs
- No gasket or additional installation hardware included
- Manufacturer does not publish long-term durability or testing data
Verdict A wallet-friendly, direct-replacement thermostat for Subaru Impreza drivers who need a straightforward DIY fix rather than a long-term performance upgrade.
5. AISIN THF-003 Engine Coolant Thermostat with Gasket
| Manufacturer | AISIN |
|---|---|
| Included components | Thermostat and gasket |
| Material | Stainless Steel |
| OEM Part Number | 21210-AA080 |
The AISIN THF-003 replaces a vehicle's thermostat to regulate coolant flow and maintain the engine at its designed operating temperature. For Subaru Impreza owners this is a direct-fit, factory-quality replacement that installs like the original and comes with the gasket needed for a proper seal. Aisin emphasizes OE-matched form, fit, and function, so the thermostat opens at the correct specified temperature and helps prevent overheating or overcooling when installed and the cooling system is properly bled. Practical benefits include durable machined construction and consistent operating temperature control that suits routine maintenance or repairs done during timing belt or coolant jobs. Some users report occasional quality defects or fitment confusion when engine variants differ, so verify compatibility with your Impreza's engine code before purchase and inspect the part on arrival. Expect straightforward installation for owners familiar with basic cooling-system work, but professional installation is sensible if you are risk averse or unfamiliar with coolant bleeding procedures.
Best for: A risk-averse mainstream buyer who wants a factory-quality, direct-replacement thermostat for routine maintenance or repair on a Subaru Impreza without experimenting with aftermarket performance parts.
Less Ideal for: Drivers seeking performance-modified thermostats or custom temperature control should look at purpose-built aftermarket options instead.
- Factory-matched design intended to replicate OEM form and operating temperature
- Includes gasket for a complete thermostat replacement and proper sealing
- Machined exterior and stainless components aimed at long service life
- Widely used part with many positive user reports for Subaru applications
- Compatibility depends on specific Impreza engine variants so fit must be confirmed before installation
- A small number of buyers report receiving damaged or nonfunctioning units
Verdict Choose the AISIN THF-003 for a reliable, OEM-matched thermostat replacement for a Subaru Impreza when you want factory-style fit and consistent temperature control.
6. Subaru Engine Coolant Thermostat (21200AA072) – Genuine Subaru OEM
| Manufacturer Part Number | 21200AA072 |
|---|---|
| Model Fitment | WRX 2002-2014 / STI 2004-2020 (includes Impreza coverage listed) |
This is the factory-quality Subaru replacement thermostat that opens and closes to regulate coolant flow and maintain correct engine temperature. It is used when a failing thermostat causes overheating, poor warm-up, or temperature fluctuations. Being a genuine Subaru part, it matches factory fit and function for compatible Impreza applications and installs in the original thermostat housing without modification. The product listing notes temperature-display-related features and manual operation, but in practical terms the unit provides the expected thermostatic operation and reliable sealing when installed with a proper gasket. The main limitation is that it does not include a replacement gasket or housing hardware, so plan to buy a gasket or inspect the housing during service. Also this is a direct factory replacement rather than an upgraded performance thermostat, so it restores stock behavior rather than changing operating temperature for tuning purposes.
Best for: Practical optimizers who want a straightforward, factory-accurate replacement that restores reliable cooling without extra fuss or modification for their Subaru Impreza.
Less Ideal for: Drivers looking to alter engine warm-up or operating temperature for performance tuning should consider an aftermarket performance thermostat instead.
- Genuine Subaru part ensures factory fit and compatibility with the Impreza cooling system.
- Restores correct thermostat operation to fix overheating and poor warm-up issues.
- Broad model coverage for Subaru Impreza model years listed, simplifying part selection.
- Users report consistent long-term performance and reliable sealing when installed correctly.
- Does not include a thermostat gasket, so a separate gasket purchase is usually required.
- Not a performance or high-temperature thermostat so it will not change operating temperature for tuning purposes.
Verdict Choose this genuine Subaru thermostat if you want a plug-in, factory-quality replacement to restore proper cooling on an Impreza rather than an upgraded performance change.
7. Subaru Engine Coolant Thermostat and Gasket (21200AA072 & 21236AA010)
| Brand | Subaru |
|---|---|
| Manufacturer Part Number | 21200AA072S |
| Is Discontinued By Manufacturer | No |
| ASIN | B01DWG0XXM |
This is the factory-quality Subaru engine coolant thermostat with its matching gasket designed as a direct replacement for a range of Subaru models including Impreza and WRX STI. It regulates coolant flow to maintain correct engine temperature and is typically installed during routine cooling-system service or when a failing thermostat causes overheating or constant fan operation. As a genuine Subaru part it fits precisely, restores correct operating temperature behavior, and avoids the calibration differences that sometimes come with aftermarket units. Buyers get the practical benefit of predictable warm-up, stable cruising temperature, and reduced risk of temperature-related drivability and seal problems. The main limitations are that you must confirm the correct temperature rating and part number for your specific Impreza year and engine before installing, and this is a single thermostat and gasket without additional hardware or seals that some repair kits include.
Best for: Balanced all-round buyers who want a reliable, factory-quality replacement for their Subaru Impreza and prefer predictable temperature control and long-term dependability.
Less Ideal for: Drivers who need a complete cooling-system repair kit with additional hoses, clamps, or aftermarket performance thermostats should look elsewhere.
- Factory-quality Subaru part designed for direct fit and correct calibration
- Includes matching gasket for a proper seal at installation
- Restores stable operating temperature and reduces unnecessary fan cycling
- Widely used across Subaru line so compatible fitment information is well documented
- Requires verifying the correct temperature rating for some model years to avoid incorrect fitment
- Sold as just the thermostat and gasket so other seals or hoses may need separate purchase
Verdict Choose this genuine Subaru thermostat and gasket when you want a precise, factory-quality replacement to restore correct engine temperature behavior in an Impreza.
Choosing the Right Subaru Impreza Thermostat: Key Factors to Consider
Correct Opening Temperature
The thermostat opening temperature determines when coolant begins circulating through the radiator, which affects warm-up time and operating temperature stability. Choosing a thermostat with the factory-recommended opening temperature helps the engine reach ideal thermal conditions quickly without running too cool or allowing excessive heat build-up.
An incorrect opening temperature can cause poor cabin heating or increased wear from prolonged cold running, while opening too late can raise overheating risk. Look for thermostats with tight manufacturing tolerances so the listed temperature reflects real-world behavior.
Fitment and Part Number Accuracy
Exact physical fitment is essential for leak-free installation and proper mechanical operation in the Subaru Impreza cooling system. Verify the part number fitment and that mounting dimensions, flange shape, and sensor passages match your vehicle to avoid installation problems.
A thermostat that does not seat correctly or requires modification can cause coolant bypass or seal issues. Clear fitment documentation reduces the chance of returns and helps ensure the replacement interfaces correctly with the housing and hoses.
Valve and Spring Design
The quality of the valve and spring assembly determines how smoothly the thermostat modulates coolant flow as temperature changes. A responsive valve with a consistent spring rate provides predictable transitions between closed, modulating, and fully open states, which helps maintain stable engine temperature.
Poorly designed valves can stick, chatter, or open inconsistently, leading to temperature swings. Durable springs and low-friction valve surfaces improve long-term reliability, particularly in engines that see frequent temperature cycling.
Sealing and Housing Interface
The thermostat housing seal prevents coolant leaks at the mounting interface. A good seal design and compatible gasket material minimize the chance of a thermostat housing seal leak and reduce the need for repeat repairs. Consider how the thermostat mates with the housing, whether a separate gasket is required, and how easy it is to achieve correct torque on the housing bolts.
Materials that resist coolant additives and temperature cycling will preserve seal integrity. Also check whether the thermostat requires a specific gasket type or O-ring to avoid compatibility issues during replacement.
Material Durability and Corrosion Resistance
Thermostats are exposed to coolant and temperature cycling, so materials that resist corrosion and fatigue extend service life. Brass, stainless steel, and coated components typically perform better than untreated metals in corrosive coolant environments.
Durable construction reduces the chance of valve leakage, sticking, or spring failure over time. Consider units that specify corrosion-resistant finishes or known long-life materials for components exposed to coolant flow.
Ease of Installation and Serviceability
How easy a thermostat is to install affects both DIY repairs and shop labor time. Thermostats that match the stock footprint, use common fasteners, and do not require extensive coolant system disassembly simplify replacement. Clear instructions and minimal ancillary parts lower the risk of installation errors.
Also consider accessibility of related components such as sensors and hoses. A design that allows straightforward reassembly and reliable torqueing of the housing helps prevent leaks and ensures the cooling system is returned to proper service quickly.
FAQ
What are common signs of a bad thermostat in a Subaru Impreza?
Common signs often include the engine running hotter than normal, slow warm-up, fluctuating temperature gauge readings, or poor heater performance. You might also notice coolant leaks near the thermostat housing or unusually high idle temperatures under load.
How can I test a thermostat without removing it?
You can monitor temperature gauge behavior and heater output during warm-up; a thermostat that stays closed will keep the engine cold and the heater weak. For a more direct test, use infrared temperature measurements at the radiator inlet and outlet to see if coolant circulation begins near the expected opening temperature. These methods are useful but less definitive than a bench test.
Will a stuck-open thermostat cause the engine to overheat?
A thermostat stuck open usually prevents the engine from reaching normal operating temperature, so overheating is unlikely from that issue alone. However, running too cool can lead to incomplete combustion and increased wear. Overheating typically results from a thermostat that fails to open or other cooling system faults.
What causes a thermostat housing seal leak?
Seal leaks commonly result from degraded gasket material, improper seating during installation, or uneven torque on the housing bolts. Corrosion or warped housing surfaces can also prevent a proper seal. Ensuring the mating surfaces are clean, using the correct gasket, and following proper torque specs reduces leak risk.
How often should I replace the thermostat in my Subaru Impreza?
There is no fixed interval, but thermostats are typically replaced when symptoms appear or during major cooling system service. Routine inspections during maintenance can identify early signs of failure. Replacing the thermostat when you replace coolant hoses or perform a head gasket repair is often practical.
Can thermostat issues affect fuel economy?
Yes. If the thermostat causes the engine to run too cold for extended periods, the engine management system may enrich the fuel mixture, which can reduce fuel economy. Conversely, a thermostat that delays warm-up can also impair fuel efficiency until normal operating temperature is reached.
How We Chose the Best Subaru Impreza Thermostat
We selected contenders based on criteria that directly affect performance and long-term reliability on Subaru Impreza engines. Key filters included documented fitment to Impreza engine families and model years, consistent opening temperature tolerance, and durable materials that resist corrosion and sticking. We also evaluated valve and spring design for predictable flow control, sealing interfaces to minimize housing leaks, and manufacturer specifications for lifespan under typical driving conditions.Units lacking clear fitment data, inconsistent temperature ratings, or signs of poor sealing design were excluded. We prioritized thermostats with straightforward mechanical designs, reliable thermal response, and clear part number fitment guidance. The final list balances correct physical compatibility, thermal performance, and construction quality so you get a replacement that restores normal operating temperature and heater function.
Our full evaluation process is outlined in our review methodology.
Conclusion
When choosing a replacement thermostat for a Subaru Impreza, prioritize correct opening temperature and confirmed part number fitment to ensure proper warm-up and leak-free installation. Balance build quality and sealing design to reduce the chance of sticking or housing leaks, and consider ease of installation to minimize labor and rework.Keep in mind the tradeoffs between rapid warm-up and coolant flow control. Focus on thermostats that provide reliable temperature regulation and durable construction to restore normal engine performance and consistent cabin heating.
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- Check out all Subaru Impreza parts & accessories
- Explore Thermostats Maintenance & Repair Guides
- Explore Cooling System
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