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If your Infiniti FX35 is taking too long to warm up or is running hotter or colder than expected, the thermostat is a common place to look. Choosing the right Infiniti FX35 thermostat matters because it controls coolant flow and affects engine warm-up time, operating temperature, and cabin heater performance. This guide covers seven shortlisted thermostats that meet common fitment and durability expectations for the FX35. We evaluated build quality, temperature calibration, and sealing to help you narrow options quickly. Read on to understand the selection criteria and which features to prioritize before installing.
Looking for other parts? See all Infiniti FX35 parts & accessories.
Top Picks at a Glance
| Best Reviewed A top-rated pick with lots of positive feedback | AISIN THN-007 Engine Coolant Thermostat with Housing |  |
| Most Popular A crowd favorite that many people choose with confidence | Toyota 90916-A3003 Genuine Thermostat |  |
| Best Quality Built to a higher standard if you want something more solid | ACDelco Gold Engine Coolant Thermostat (19474058) |  |
| Best Budget The cheapest option that still gets the job done reliably | Dorman Engine Coolant Thermostat Housing 902-5900 |  |
| 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 | Dorman Engine Coolant Thermostat Housing Assembly (902-5140) |  |
| Best Overall The best all-around choice for most situations | Honda Genuine Thermostat Assembly (19301-RNA-315) |  |
1. AISIN THN-007 Engine Coolant Thermostat with Housing
| Manufacturer | AISIN |
|---|---|
| Item model number | THN-007 |
| Exterior | Machined |
| OEM Part Number | 21200-31U1B |
| Position | Center |
The AISIN THN-007 replaces the engine coolant thermostat and housing to restore proper temperature regulation on compatible Infiniti vehicles. It installs in place of the factory unit and is intended for direct replacement to correct overheating or fluctuating temperature gauge readings. Notable features include AISIN's factory-matched design and correct specified opening temperature, plus a machined housing that helps ensure a precise fit. In practice this means straightforward installation for anyone doing a thermostat job and predictable temperature control once fitted. Reviewers report OEM-like appearance and durable construction, and the included housing removes the need to transfer the old part. Important limitations are that the unit does not include a gasket so buyers must source a compatible seal separately, and fitment can vary by engine and year so verifying the exact application for an FX35 is essential. For reliability-focused buyers who prefer factory-quality replacements, this thermostat is a practical way to restore cooling system performance.
Best for: Reliability-focused buyers who prioritize factory-quality replacements and want a durable, direct-fit thermostat to restore consistent cooling performance in their Infiniti FX35.
Less Ideal for: Buyers looking for a complete gasket kit or a universal aftermarket option should consider alternatives that include seals or broader fitment coverage.
- Factory-matched design for correct operating temperature and fit
- Includes thermostat housing for a more complete replacement
- Machined exterior for consistent fit and durable construction
- Widely reported OEM-like fit and reliable temperature control
- Does not include a gasket so a separate seal purchase is required
- Fitment depends on engine/year so verification for specific FX35 variants is necessary
Verdict A reliable, factory-matched thermostat and housing intended to restore proper cooling behavior on the Infiniti FX35, but buy a gasket separately and confirm exact engine fitment.
2. Toyota 90916-A3003 Genuine Thermostat
| Brand | Toyota |
|---|---|
| Item model number | 90916-A3003 |
| Manufacturer Part Number | 90916-A3003 |
This Toyota 90916-A3003 is a factory-quality replacement thermostat that regulates coolant flow to keep engine temperature steady. It installs where the original thermostat sits and is used during routine cooling-system repairs or when the engine is running hot or overcooling. The unit is sold as the bare thermostat without a housing gasket, so expect to reuse the old gasket or purchase the correct seal separately. Buyers report straightforward fit and reliable operation when matched to compatible engines, and the genuine part construction reduces uncertainty about tolerances and valve behavior. The small size and OEM fit make it practical for a home mechanic or technician swapping the part during a coolant service. The main limitation is the missing gasket which adds an extra step and small additional cost, and installation can be tight on some engines where surrounding components must be removed for access.
Best for: Convenience-driven buyers who prioritize getting a factory-quality replacement with minimal compatibility questions and prefer a straightforward swap during routine maintenance.
Less Ideal for: Drivers who want a complete kit with gasket and hardware included or those who prefer aftermarket thermostats tuned for altered cooling characteristics should look elsewhere.
- Genuine Toyota factory-quality part for predictable fit and function
- Widely reported to match original fit and valve operation
- Compact, direct-replacement design suited for standard thermostat locations
- Does not include the thermostat housing gasket so a separate seal may be required
- Small part can be awkward to install in cramped engine bays requiring removal of nearby components
Verdict A dependable genuine Toyota replacement thermostat that fits like the factory unit but requires you to supply a gasket for installation.
3. ACDelco Gold Engine Coolant Thermostat (19474058)
| Manufacturer | ACDelco |
|---|---|
| Included Components | Engine Coolant Thermostat |
| Power Source | Not Powered |
| Exterior | Machined |
This ACDelco Gold thermostat is a factory-quality replacement designed to regulate coolant flow and return an engine to its correct operating temperature. It installs in place of the original thermostat and is typically used when a stuck-open or stuck-closed thermometer causes overheating or chronically low running temperature. The unit is machined and supplied with the necessary gasket, so fit and sealing are straightforward when installed to factory procedures. Buyers will appreciate its compatibility with GM-backed parts manufacturing and the consistent temperature control that prevents prolonged fan operation and related drivability codes. Installation is a routine job for a home mechanic with common hand tools, though expect to drain some coolant and take care with hose clamps to limit spills. There are no electronic components or advanced features here, so it simply restores proper thermostat function rather than adding diagnostics or performance tuning.
Best for: Performance-oriented Infiniti FX35 owners who prioritize reliable, factory-spec engine temperature control to maintain engine performance and avoid cooling-related drivability issues.
Less Ideal for: Drivers looking for aftermarket thermostats that change operating temperature or add diagnostic features should consider performance-tuned alternatives instead.
- Factory-quality design that matches original fit and function
- Included gasket simplifies installation and sealing
- Machined exterior for consistent build and durability
- Proven to correct temperature-related issues like constant fan run or low coolant temperature
- No electronic features or temperature adjustability beyond factory spec
- Requires coolant drain and basic mechanical work to install
Verdict A direct factory-quality replacement that restores correct coolant control for drivers who need reliable, OEM-level temperature management.
4. Dorman Engine Coolant Thermostat Housing 902-5900
| Manufacturer | Dorman Products |
|---|---|
| Manufacturer Part Number | 902-5900 |
| OEM Part Number | 815421; 85421; 9339; CH6900; CO34869; 13049AR000 |
| Included Components | Dorman – WATER OUTLETS |
| Position | Center |
This Dorman thermostat housing is a straightforward, budget-focused replacement for vehicles that use the listed part number, including Infiniti V8 applications in the specified model years. It replaces the factory housing and provides the same mounting and coolant outlet locations so installation is a simple swap for a competent DIYer or technician. The housing is engineered to tolerate extreme temperature changes to reduce cracking and leaking and is made from metal materials intended to be more durable than some plastic housings. Customers report consistent fitment and quick installation with factory-aligned connections. The practical benefit is a lower-cost alternative to dealer parts that still restores cooling system integrity. Limitations are that this is an aftermarket replacement rather than a factory part so surface finish and exact material composition may differ from the original, and fit should be double-checked for year and trim before ordering to avoid returns or additional labor.
Best for: A budget-conscious buyer who prioritizes fixing a coolant leak or replacing a failed housing quickly and affordably while accepting a reliable aftermarket alternative to factory parts.
Less Ideal for: Owners seeking an exact factory-branded component or those who require OEM-matched materials and finish for concours or long-term original-spec restoration should consider dealer parts instead.
- Direct replacement design that matches factory mounting and outlet locations
- Constructed to withstand extreme temperature changes to reduce cracking and leaking
- Lower cost than dealer or factory replacement while restoring cooling system function
- Many buyers report straightforward installation and good fitment
- Not a factory-brand part so finish and material may differ from the original
- Fit depends on correct year and trim selection so verification is required before purchase
Verdict A cost-effective, durable aftermarket thermostat housing that restores cooling system reliability for qualifying Infiniti FX applications when you want a budget-friendly replacement.
5. MotoRad 7416-203 203°F Fail-Safe Thermostat
| Included components | (1) Fail-Safe Thermostat, 203 °F |
|---|---|
| Voltage | 230 Volts |
| Power Source | Battery Powered |
| Material | Plastic, Metal, Abs |
The MotoRad 7416-203 is a 203°F fail-safe thermostat intended as a direct-fit replacement that prevents thermostat overheating failure by locking closed at its rated temperature and includes a lockable design. It is typically used as a factory-quality replacement during routine cooling-system service or when troubleshooting overheating on compatible vehicles. The unit is fully tested and calibrated at the factory and is described as manufactured to meet or exceed factory specifications. In practice it installs like a standard thermostat and gives straightforward temperature control with a mechanical, analog operation so there are no electronics to program. Some buyers report reliable performance and easy fit, while others have reported units that failed to open in bench tests or arrived with corrosion, so pre-install testing is advisable. This thermostat offers a simple, affordable way to restore correct engine warm-up and cooling behavior, but mixed customer reports mean you should verify operation before driving heavy use or long trips.
Best for: Risk-averse mainstream buyers who prioritize restoring dependable, factory-quality cooling performance with a straightforward replacement part and who prefer tested, vehicle-specific components.
Less Ideal for: Drivers who need a performance or modified cooling setup, or those unwilling to bench-test a replacement thermostat before installation should look for alternatives or professional-sourced parts.
- Factory-style 203°F rating for predictable cooling control
- Fail-safe locking design intended to prevent catastrophic overheating
- Manufactured and calibrated by a known supplier to meet factory standards
- Direct-fit, vehicle-specific design simplifies installation
- Some customer reports of units that failed to open in bench testing
- Occasional reports of corrosion or used-condition parts on arrival
Verdict A budget-friendly, vehicle-specific 203°F fail-safe thermostat that aims to match factory performance but should be tested before installation due to some inconsistent reports.
6. Dorman Engine Coolant Thermostat Housing Assembly (902-5140)
| Included Components | 1 Thermostat Housing, 1 Gasket, 1 Thermostat |
|---|---|
| Material | Metal, Plastic |
| Manufacturer | Dorman Products |
| Position | Center |
This Dorman thermostat housing assembly replaces the water outlet and thermostat on compatible Infiniti models and is sold as a complete assembly with a gasket and thermostat. It is designed for a direct fit to the factory location on the FX35 and built to resist cracking and leaking under the rapid temperature cycles of engine operation. Typical use is straightforward: remove the old housing, transfer coolant, install the assembly and refill the cooling system; having the thermostat and gasket included reduces parts-sourcing and can save installation time. Buyers will appreciate the machined exterior and the assembly approach from a long-standing aftermarket brand, which helps streamline repairs and restores cooling system function without modifying the vehicle. Some users report the included gasket may not match every model year precisely, so confirm fitment for your FX35 year before installing. This part is an aftermarket replacement rather than a factory part so finish and minor fit differences are possible compared with a factory-quality unit.
Best for: Practical optimizers who want a cost-effective, ready-to-install replacement that reduces parts hunting and gets an Infiniti FX35 back on the road quickly.
Less Ideal for: Owners seeking a strict factory-produced component or concours-level finish should consider buying a factory-quality replacement instead.
- Includes thermostat and gasket so you get a ready-to-install assembly
- Engineered to handle extreme temperature changes to reduce cracking and leaks
- Machined exterior and aftermarket engineering aimed at matching factory fit
- Included gasket has been reported to be incorrect for some model years
- Aftermarket part may have minor fit or finish differences versus a factory-quality component
Verdict A budget-friendly, complete housing assembly that simplifies FX35 thermostat replacement while offering generally reliable performance with occasional gasket fit issues.
7. Honda Genuine Thermostat Assembly (19301-RNA-315)
| Brand | Honda |
|---|---|
| Model | 19301-RNA-315 |
| Voltage | 12 Volts |
This thermostat assembly is a factory-quality replacement designed to restore proper coolant flow and temperature regulation. Installed in place of a failed unit, it opens and closes at the correct temperature to eliminate underheating codes and return the engine to normal operating range. Buyers will notice a straightforward, OEM-style fit and finish that simplifies installation and reduces the risk of leaks when compared with many aftermarket alternatives. The practical benefit is predictable engine warm-up and improved fuel economy when the original thermostat has been stuck open or degraded. The part does not include hoses or gaskets beyond the unit itself, so expect to reuse or separately replace associated hardware during service. Also the listing specifies Honda as the brand, so confirm compatibility for the Infiniti FX35 before purchase since this is a Honda part and may require cross-reference or an appropriate part number for the FX35 application.
Best for: Practical, balanced buyers who prioritize reliability and long-term consistency over low-cost aftermarket options and want a straightforward replacement that restores proper engine temperature behavior.
Less Ideal for: Buyers seeking an Infiniti-specific aftermarket performance thermostat or a complete installation kit may prefer a part made and sold explicitly for the FX35 rather than this Honda-branded assembly.
- Factory-quality construction that matches original manufacturer specifications
- Consistent temperature control to resolve underheating codes and improve warm-up
- Durable build and smooth exterior that reduces leak risk when installed properly
- Listed as a Honda part so direct fit for Infiniti FX35 is not guaranteed without cross-reference
- Does not include additional sealing hardware which may need to be replaced separately
Verdict A durable, factory-quality thermostat assembly that restores reliable temperature control but requires verifying fitment for the Infiniti FX35 before buying.
Choosing the Right Infiniti FX35 Thermostat: Key Factors to Consider
Opening Temperature and Calibration
The thermostats opening temperature determines when coolant begins circulating through the engine and radiator. A thermostat that opens too early can cause the engine to run cold, reducing efficiency and cabin heat, while one that opens too late can allow prolonged overheating and increased engine wear.
Look for thermostats specified to match the vehicle manufacturers temperature rating. Consistent calibration matters because small deviations change warm-up time and cruising temperature, affecting emissions and performance. Consider whether the part lists a tolerance range and how that aligns with your local climate and driving patterns.
Material and Corrosion Resistance
Thermostats and their housings sit in a harsh environment of hot coolant and additives, so construction materials matter. Metal components should resist corrosion, and any rubber seals or gaskets should tolerate coolant additives and temperature cycling without hardening or cracking.
Longevity depends on both the valve mechanism and the surrounding housing. Parts with stamped stainless components or higher-grade alloys generally offer better resistance to pitting and corrosion. Inspecting material specifications and expected service life helps avoid premature failures that lead to leaks or sticking.
Sealing and Gasket Design
A secure seal at the thermostat housing prevents coolant loss and maintains system pressure. Poorly designed seals or mismatched gasket thickness can cause weeping or full leaks, which quickly lead to low coolant levels and overheating concerns.
Consider whether the thermostat includes a dedicated gasket or O-ring designed for the FX35 housing. Some designs integrate a flange with a molded seal to simplify installation, while others require a separate gasket. Proper compression and correct torque on mounting fasteners are also important to avoid deformation and ensure a leak-free fit.
Fitment and Mounting Accuracy
Accurate fitment ensures the thermostat aligns with the housing, bypass passages, and any mounting studs or bolts. A unit that is slightly off in dimensions can stress the housing, misalign seals, or impede coolant flow.
Verify that the part explicitly lists compatibility with the Infiniti FX35 and matches OEM mounting points. Check for notes on flange orientation, sensor boss locations, and clearance for surrounding components. Fitment accuracy reduces installation time and the risk of post-install leaks or airflow restrictions within the cooling circuit.
Valve Mechanism and Responsiveness
The internal valve and sensing element govern how predictably the thermostat reacts to temperature changes. A well-designed wax pellet or temperature-actuated mechanism opens and closes smoothly without sticking.
Responsiveness affects warm-up time and helps avoid rapid cycling that can stress the cooling system. Consider whether the design minimizes hysteresis and whether the mechanism is protected from debris. Reliable actuation reduces the chance of symptoms like overheating or the engine running cold due to a thermostat stuck open or closed.
Ease of Installation and Serviceability
Some thermostats are straightforward swaps using common hand tools, while others require partial disassembly of surrounding components. Ease of installation reduces labor time and the risk of installation errors that can cause leaks or improper seating.
Look for parts that come with new seals or gaskets and clear mounting orientation indicators. Availability of replacement gaskets and accessibility of the housing bolts affect how practical a DIY replacement will be. Also consider whether any coolant system bleeding steps are needed after installation and how that affects the overall service procedure.
FAQ
What are common thermostat symptoms on an Infiniti FX35?
Common signs often include slow warm-up, fluctuating temperature gauge readings, or heater output that is weak. You may also notice the engine temperature climbing past the normal range under load. These symptoms can result from a thermostat that is stuck open, stuck closed, or not sealing properly.
How can I test a thermostat in my vehicle?
A basic test is to monitor coolant temperature changes during warm-up and look for steady, predictable movement on the temperature gauge. For a hands-on test, remove the thermostat and place it in heated water to observe the valve opening at the rated temperature, but only if you are comfortable with that procedure. Many professionals use infrared thermometers or pressure test equipment to check system behavior without removal.
Will a thermostat housing seal leak cause overheating?
Yes, a leaking housing seal can lower coolant level and system pressure, which reduces boiling point and cooling efficiency. Small leaks may first show as dampness or coolant residue near the housing and later cause higher operating temperatures if not addressed. Ensuring a proper seal and correct torque on fasteners helps prevent this issue.
What causes an engine to run cold if the thermostat is stuck open?
If the thermostat is stuck open, coolant circulates continuously through the radiator, preventing the engine from reaching its intended operating temperature. This reduces heater performance, can increase fuel consumption, and may cause higher emissions. The condition often results from a failed valve mechanism or degraded sensing element.
How do I choose the right thermostat part number fitment for my FX35?
Match the thermostat to the Infiniti FX35 by checking OEM part references, physical dimensions, and mounting flange orientation. Ensure the listed opening temperature matches manufacturer specifications and that the product includes the correct gasket or seal format. If in doubt, compare technical drawings or fitment notes provided by the supplier.
How We Chose the Best Infiniti FX35 Thermostat
We focused on components that directly affect cooling performance and long-term reliability for the Infiniti FX35. Key evaluation criteria included: material and construction quality to resist corrosion and wear; thermostat opening temperature and calibration to maintain proper engine operating range; housing and gasket design to prevent coolant leaks and ensure a secure seal; fitment accuracy to match OEM mounting points and clearances; and ease of installation, including whether common tools suffice. We excluded parts with unclear fitment information, nonstandard temperature ratings, or suspect construction that could compromise cooling control. We also filtered out units lacking replacement seals or using untested valve mechanisms. The resulting shortlist prioritizes thermally accurate, durable thermostats with reliable sealing and straightforward installation for the FX35.
Our full evaluation process is outlined in our review methodology.
Conclusion
Prioritize a thermostat that matches the FX35s specified opening temperature, offers durable materials, and includes a reliable sealing solution. Fitment accuracy and a responsive valve mechanism reduce the chance of post-install problems and ensure predictable warm-up and operating temperatures. Balance ease of installation against long-term reliability when deciding. Addressing common failure modes such as leaks, corrosion, and stuck valves will keep the cooling system functioning and help maintain engine performance and heater operation.
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