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How do I know if my Subaru thermostat is bad?

In short, a failing Subaru thermostat usually shows as the engine overheating or not warming up properly, often with weak cabin heat and a fluctuating temperature gauge.

The thermostat regulates coolant flow in the cooling system, helping the engine reach and stay at its optimal temperature. When it fails—whether it sticks closed or opens too early—it can lead to overheating, poor heater performance, and other cooling-system issues. This article explains the key signs, basic home checks, and when to replace the unit.

What the thermostat does

The thermostat sits between the engine and radiator in the cooling system. It senses coolant temperature and opens to allow hot coolant to flow to the radiator. When cold, it stays closed to help the engine warm up quickly. At normal operating temperature, it should be partially open to regulate heat flow. A stuck closed thermostat causes rapid overheating and possible engine damage, while a stuck open thermostat can prevent the engine from reaching operating temperature and reduce heater efficiency.

Common warning signs

Look for these telltale signs that point to a thermostat issue rather than a separate cooling-system problem.

Engine overheats or climbs well above the normal temperature range

Temperature gauge stays in the hot zone or spikes during drive

Engine takes a long time to warm up, or never reaches normal operating temp

Cabin heater blows weak or cool air even when the cabin is set to heat

Coolant leaks or a damp area around the thermostat housing or upper radiator hose

Cooling fans run more often or stay on longer than usual

Inconsistent or fluctuating temperature readings while driving

Warning lights or codes related to the cooling system may appear

Seen any of these signs? They strongly point toward a thermostat issue, but other cooling-system faults can mimic them. A professional diagnostic can confirm the source.

Home diagnosis steps

If you’re comfortable with basic car maintenance, you can perform a few checks to gauge whether the thermostat is at fault before replacing parts.

In-car observations

Start from a cold engine and monitor how quickly it warms up, how the heater performs, and how the cooling system behaves. Note whether the gauge reaches the normal range, stays cold, or overheats. Also pay attention to how long the radiator fans run after start-up.

With the engine cold, check the coolant level in the expansion tank and radiator (if accessible). Top up only with the correct mix for your model (often 50/50 water and antifreeze). Do not open the radiator cap when the engine is hot.

Inspect for external leaks around the thermostat housing and upper radiator hose. Wipe away residue to better spot drips.

Start the engine and observe the temperature gauge as it warms. If the gauge stays matter-of-factly cold or takes unusually long to rise, note the behavior for later comparison.

Test the thermostat outside the engine (optional but informative). Locate the housing and remove the thermostat. Submerge it in a pot of water on the stove and heat gradually. Use a thermometer to verify the opening temperature (typical Subaru thermostats open around 82–88°C or 180–190°F, but check your model’s spec). The valve should begin to open at the rated temperature and open fully as the water rises. If it does not open, or sticks open/closed, the thermostat is faulty and should be replaced.

Reinstall the thermostat with a new gasket or O-ring if needed, refill and bleed the cooling system to remove air pockets, and run the engine to normal operating temperature. Recheck for leaks and verify cabin heat performance.

Note: This home test requires caution. Do not work near a hot cooling system, and if you’re unsure, consult a professional to avoid burns or damage.

When to replace

A thermostat does not have a fixed replacement interval. It can last many years, but failure is common with wear, buildup, or corrosion. If symptoms point to a faulty valve, replacement is typically the best course. When replacing, use a quality thermostat (OEM or equivalent) matched to your Subaru model, and replace any associated gaskets and seals. After replacement, ensure the cooling system is properly bled and refilled to prevent air pockets that can cause overheating or poor heater performance.

Professional diagnosis and next steps

If you’re not confident performing the tests or if the cooling system shows signs of leaks, abnormal pressure, or persistent overheating, seek professional service. A trained technician can perform a controlled pressure test, inspect the thermostat housing and water pump, verify component operation, and ensure the cooling system is correctly bled and filled.

Summary

In summary, a bad Subaru thermostat commonly presents as overheating, poor or no cabin heat, and a fluctuating temperature gauge, sometimes with coolant leaks near the thermostat housing. Basic home checks can help you narrow down the cause, but definitive confirmation and safe replacement typically require professional service. Replacing with a correct-spec thermostat and ensuring the cooling system is properly bled will help restore normal engine temperature control and heater performance.

How to tell the difference between a bad thermostat and a water pump?

A bad thermostat typically causes symptoms like your engine overheating or taking too long to warm up, while a bad water pump is most often indicated by coolant leaks from the front of the engine or strange noises like grinding or whining. Both can cause your engine to overheat, but a leaking water pump is a more direct sign of a water pump failure.
This video explains the symptoms of a bad water pump and how to identify it: 46sAuto V Fix YouTube · Apr 16, 2025
Bad thermostat symptoms

Overheating: The most common sign, especially if the thermostat is stuck closed.
Slow warm-up: The engine takes a long time to reach its normal operating temperature or never fully warms up, which can happen if the thermostat is stuck open.
Temperature gauge fluctuations: The temperature gauge may jump up and down erratically.
Poor cabin heat: The car's heater may not get very warm because the coolant isn't circulating properly.
Check engine light: A code like P0128cap P 0128𝑃0128 may appear if the thermostat is stuck open.

Bad water pump symptoms

Coolant leaks: Look for puddles of coolant (often green, orange, or pink) under the front of the engine, potentially from a "weep hole".
Grinding or whining noises: These sounds, especially when they change with engine speed, often point to a failing bearing in the pump.
Wobbly pulley: If the pump's pulley feels loose when the engine is off, the bearing has likely failed.
Overheating: If the pump fails to circulate coolant, the engine will overheat, though this is also a symptom of a bad thermostat.
Deposit build-up: You might see rust or mineral deposits around the pump due to a slow leak.

How to tell the difference

Focus on the leak: A coolant leak is the most direct indicator of a bad water pump. If you see a leak, it's very likely the water pump is the problem.
Focus on temperature: If the engine overheats or won't warm up but there are no coolant leaks, the thermostat is the more probable cause.
Check both: It's possible both are failing. A malfunctioning thermostat can put stress on the water pump, causing it to leak.

How to tell if a vehicle thermostat is bad?

A bad vehicle thermostat can be identified by engine overheating or overcooling, erratic temperature gauge readings, coolant leaks, strange noises, and issues with the heater system. If the engine doesn't reach its normal operating temperature, it may be stuck open, and if it runs hotter than usual, it could be stuck closed.

This video explains the symptoms of a bad thermostat: 59sEasyAutoFixYouTube · Nov 11, 2023
Common symptoms

Engine overheating: A common sign that the thermostat is stuck closed, which prevents coolant from reaching the radiator to be cooled.
Engine takes a long time to warm up: If the engine stays cold for an extended period, the thermostat might be stuck open.
Erratic temperature gauge readings: The temperature gauge may fluctuate wildly between cold and hot, or suddenly jump to the hot zone.
Poor cabin heating: If the heater blows cold air or takes a long time to warm up, the thermostat may be stuck open, preventing hot coolant from circulating to the heater core.
Coolant leaks: While not a direct sign of a bad thermostat, it can be a symptom of an overall aging or corroded cooling system, which can also affect the thermostat.
Unusual noises: A gurgling sound from the engine can sometimes indicate a problem with the thermostat.
Poor fuel economy: An engine that runs cold for too long due to a stuck-open thermostat can lead to a richer fuel mixture, decreasing fuel efficiency.

You can watch this video to see how to check the hoses for temperature difference indicating a bad thermostat: 57sYou WrenchYouTube · Mar 29, 2021
How to diagnose

Watch the temperature gauge: Pay attention to how quickly the engine warms up and if the temperature stays stable during operation.
Check heater performance: Use the heater on a cold day and see if it provides sufficient heat. A weak or absent heater could indicate a bad thermostat.
Listen for noises: Unusual sounds from the engine bay, especially while the engine is running, can point to a problem.

This video demonstrates how to test a thermostat in boiling water: 49sTimeless Steel GarageYouTube · Dec 29, 2024
DIY test (advanced)

After the engine has cooled completely, you can remove the thermostat and place it in a pot of boiling water.
A functioning thermostat will open at its rated temperature, which can be found on the thermostat itself.
If the thermostat does not open in the boiling water, it is likely bad and needs to be replaced.

This video explains how to test a bad thermostat by checking the radiator hoses: 58sBackyard ChevyYouTube · Sep 11, 2021

How do I know if my thermostat is stuck open or closed?

You can tell if your thermostat is stuck open or closed by observing your engine's temperature gauge and by feeling the radiator hoses. A stuck closed thermostat causes the engine to overheat, leading to a gauge in the red zone and the upper radiator hose being hot while the bottom hose is still cold. A stuck open thermostat causes the engine to take a long time to warm up and the temperature gauge to stay low, and you'll find both the upper and lower radiator hoses are warm-to-hot during normal operation, as shown in this YouTube video.
Stuck open vs. Stuck closed
This video demonstrates how to test a thermostat and shows symptoms of a stuck closed thermostat: 1mYou WrenchYouTube · Mar 29, 2021

	Stuck Closed	Stuck Open
Engine Temperature	Overheats quickly; gauge in the red zone	Takes a long time to warm up; gauge stays low or drops when coasting
Radiator Hoses	Upper hose is hot, lower hose is cold	Both upper and lower hoses are warm-to-hot
Heating	Heater may blow cold air	Heater may blow lukewarm or cold air because the coolant isn't hot enough
Performance	Potential for reduced power, rough idle, or stalling	May have reduced fuel efficiency
Check Engine Light	May or may not illuminate	Often illuminates with code P0128
Other Signs	Steam from the engine may occur	Temperature gauge may drop when driving downhill

Start the engine: and let it run until it reaches operating temperature.
Carefully: feel the upper and lower radiator hoses. Be cautious, as they will be hot.
If the upper hose is hot and the lower hose is cold, the thermostat is stuck closed, preventing the coolant from circulating to the radiator.
If both the upper and lower hoses are hot: after the engine has warmed up, the thermostat is likely stuck open.

This video demonstrates how to check a thermostat by feeling the radiator hoses: 52sMake It Easy MechanicYouTube · Jul 15, 2023

How to tell if a Subaru is overheating?

Other signs that your Subaru engine is overheating are a lot more obvious. For example, noticing steam coming from under your hood is a sure sign that you have an overheating problem. There could also be burning smells as the super-heated engine melts rubber components in the system.

Kevin Bennett

Company Owner

Kevin Bennet is the founder and owner of Kevin's Autos, a leading automotive service provider in Australia. With a deep commitment to customer satisfaction and years of industry expertise, Kevin uses his blog to answer the most common questions posed by his customers. From maintenance tips to troubleshooting advice, Kevin's articles are designed to empower drivers with the knowledge they need to keep their vehicles running smoothly and safely.