How to check thermostat on Subaru Outback?
The thermostat in a Subaru Outback typically begins to open around 180°F (82°C). If it sticks closed, the engine can overheat; if it opens too early or too late, you may notice poor heating or cooling performance. This guide walks you through locating, diagnosing, testing, and replacing a thermostat on a Subaru Outback, with steps you can perform at home or with basic tools.
Where the thermostat sits in a Subaru Outback
The thermostat is housed in the thermostat housing, usually connected to the upper radiator hose at the front of the engine. Access varies by model year and engine (for example the 2.5L EJ/FB series and the 2.4L turbo variants). You may need to remove some splash shields or engine covers to reach the housing. Always consult your specific year/model service manual for the exact location, bolt sizes, and torque specs.
Common signs your thermostat may be failing
Before you disassemble anything, look for a pattern of symptoms that often point to a faulty thermostat. The list below summarizes typical behavior you might observe.
- Engine overheating or running hotter-than-normal, especially after idle or short trips
- Insufficient cabin heat or a heater that blows cold air
- Thermostat seems to stick closed (rad hose stays cool, then suddenly heats up)
- Rising temperature gauge that doesn’t settle near the normal range
- Coolant leaks around the thermostat housing or a visible gasket issue
These indicators don’t prove the thermostat is bad on their own, but they guide your next diagnostic steps—whether you test in-car, bench-test the thermostat, or proceed to replacement.
In-car diagnostic steps to check thermostat operation
Use these in-car checks to gauge whether the thermostat is behaving as it should without removing it. Focus on engine temperature, heater performance, and upper radiator hose temperature as the engine warms up.
- Park on level ground, with the engine cold. Start the engine and let it idle with the radiator cap off if you’re comfortable doing so, or monitor through the reservoir.
- Watch the temperature gauge as the engine warms up. The gauge should move from cool toward normal and stabilize once the thermostat opens.
- Feel or carefully inspect the upper radiator hose as the engine reaches operating temperature. It should begin to warm as the thermostat opens and coolant circulates.
- Set the heater to the maximum temperature and fan speed. If the heater stays warm throughout, the thermostat is likely opening correctly; if the heater remains intermittently cold, it may be misbehaving or there could be an air pocket or coolant flow issue.
- If the engine overheats quickly or the gauge stays high while the heater remains weak, a stuck-closed thermostat is a likely cause and warrants further testing or replacement.
These steps help you decide whether to perform a bench test or move straight to replacement. If you’re unsure or the engine shows signs of overheating, stop driving and seek professional help to avoid head-gasket or engine damage.
Bench-testing the thermostat outside the engine
Bench-testing verifies whether the thermostat opens at the correct temperature and moves freely. This is a reliable check if you’re troubleshooting a suspected faulty unit without engine disassembly.
- Drain some coolant and remove the thermostat from the housing, ensuring you have a clean workspace and gloves for handling hot parts.
- Place the thermostat in a pot of water on the stove. Heat gradually and monitor with a reliable thermometer.
- Note the temperature at which the thermostat begins to open. For many Subaru thermostats, an opening around 180°F (82°C) is expected; full opening should occur a bit higher (roughly 190–200°F / 88–93°C).
- Observe that the valve opens smoothly and that coolant can flow through the valve. Then remove the thermostat to cool and close; it should close fully as the water cools.
- Inspect the thermostat for sticking, cracks, or obvious damage. If movement is sluggish or incomplete, replace it.
If the thermostat fails to open within the expected temperature range or remains stuck closed, replacement is recommended. If it opens too early or too late, replacement is also advisable to restore proper cooling behavior.
Replacing the thermostat and bleeding the cooling system
If testing indicates a faulty thermostat, replace it using OEM parts and follow a proper cooling-system bleed procedure to remove air pockets. Plan to replace the gasket or O-ring, and refresh the coolant to the specification recommended in your owner’s manual.
Replacement steps (overview)
Follow these general steps, then consult your model-year specific service manual for torque specs and any model-specific steps.
- Cool the engine completely, then disconnect the battery and drain the cooling system.
- Access the thermostat housing (may require removing engine covers or other components). Remove the housing bolts and carefully lift off the housing to expose the thermostat.
- Remove the old thermostat and gasket. Clean the mating surfaces on the engine and housing to ensure a good seal.
- Install the new thermostat in the correct orientation (the spring side typically faces toward the engine; check the old thermostat or service manual for the exact orientation).
- Install a new gasket or O-ring, reattach the housing, and torque bolts to the manufacturer’s specification.
- Refill the cooling system with the recommended coolant and bleed air from the system. Run the engine with the radiator cap off (if safe to do so) and the heater on high until the thermostat opens and bubbles are expelled, then top off coolant as needed.
- Check for leaks, monitor the temperature gauge, and ensure the engine reaches and holds normal operating temperature with a properly functioning heater.
After replacement, avoid opening the radiator cap while hot and keep an eye on the temperature gauge during the first drive to ensure the cooling system is functioning correctly. If you’re not comfortable performing cooling-system work, or if you encounter unusual signs (leaks, overheating after replacement, or persistent heater issues), contact a professional technician.
Bleeding and coolant considerations
Bleeding removes air that can become trapped in the cooling system. Subaru vehicles often require a specific bleed sequence or procedures described in the owner’s manual or service manual. Typical steps include running the engine with the heater on high, removing the cap briefly to release air bubbles, and gradually topping off coolant while watching the level in the reservoir.
Always use the coolant type recommended for your Outback’s year and engine. Refill to the correct level, check for leaks after installation, and dispose of old coolant properly.
Summary
In most Subaru Outbacks, the thermostat opens around 180°F (82°C). A combination of in-car observations (gauge behavior and heater performance) and bench tests can verify thermostat function. If the thermostat fails, replacing it with a new OEM unit and properly bleeding the cooling system usually resolves overheating or heater-related issues. When in doubt, especially with persistent overheating, seek professional help to avoid engine damage.
How do I test to see if my thermostat is bad?
A thermostat may be bad if your HVAC system is not turning on, won't turn off, or is "short-cycling" (turning on and off rapidly). Other signs include the display being unresponsive or showing an incorrect room temperature, and the thermostat forgetting programmed settings. For a car, a bad thermostat can cause engine overheating or, conversely, overcooling (a prolonged warm-up period), as well as coolant leaks and poor fuel efficiency.
Home HVAC thermostat
- System won't turn on or off: The most direct sign is that the heating or cooling won't start, or it runs continuously after the set temperature is reached.
- Unresponsive controls: The screen may be blank, or the buttons might not work when you try to adjust settings.
- Incorrect temperature: The temperature on the display may not match the actual room temperature, or it may fluctuate erratically.
- Program settings are lost: If your thermostat has a programmable schedule, it shouldn't constantly forget the settings.
- Short-cycling: The system turns on and off in quick succession instead of running through a normal cycle.
Car thermostat
- Engine overheating: The temperature gauge consistently runs high or rapidly climbs to the red zone. This is often because the thermostat is stuck closed and not letting coolant flow to the radiator.
- Engine is too cool: The temperature gauge stays at the low end of the normal range, or the engine takes a very long time to warm up. This can happen if the thermostat is stuck open.
- Coolant leaks: Dripping liquid under your car can indicate a leak, which is sometimes related to a failing thermostat.
- Heater problems: The heater blows cold air even after the engine has been running for a while.
- Poor fuel efficiency: The engine runs inefficiently for too long because it doesn't reach its optimal operating temperature.
What is the most common problem with a Subaru Outback?
Common problems for the Subaru Outback include weak windshields, excessive oil consumption, and issues with the cooling system and CVT transmission, especially in specific model years. Other frequent complaints involve airbag defects, electrical problems like battery drain, and vibration during acceleration.
You can watch this video to learn about the history of Subaru head gasket issues: 1mMrSubaru1387YouTube · Aug 11, 2024
Engine and powertrain
- Excessive oil consumption: Many models, particularly older 2.5L engines, may burn oil between oil changes due to leaks from seals.
- Cooling system failures: Overheating can lead to blown head gaskets, and the cooling system itself is a common point of failure.
- CVT transmission problems: Some model years are more susceptible to issues like rough shifting or failure.
- Vibration during acceleration: Some owners report a shudder or vibration when accelerating.
- Older head gasket issues: The EJ-series engines (roughly 1997–2012) had an inherent head gasket failure problem, though this was mostly resolved in later models.
Body and electrical
- Weak windshields: The glass is prone to cracking from stress or minor impacts.
- Airbag defects: Some models have a history of airbag problems, leading to recalls.
- Electrical issues: Problems can include unexpected battery drain and issues with the ABS control module.
Other common issues
- Oxygen sensor failure: The front O2 sensor can fail, sometimes leading to a recall.
- Premature brake wear: Some models experience faster-than-average brake wear.
- Suspension wear and tear: Like many vehicles, suspension components will eventually wear out.
How to tell if a Subaru thermostat is bad?
Temperature Fluctuations: One of the most common signs of a bad thermostat is erratic temperature readings on your dashboard gauge. If you notice that the temperature gauge moves from normal to hot quickly or fluctuates unexpectedly, it's a clear indication that your thermostat may be malfunctioning.
How do I check the temperature of my Subaru?
The temperature gauge on your Subaru's dashboard is a key indicator of engine temperature. Pay attention to the temperature gauge and ensure it stays within the normal range. If the needle consistently rises into the red zone or shows a higher temperature than usual, it is a clear sign of engine overheating.
