🔧 Ascent - Thermostat Replacement

This is an involved cooling-system repair, not the quick two-bolt thermostat job found on older Subarus. The thermostat sits at the engine-side water-pump area, so you must work from underneath, drain the coolant, remove the lower covers, and carefully refill and bleed the system.

Difficulty Level: Advanced | Estimated Time: 3-5 hours

Important: The thermostats currently displayed in the parts sidebar list older BRZ, Forester, Impreza, and related applications. Their supplied fitment data does not confirm them for your Ascent, so I would not order one of those three. Subaru’s application catalog identifies a different thermostat for the 2.4L turbo cooling system. parts.subaruonlineparts.com

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⚠️ Safety & Precautions

• ⚠️ Work only when the engine is completely cold. Hot coolant can cause severe burns.
• 🛑 Never remove the pressure cap while the cooling system is warm or pressurized.
• 🧤 Wear gloves and safety glasses because coolant can splash during hose removal.
• 🏗️ Support your Ascent on properly rated jack stands. Never work beneath a vehicle supported only by a jack.
• 🐾 Keep drained coolant away from children and animals. It is poisonous and may taste sweet.
• 🔋 Battery disconnection is not normally required unless an electrical component must be moved for access.
• 🌡️ Sonīpat heat makes complete air bleeding especially important. Stop immediately if the temperature warning appears afterward.

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🔧 Required Tools

You'll need the following tools for this repair:

• 3-ton floor jack
• 3-ton jack stands
• Wheel chocks
• 10mm socket
• 12mm socket
• 14mm socket
• 3/8-inch ratchet
• 3-inch socket extension
• 6-inch socket extension
• 3/8-inch torque wrench rated 5-80 ft-lbs
• Flat-blade trim tool
• Angled hose-clamp pliers
• Plastic gasket scraper
• Large coolant drain pan
• Spill-free coolant funnel kit (specialty)
• Cooling-system vacuum filler (specialty)
• Shop towels
• Nitrile gloves
• Safety glasses

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• FA24 turbo engine coolant thermostat - Qty: 1
• Thermostat rubber gasket - Qty: 1
• Subaru Super Coolant premix - Qty: 2 gallons
• Replacement hose clamp - Qty: 1 if the original is damaged

Use a factory-temperature thermostat. Don’t install the 68°C racing thermostat shown in the sidebar. A colder racing thermostat isn’t a repair upgrade for a stock daily-driven Ascent.

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📋 Before You Begin

• 🅿️ Park on level ground, select Park, apply the parking brake, and chock the rear wheels.
• ❄️ Leave the engine off until the upper radiator hose and pressure cap are completely cold.
• 🧴 Use Subaru-compatible premixed coolant. Do not mix green universal coolant with Subaru Super Coolant.
• 📦 Compare the replacement thermostat and gasket with the removed parts before installation.
• 📱 No infotainment service mode, battery registration, EPB retraction, or scan-tool command is required.
• 🔍 Inspect the water-pump area first. Coolant from a leaking pump or housing can look like a failed thermostat.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Raise and secure the vehicle

• Place the wheel chocks behind both rear wheels.
• Use the 3-ton floor jack at the approved front-center lifting point.
• Lower your Ascent securely onto the 3-ton jack stands at the front side support points.
• Push gently on the body to confirm the vehicle is stable before going underneath.

Step 2: Remove the lower engine covers

• Use the flat-blade trim tool to release the plastic push clips.
• Use the 10mm socket, extension, and ratchet to remove the under-cover bolts.
• Support each panel with one hand while removing its final fastener.
• Arrange the clips and bolts by location so they return to the correct holes.

Step 3: Drain the engine coolant

• Position the large coolant drain pan beneath the radiator drain area.
• Remove the cold pressure cap slowly.
• Open the radiator drain fitting carefully. Use hand pressure where possible because the plastic fitting is easy to damage.
• Allow the system to drain until coolant stops flowing steadily.
• Move the drain pan beneath the thermostat and water-pump area before disconnecting any hose.
• Coolant always finds the cleanest sleeve.

Step 4: Gain access to the thermostat housing

• Locate the thermostat housing at the engine-side water-pump assembly, following the lower coolant hose toward the engine.
• Use the angled hose-clamp pliers to compress the spring clamp and slide it several inches back.
• Twist the hose gently to break the seal, then pull it from the housing.
• Do not pry against the aluminum sealing surface.
• If a bracket blocks direct access, use the matching 10mm, 12mm, or 14mm socket to remove it and record its position.

Step 5: Remove the thermostat cover

• Use the 10mm socket, extension, and ratchet to loosen the cover bolts evenly.
• Hold the cover as the final bolt is removed because additional coolant will drain.
• Note the thermostat’s exact direction and gasket position before pulling it out.
• Take a clear phone photo before removal. This is cheap insurance against installing it backward.

Step 6: Clean and inspect the housing

• Remove the old thermostat and rubber gasket by hand.
• Use the plastic gasket scraper and shop towels to clean the mating surfaces.
• Do not use a metal scraper, abrasive disc, or excessive sealant.
• Inspect the cover for cracks, corrosion, distortion, and a damaged hose connection.
• Inspect the removed thermostat for sticking, a displaced seal, or physical damage.

Step 7: Install the new thermostat

• Install the new rubber gasket onto the new thermostat without stretching or twisting it.
• Position the thermostat exactly like the original. The valve and any air-bleed feature must retain the factory orientation.
• Seat the thermostat and gasket fully in their recess.
• Install the cover straight onto the engine without allowing the gasket to slip.
• Start every cover bolt by hand using the 10mm socket and extension.
• Tighten the bolts evenly with the 3/8-inch torque wrench.
• Do not use a guessed torque value. These are small fasteners in an aluminum housing. Confirm the current Subaru STIS thermostat-cover specification before final tightening, because Subaru directs technicians to use its latest service information. static.nhtsa.gov)

Step 8: Reconnect the coolant hose

• Push the hose completely over the housing neck by hand.
• Use the angled hose-clamp pliers to return the clamp to its original position.
• Verify the clamp sits behind the raised retaining bead and is not cocked sideways.
• Reinstall any removed bracket using the appropriate 10mm, 12mm, or 14mm socket.

Step 9: Close the drain and refill the system

• Close the radiator drain fitting by hand. Do not overtighten the plastic fitting.
• Install the spill-free coolant funnel kit at the fill neck.
• Slowly add Subaru Super Coolant premix until the level remains steady.
• Fill the expansion reservoir to the cold full mark.
• A cooling-system vacuum filler evacuates trapped air before drawing coolant into the system. This is the safer method when available.

Step 10: Bleed trapped air

• Keep the spill-free coolant funnel kit installed and start the engine.
• Set the cabin temperature to maximum heat and the blower to low.
• Allow the engine to warm at idle while watching the coolant level and temperature display.
• Add coolant as the level drops. Do not allow the funnel to run empty.
• Watch for steady cabin heat, normal operating temperature, and the absence of large air bubbles.
• Do not stand directly over the funnel or place your hands near the cooling fans.
• Shut the engine off immediately if the temperature warning appears, coolant surges violently, or the cabin heat suddenly becomes cold.

Step 11: Check for leaks and install the covers

• Switch off the engine and allow it to cool completely.
• Use a flashlight and shop towels to inspect the thermostat cover, hose, drain fitting, and surrounding water-pump area.
• Correct the coolant level after the engine is cold.
• Reinstall the lower covers using the 10mm socket and flat-blade trim tool.
• Use the 3-ton floor jack to raise the vehicle, remove the jack stands, and lower it carefully.

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✅ After Repair

• ✅ Run the engine until fully warm and confirm the cabin heater produces steady heat.
• 🔍 Inspect beneath your Ascent and around the thermostat housing for fresh coolant.
• 🌡️ Confirm the temperature remains normal during a short local drive.
• 🧊 Let the engine cool completely, then recheck the reservoir level and top up only with matching premixed coolant.
• 📅 Recheck the level and housing after the next two or three complete heat cycles.
• 🛑 Stop driving if the temperature warning appears, the heater turns cold, or coolant begins bubbling out of the reservoir.
• 🧪 If overheating continues, pressure-test the system and check the radiator fans, water pump, temperature sensor, combustion gases, and coolant circulation rather than replacing more parts blindly.

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💰 DIY vs Shop Cost

Shop Cost: $450-$850 (parts + labor)

DIY Cost: $80-$180 (parts and coolant)

You Save: $270-$670 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2.5-4 hours.

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🎯 Ready to get started?

HowToo makes it easy: same-day/2-day shipping on every part, plus all the tools and specialty tools you need! Check out the parts and tools sections below to add everything to your cart. Hold off on the three thermostats currently shown until the catalog returns one explicitly matched to the Ascent’s FA24 turbo engine.

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