🔧 Forester - Thermostat Replacement

Your Forester’s thermostat controls coolant flow to keep engine temperature stable. If it’s stuck open (runs cool) or stuck closed (overheats), replacing it restores proper temperature control and heater performance.

Difficulty Level: Intermediate | Estimated Time: 2-3 hours

Assumption: thermostat is in the water inlet/thermostat cover at the lower radiator hose.

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⚠️ Safety & Precautions

• ⚠️ Never open the radiator cap on a hot engine; scalding coolant can spray out.
• ⚠️ Support the vehicle with jack stands; never rely on a floor jack alone.
• ⚠️ Coolant is toxic; keep it away from kids/pets and clean spills immediately.
• ⚠️ Keep hands/tools clear of cooling fans; they can turn on automatically.
• ⚠️ Battery disconnect is not required for this job.

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🔧 Required Tools

You'll need the following tools for this repair:

• Floor jack (rated 3-ton minimum)
• Jack stands (rated 3-ton minimum)
• Wheel chocks
• Safety glasses
• Nitrile gloves
• Drain pan (at least 10-quart)
• Spill-free coolant funnel kit (specialty)
• Shop towels
• Trim clip removal tool
• Ratchet (3/8" drive)
• Socket extension (3")
• 10mm socket
• 12mm socket
• Torque wrench (5-30 Nm range)
• Hose clamp pliers
• Flathead screwdriver
• Pick tool
• Plastic scraper

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Engine thermostat - Qty: 1
• Thermostat gasket / O-ring - Qty: 1
• Subaru-compatible coolant (phosphate-free, premixed 50/50) - Qty: 2 gallons
• Lower radiator hose clamp - Qty: 1

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📋 Before You Begin

• 🧰 Park on level ground, set the parking brake, and let the engine cool completely (ideally overnight).
• 🧰 Set the heater to full HOT later during bleeding; no infotainment steps are required.
• 🧰 Raise the front of your Forester with a floor jack and support it securely on jack stands.
• 🧰 Place a drain pan under the radiator area before opening any drains or hoses.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the lower engine undercover

• Use a trim clip removal tool to remove the plastic push-clips.
• Use a 12mm socket and ratchet to remove any bolts holding the undercover on.
• Set the undercover and hardware aside in a small pile so nothing gets lost.

Step 2: Relieve pressure and prepare to drain coolant

• Place the drain pan (at least 10-quart) under the radiator.
• Slowly loosen the radiator cap to the first stop (if equipped) using your hand and nitrile gloves, then remove it once you’re sure there is no pressure.
• Open the coolant reservoir cap by hand.
• If it hisses, wait until fully cool.

Step 3: Drain the coolant

• Locate the radiator drain cock at the lower portion of the radiator.
• Open it by hand or with a flathead screwdriver (depending on style) and drain into the pan.
• Once flow slows down, close the drain cock snugly by hand (do not over-tighten).

Step 4: Access the thermostat (lower radiator hose area)

• Follow the lower radiator hose to the engine-side connection (this is typically where the thermostat cover is).
• Use hose clamp pliers to compress the clamp and slide it back on the hose.
• Twist the hose gently to break it free, then pull it off. Keep the drain pan under it for extra coolant.
• Twist first, pull second to avoid tearing.

Step 5: Remove the thermostat cover

• Use a 10mm socket, ratchet, and socket extension (3") to remove the thermostat cover bolts.
• Carefully separate the cover. If it’s stuck, use a plastic scraper to gently pry at the seam.
• Remove the thermostat and the old thermostat gasket / O-ring.

Step 6: Install the new thermostat and gasket

• Clean the mating surfaces using a plastic scraper and shop towels. Do not gouge the aluminum.
• Install the new gasket/O-ring. If it keeps popping out, use a pick tool gently to seat it fully.
• Install the new thermostat in the same orientation as removed.
• If your thermostat has a small “jiggle valve,” position it at the top (12 o’clock) so trapped air can escape.

Step 7: Reinstall the thermostat cover and hose

• Reinstall the thermostat cover bolts by hand first to avoid cross-threading.
• Use a torque wrench (5-30 Nm range) to tighten the thermostat cover bolts evenly: Torque to 10 Nm (7 ft-lbs).
• Reinstall the lower radiator hose fully onto its neck.
• Use hose clamp pliers to move the clamp back into its original position.

Step 8: Refill coolant and bleed air from the system

• Install the spill-free coolant funnel kit (specialty) onto the radiator fill neck. (A spill-free funnel seals to the neck so you can run the engine without spilling while air burps out.)
• Slowly add Subaru-compatible coolant (phosphate-free, premixed 50/50) until the funnel stays about half full.
• Start the engine and let it idle.
• Set climate controls to maximum heat and medium fan to help purge air through the heater core.
• Watch for bubbles in the funnel and keep topping up so the funnel never runs dry.
• When the upper radiator hose becomes hot and you see fewer bubbles, lightly raise RPM to about 1,500-2,000 for 10-15 seconds a few times (do not race a cold engine).
• Continue until the cooling fans cycle on and off at least once and bubbles are minimal.
• Shut the engine off and let it cool. Remove the funnel and install the radiator cap.
• Top the reservoir to the “FULL” line using coolant.

Step 9: Reinstall the undercover

• Reinstall the undercover using the 12mm socket, ratchet, and the original clips.
• Lower the vehicle using the floor jack (rated 3-ton minimum).

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✅ After Repair

• 🧪 Start the engine and confirm the temperature gauge stays normal and the heater blows hot.
• 🧪 Check for leaks around the thermostat cover and lower radiator hose with safety glasses on.
• 🧪 After the engine fully cools, recheck coolant level in the reservoir and top off to “FULL” if needed.
• 🧪 Over the next 1-2 drives, monitor for any coolant smell, wet spots, or temperature fluctuations.

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💰 DIY vs Shop Cost

Shop Cost: $400-$700 (parts + labor)

DIY Cost: $60-$180 (parts only)

You Save: $220-$640 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2-3 hours.

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🎯 Ready to get started?

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