🔧 Forester - Thermostat Replacement

This repair replaces the engine thermostat, which controls coolant flow so your Forester warms up properly and stays at the correct operating temperature. On your Forester, the thermostat sits in the lower coolant outlet area, so coolant must be drained before removal.

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

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⚠️ Safety & Precautions

• ⚠️ Only work on a completely cold engine. Hot coolant can cause serious burns.
• ⚠️ Never remove the radiator cap or coolant reservoir cap while the engine is hot.
• ⚠️ Keep coolant away from pets and children. It is toxic and tastes sweet.
• ⚠️ Support your Forester securely with jack stands before working underneath.
• ⚠️ Do not mix coolant types. Use Subaru-compatible blue premixed long-life coolant.
• ⚠️ Battery disconnect is not required for this repair.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 3/8-inch ratchet
• 3/8-inch extension 6-inch
• Torque wrench 5-50 Nm
• Phillips screwdriver #2
• Flat trim clip remover
• Hose clamp pliers
• Drain pan 2-gallon minimum
• Funnel spill-free coolant filling kit (specialty)
• Plastic scraper
• Shop towels
• Floor jack 2-ton minimum
• Jack stands rated 2-ton minimum
• Nitrile gloves
• Safety glasses

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Engine thermostat - Qty: 1
• Thermostat seal/gasket - Qty: 1
• Subaru-compatible blue premixed engine coolant - Qty: 2 gallons
• Radiator drain plug gasket - Qty: 1

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📋 Before You Begin

• Park your Forester on level ground and let the engine cool completely.
• Set the parking brake and place the transmission in Park.
• Raise the front of your Forester with a floor jack and support it with jack stands.
• Remove the lower engine splash shield to access the lower radiator area and thermostat housing.
• A spill-free coolant filling kit is a funnel that locks onto the radiator or reservoir opening and helps remove trapped air while refilling.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the Lower Splash Shield

• Put on safety glasses and nitrile gloves.
• Use a flat trim clip remover to remove the plastic clips from the lower splash shield.
• Use a 10mm socket and 3/8-inch ratchet to remove any 10mm bolts holding the shield.
• Lower the splash shield and set it aside.
• Keep clips grouped by location.

Step 2: Drain the Engine Coolant

• Place a drain pan 2-gallon minimum under the radiator drain area.
• Slowly remove the coolant reservoir cap by hand only after confirming the engine is cold.
• Open the radiator drain plug carefully by hand or with a Phillips screwdriver #2, depending on plug style.
• Let coolant drain until the flow slows to a drip.
• Close the radiator drain plug by hand. Do not overtighten plastic drain plugs.

Step 3: Locate the Thermostat Housing

• From underneath the front of your Forester, follow the lower radiator hose to the engine.
• The thermostat housing is the metal outlet where the lower radiator hose connects to the engine.
• Use shop towels to wipe the area clean before opening it.

Step 4: Remove the Lower Radiator Hose

• Use hose clamp pliers to squeeze the spring clamp on the lower radiator hose.
• Slide the clamp back on the hose.
• Twist the hose gently by hand to break it loose, then pull it off the thermostat housing.
• Keep the drain pan 2-gallon minimum under the hose because more coolant may drain out.
• Do not pry on plastic radiator necks.

Step 5: Remove the Thermostat Housing

• Use a 12mm socket, 3/8-inch extension 6-inch, and 3/8-inch ratchet to remove the thermostat housing bolts.
• Hold the housing while removing the last bolt so it does not drop.
• Pull the housing away from the engine.
• Note the direction of the old thermostat before removing it. The spring side faces into the engine.

Step 6: Clean the Sealing Surface

• Remove the old thermostat and seal by hand.
• Use a plastic scraper to gently clean the thermostat housing and engine sealing surfaces.
• Wipe both surfaces with shop towels.
• Do not gouge the aluminum surface. A scratch can cause a coolant leak.

Step 7: Install the New Thermostat

• Install the new thermostat seal/gasket onto the new engine thermostat.
• Place the thermostat into the engine with the spring side facing inward.
• If the thermostat has a small jiggle valve or bleed hole, position it upward.
• Hold the thermostat steady so it stays seated correctly.

Step 8: Reinstall the Thermostat Housing

• Position the thermostat housing over the thermostat by hand.
• Start all housing bolts by hand first to avoid cross-threading.
• Use a 12mm socket, 3/8-inch extension 6-inch, and torque wrench 5-50 Nm to tighten the housing bolts evenly.
• Torque to 7.5 Nm (5.5 ft-lbs).
• Small bolts strip easily.

Step 9: Reconnect the Lower Radiator Hose

• Push the lower radiator hose fully onto the thermostat housing by hand.
• Use hose clamp pliers to move the spring clamp back to its original position.
• Make sure the clamp sits behind the raised bead on the housing neck.

Step 10: Refill the Cooling System

• Install the funnel spill-free coolant filling kit (specialty) at the coolant fill point.
• Slowly pour Subaru-compatible blue premixed engine coolant into the funnel.
• Fill until the coolant level stabilizes.
• Squeeze the upper radiator hose gently by hand several times to help move trapped air.

Step 11: Bleed Air from the Cooling System

• Leave the funnel spill-free coolant filling kit (specialty) installed.
• Start the engine and set the cabin heater to full hot with the fan on low.
• Let the engine idle while watching the coolant level in the funnel.
• Add coolant as the level drops.
• Wait for the radiator fans to cycle on, then off, while monitoring the temperature gauge.
• If large bubbles continue, keep idling until bubbling slows.
• Shut the engine off and let it cool.

Step 12: Reinstall the Lower Splash Shield

• Use a 10mm socket and 3/8-inch ratchet to reinstall any splash shield bolts.
• Use a flat trim clip remover or your fingers to reinstall the plastic clips.
• Torque small splash shield bolts to 5 Nm (44 in-lbs).

Step 13: Lower the Vehicle

• Use the floor jack 2-ton minimum to raise your Forester slightly off the jack stands.
• Remove the jack stands rated 2-ton minimum.
• Lower your Forester carefully to the ground.

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✅ After Repair

• ✅ Check for leaks around the thermostat housing, lower radiator hose, and radiator drain plug.
• ✅ After the engine cools completely, check the coolant level again and top off if needed.
• ✅ Verify the cabin heater blows hot air at idle.
• ✅ Watch the temperature gauge during the first drive. It should stay steady in the normal range.
• ✅ Recheck coolant level the next morning when the engine is cold.
• ✅ Dispose of old coolant at a recycling or hazardous waste facility. Do not pour it on the ground or into drains.

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💰 DIY vs Shop Cost

Shop Cost: $280-$500 (parts + labor)

DIY Cost: $45-$120 (parts only)

You Save: $235-$380 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 1.5-2.5 hours.

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