🔧 Impreza - Thermostat Replacement

The thermostat controls coolant flow to keep your Impreza at the correct operating temperature. Replacing it fixes issues like overheating, slow warm-up, or a temperature gauge that swings around.

Difficulty Level: Intermediate | Estimated Time: 1.5-3 hours

Assumption: stock cooling system, FB20 2.0L layout.

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⚠️ Safety & Precautions

• ⚠️ Never open the radiator cap on a hot engine; hot coolant can spray out.
• ⚠️ Support the car with jack stands before going underneath.
• ⚠️ Coolant is toxic; keep away from kids and pets and clean spills.
• 🔋 Battery disconnect is not required, but keep tools away from the fan area.

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🔧 Required Tools

You'll need the following tools for this repair:

• Floor jack (rated 2-ton minimum)
• Jack stands (rated 2-ton minimum)
• Wheel chocks
• Safety glasses
• Nitrile gloves
• Drain pan (at least 2-gallon)
• 10mm socket
• 3/8" drive ratchet
• 3" extension (3/8" drive)
• Torque wrench (in-lb or low-range ft-lb)
• Hose clamp pliers
• Trim clip removal tool
• Flat-blade screwdriver
• Funnel
• Spill-free coolant funnel kit (specialty)
• Shop rags

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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-approved coolant (blue premix equivalent) - 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 place wheel chocks behind the rear wheels.
• Let the engine cool fully (ideally overnight).
• Set the HVAC to heat (HOT) and fan LOW later during bleeding (this helps move coolant through the heater core).
• Raise the front of the car with a floor jack and support it securely on jack stands.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the lower splash shield (if equipped)

• Place a drain pan under the front of the engine.
• Use a trim clip removal tool and flat-blade screwdriver to remove the plastic clips/screws holding the undertray.
• Set the shield and clips aside so you don’t lose them.

Step 2: Drain the coolant

• Remove the radiator cap slowly (engine must be cold). Use a shop rag for grip.
• At the bottom of the radiator, open the drain cock (petcock) using a flat-blade screwdriver if needed.
• Let coolant drain into the drain pan.

Step 3: Access the thermostat housing (lower radiator hose area)

• Locate the lower radiator hose (the larger hose coming off the bottom of the radiator and going to the engine).
• Use hose clamp pliers to compress the clamp and slide it back on the hose.
• Twist the hose gently to break it loose, then pull it off. Twist first—don’t pry hard.

Step 4: Remove the thermostat housing

• Place the drain pan under the housing—more coolant will spill.
• Use a 10mm socket, 3/8" ratchet, and extension to remove the thermostat housing bolts.
• Carefully pull the housing off and note how the thermostat sits before removing it.

Step 5: Replace the thermostat and gasket

• Remove the old thermostat and old gasket/O-ring by hand (use shop rags to catch drips).
• Clean the mating surfaces with a shop rag (no deep scratching).
• Install the new thermostat in the same orientation as the old one.
• Install the new gasket/O-ring (do not reuse the old seal).

Step 6: Reinstall the thermostat housing

• Reinstall the housing and start the bolts by hand to avoid cross-threading.
• Tighten with a 10mm socket until snug.
• Final tighten with a torque wrench: Torque to 10 Nm (7.4 ft-lbs).

Step 7: Reinstall the lower radiator hose

• Push the hose fully onto the fitting.
• Use hose clamp pliers to move the clamp back into its original position.
• Wipe the area with shop rags so leaks are easy to spot later.

Step 8: Refill and bleed the cooling system

• Close the radiator drain cock.
• Install a spill-free coolant funnel kit (specialty) on the radiator fill neck (this is a funnel that seals so air can burp out without spilling).
• Slowly add Subaru-approved coolant (blue premix equivalent) until the level stays near the top.
• Start the engine and let it idle. Keep the funnel at least half full.
• Turn the heater to HOT and fan on LOW.
• Watch for air bubbles in the funnel; add coolant as the level drops.
• When the thermostat opens, the upper radiator hose will get hot and you’ll usually see a bigger “burp” of bubbles.
• After bubbles stop and the cooling fans cycle once, shut the engine off and let it cool.
• Remove the funnel and install the radiator cap.
• Fill the overflow tank to the FULL line using a funnel.

Step 9: Reinstall the splash shield and lower the car

• Reinstall the undertray using the trim clip removal tool and flat-blade screwdriver.
• Lower the car from the jack stands using the floor jack.

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✅ After Repair

• Start the engine and let it warm up while watching the temperature gauge for normal operation.
• Check for leaks around the thermostat housing and lower radiator hose.
• After the first complete cool-down, recheck the overflow tank and top off to the FULL line if needed.
• If the heater blows cold or the gauge fluctuates, you likely still have air—repeat the bleed process.

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💰 DIY vs Shop Cost

Shop Cost: $250-$500 (parts + labor)

DIY Cost: $50-$140 (parts only)

You Save: $200-$360 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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🎯 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.