🔧 Impreza - Thermostat Replacement

The thermostat controls coolant flow to help your engine warm up quickly and prevent overheating. If it’s stuck open you may get weak heat and low temp; stuck closed can cause overheating. On your Impreza, the thermostat sits at the water pump inlet where the lower radiator hose connects.

Difficulty Level: Intermediate | Estimated Time: 1.5-3.0 hours

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⚠️ Safety & Precautions

• ⚠️ Never open the coolant cap on a hot engine; hot coolant can spray and burn.
• ⚠️ Support the car with jack stands; never rely on a floor jack alone.
• ⚠️ Coolant is toxic—keep it off the ground and away from pets/kids.
• ⚠️ Let the engine cool fully before starting (at least 2-3 hours).
• Battery disconnect is not required for this repair.

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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 10-quart)
• Funnel
• Spill-free coolant funnel kit (specialty)
• 10mm socket
• 12mm socket
• 3/8" drive ratchet
• 3/8" drive torque wrench (5–50 Nm range)
• 6" extension (3/8" drive)
• Flathead screwdriver
• Hose clamp pliers (specialty)
• Trim clip removal tool
• Shop rags

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Engine thermostat - Qty: 1
• Thermostat seal / O-ring - Qty: 1
• Engine coolant (Subaru Super Coolant compatible, 50/50 premix) - Qty: 2 gallons
• Lower radiator hose clamp (optional) - Qty: 1

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📋 Before You Begin

• Park on level ground, set the parking brake, and chock the rear wheels.
• Let the engine cool completely before touching hoses or the radiator cap.
• Set the HVAC to HOT during bleeding later so coolant flows through the heater core.
• Take a photo of hose routing first.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Raise the front and remove the lower splash shield

• Use a floor jack (rated 2-ton minimum) to lift the front at the center jack point.
• Place jack stands (rated 2-ton minimum) under the proper front support points and lower onto them.
• Use a trim clip removal tool and 10mm socket to remove the engine under cover/splash shield fasteners.

Step 2: Drain the coolant

• Place a drain pan (at least 10-quart) under the radiator drain.
• Slowly loosen the radiator cap (only if fully cool), then remove it to help it drain.
• Open the radiator drain cock using a flathead screwdriver as needed and let coolant drain completely.
• Close the drain cock when finished (snug only—do not overtighten).

Step 3: Access the thermostat at the lower radiator hose

• Locate the lower radiator hose going to the front of the engine (water pump inlet).
• Use hose clamp pliers (specialty) to compress the clamp and slide it back on the hose.
• Twist the hose gently to break it loose, then pull it off. Use shop rags to catch remaining coolant.
• Don’t pry hard—plastic fittings can crack.

Step 4: Remove the thermostat cover

• Remove the thermostat housing/cover bolts using a 10mm socket with a 3/8" drive ratchet and 6" extension.
• Pull the cover straight off and set it aside.
• Note the thermostat orientation before removing it (how it sits in the housing).

Step 5: Replace the thermostat and seal

• Remove the old thermostat by hand and remove the old seal / O-ring.
• Clean the sealing surfaces with a shop rag (no gouging or scraping).
• Install the new seal/O-ring onto the new thermostat (match how the old one sat).
• Install the new thermostat into the housing in the same orientation as removed.

Step 6: Reinstall the thermostat cover and hose

• Reinstall the cover and start bolts by hand to avoid cross-threading.
• Tighten bolts evenly using a 10mm socket.
• Final tighten with a 3/8" drive torque wrench (5–50 Nm range): Torque to 9 Nm (80 in-lbs).
• Reinstall the lower radiator hose and position the clamp using hose clamp pliers (specialty).

Step 7: Reinstall the splash shield and lower the car

• Reinstall the under cover using the 10mm socket and trim clip removal tool.
• Raise slightly with the floor jack (rated 2-ton minimum), remove jack stands (rated 2-ton minimum), and lower the car.

Step 8: Refill and bleed the cooling system

• Install a spill-free coolant funnel kit (specialty) at the radiator fill neck.
• Slowly add engine coolant (Subaru Super Coolant compatible, 50/50 premix) using a funnel until the radiator stays full.
• Fill the overflow reservoir to the FULL line.
• Start the engine and set the heater to HOT, fan on low.
• Let the engine idle; watch for air bubbles in the funnel. Add coolant as the level drops.
• Once the radiator fan cycles and the upper hose gets hot, shut the engine off and let it cool.
• After cooling, top off the radiator (if needed) and set the overflow to the proper level.
• Small bubbles are normal at first.

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✅ After Repair

• Start the engine and verify the cabin heater blows hot air at idle.
• Check for leaks at the thermostat cover and lower radiator hose connection.
• Test drive 10–15 minutes, then recheck the overflow level after the engine cools.
• If the temperature gauge climbs abnormally, shut down and re-bleed the system.

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💰 DIY vs Shop Cost

Shop Cost: $350-$650 (parts + labor)

DIY Cost: $60-$140 (parts only)

You Save: $290-$510 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 1.5-3.0 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.