🔧 WRX - Thermostat Replacement

The thermostat controls coolant flow so your engine warms up correctly and doesn’t overheat. On your WRX, it’s mounted in the lower coolant inlet/thermostat housing at the front of the engine, so you’ll drain coolant, swap the thermostat and seal, then refill and bleed air.

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

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⚠️ Safety & Precautions

• ⚠️ Never open the radiator cap on a hot engine; hot coolant can spray and burn.
• ⚠️ Support the car with jack stands before going underneath; never rely on a jack alone.
• ⚠️ Keep hands/tools away from radiator fans; they can turn on automatically.
• ⚠️ Coolant is toxic—clean spills immediately and dispose of old coolant properly.
• ⚠️ Battery disconnect is not required, but recommended if you’ll work close to the fans or wiring.

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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 2-gallon)
• Funnel
• Spill-free coolant funnel kit (specialty)
• Socket wrench
• 10mm socket
• 12mm socket
• 3/8" torque wrench (5-60 Nm range)
• Flat-blade 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
• Subaru-approved coolant (pre-mixed) - Qty: 2 gallons
• Coolant hose clamp (optional) - 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 completely (ideally overnight).
• Set the cabin heat to HOT (this helps coolant circulate through the heater core during bleeding).
• Hose clamp pliers pinch spring clamps safely.
• Spill-free funnel helps purge air pockets.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Raise the front and remove the undertray

• Use a floor jack (rated 3-ton minimum) to lift the front of the car at the front jack point.
• Set it down securely on jack stands (rated 3-ton minimum) and keep wheel chocks in place.
• Remove the engine undertray/splash shield using a trim clip removal tool and 10mm socket (depending on fastener type).

Step 2: Relieve pressure and drain the coolant

• Place a drain pan (at least 2-gallon) under the radiator drain area.
• Slowly loosen the radiator cap at the top of the radiator (use nitrile gloves and safety glasses).
• Open the radiator drain cock using a flat-blade screwdriver (if equipped with a slotted drain) and let coolant drain fully.
• Close the drain cock once draining slows to a drip using a flat-blade screwdriver.

Step 3: Access the thermostat housing

• Locate the lower radiator hose where it connects to the front-lower area of the engine (thermostat housing/water inlet).
• Use hose clamp pliers (specialty) to compress the spring clamp and slide it back on the hose.
• Twist and pull the hose off the housing by hand; use a shop rag to catch remaining coolant.

Step 4: Remove the thermostat housing cover

• Remove the thermostat housing fasteners using a 10mm socket (some housings may use 12mm socket depending on fastener style).
• Gently separate the housing cover; use a flat-blade screwdriver only if needed, and pry carefully to avoid gouging aluminum.
• Note the thermostat orientation before removal (spring side goes toward the engine).

Step 5: Replace the thermostat and seal

• Remove the old thermostat by hand and remove the old seal/O-ring.
• Wipe the mating surfaces clean using shop rags (no deep scraping).
• Install the new seal/O-ring onto the new thermostat, then place the thermostat in the same orientation as removed.
• Wrong orientation can cause overheating.

Step 6: Reinstall the housing and hose

• Reinstall the thermostat housing cover and start bolts by hand.
• Tighten evenly using a 10mm socket, then final-tighten using a 3/8" torque wrench (5-60 Nm range): Torque to 10 Nm (7 ft-lbs).
• Reinstall the lower radiator hose and position the clamp in the original spot using hose clamp pliers (specialty).

Step 7: Refill and bleed air from the cooling system

• Install a spill-free coolant funnel kit (specialty) at the radiator fill neck.
• Slowly add Subaru-approved coolant (pre-mixed) using the funnel until the radiator stays full.
• Fill the overflow reservoir to the FULL line using a funnel.
• Start the engine and let it idle with the heat on HOT.
• As the engine warms up, watch the funnel level and add coolant as needed using the spill-free coolant funnel kit (specialty).
• When the thermostat opens, the upper radiator hose will get hot and the coolant level may drop—top off again.
• Let the radiator fans cycle on and off at least once, then shut the engine off.
• After it cools, remove the funnel and install the radiator cap firmly.

Step 8: Reinstall the undertray and lower the car

• Reinstall the undertray using the trim clip removal tool and 10mm socket.
• Raise slightly with the floor jack (rated 3-ton minimum), remove jack stands (rated 3-ton minimum), then lower the car.

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✅ After Repair

• With the engine idling, check for leaks at the thermostat housing and lower hose connection.
• Verify the temperature gauge stays normal during a 10-15 minute drive.
• After the engine fully cools, recheck the overflow reservoir and top off to the FULL line if needed.
• If you hear gurgling behind the dash or see temp swings, there may be trapped air—repeat the bleed step with the spill-free coolant funnel kit (specialty).

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💰 DIY vs Shop Cost

Shop Cost: $350-$700 (parts + labor)

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

You Save: $170-$640 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?

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