🔧 Forester - Thermostat Replacement

The thermostat controls engine temperature by opening and closing to let coolant flow to the radiator. If it sticks closed, your A4 can overheat; if it sticks open, it may run cool and have weak cabin heat. On your Forester, the thermostat sits behind the lower radiator hose at the thermostat housing.

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

---

⚠️ Safety & Precautions

• ⚠️ Never open the radiator cap on a hot engine; scalding coolant can spray out.
• ⚠️ Support the Forester on jack stands; never rely on a floor jack alone.
• ⚠️ Coolant is toxic—keep it away from kids/pets and clean spills immediately.
• Use only Subaru-approved coolant type; mixing coolants can cause corrosion and leaks.
• Battery disconnect is not required for this repair.

---

🔧 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)
• Funnel (spill-free coolant funnel preferred)
• 10mm socket
• 3/8" ratchet
• 6" extension (3/8" drive)
• Flathead screwdriver
• Hose clamp pliers
• Torque wrench (inch-pound or low-range Nm)
• Shop rags

---

🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Engine thermostat - Qty: 1
• Thermostat gasket / O-ring - Qty: 1
• Subaru Super Coolant (premixed) - Qty: 2 gallons

---

📋 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 sit overnight).
• Set the HVAC to HOT and fan LOW for later bleeding (this helps coolant circulate through the heater core).
• Plan for coolant disposal—most parts stores accept used coolant.

---

🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Raise the front of the Forester safely

• Use a floor jack (rated 3-ton minimum) to lift the front at the proper front jack point.
• Set the vehicle onto jack stands (rated 3-ton minimum) and give it a firm shake test.

Step 2: Remove the lower splash shield (if equipped)

• Use a 10mm socket and flathead screwdriver to remove the bolts and plastic clips.
• Set the hardware aside so it doesn’t get lost.

Step 3: Drain the coolant

• Place a drain pan (at least 10-quart) under the radiator drain area.
• Slowly remove the radiator cap to release any leftover pressure (engine must be cool).
• Open the radiator drain cock using a flathead screwdriver (if it has a slot) and let coolant drain into the pan.
• Tip: Keep the drain pan centered—flow can shift.

Step 4: Access the thermostat at the lower radiator hose

• Locate the lower radiator hose (the larger hose going to the bottom of the radiator).
• Move the hose clamp back using hose clamp pliers (these squeeze spring clamps open so you can slide them).
• Twist the hose to break it loose, then pull it off the thermostat housing.
• Use shop rags to catch remaining coolant drips.

Step 5: Remove the thermostat housing

• Remove the thermostat housing bolts using a 10mm socket, 3/8" ratchet, and 6" extension.
• Carefully separate the housing (a small amount of coolant may spill).

Step 6: Replace the thermostat and gasket

• Note how the old thermostat sits, then remove it by hand.
• Clean the mating surfaces with a shop rag (no gouging or scratching).
• Install the new thermostat in the same orientation and install the new gasket / O-ring.

Step 7: Reinstall the thermostat housing and hose

• Reinstall the housing bolts by hand first to avoid cross-threading.
• Tighten with a torque wrench (inch-pound or low-range Nm): Torque to 10 Nm (7 ft-lbs).
• Reinstall the lower radiator hose and slide the clamp back into its original position using hose clamp pliers.

Step 8: Refill coolant

• Close the radiator drain cock (snug only) using a flathead screwdriver if needed.
• Use a funnel (spill-free coolant funnel preferred) to fill the radiator with Subaru Super Coolant (premixed) until full.
• Fill the overflow reservoir to the FULL line.

Step 9: Bleed air from the cooling system

• Start the engine and let it idle with the radiator cap off (or spill-free funnel installed).
• Turn the heater to HOT and fan on low; watch for heat from the vents.
• As the engine warms up, gently squeeze the upper radiator hose a few times (with nitrile gloves) to help push trapped air out.
• When the thermostat opens, the coolant level may drop—top off as needed using the funnel.
• Let the engine reach operating temp and wait for the radiator fan to cycle at least once.
• Shut the engine off, let it cool, then top off the radiator and reservoir again.
• Tip: Gurgling = air; keep bleeding until stable.

Step 10: Reinstall the splash shield and lower the vehicle

• Reinstall the shield using a 10mm socket and flathead screwdriver.
• Lift slightly with the floor jack (rated 3-ton minimum), remove the jack stands, then lower the Forester.

---

✅ After Repair

• Check for leaks around the thermostat housing and lower hose with the engine running.
• Drive 10–15 minutes, then recheck the coolant level after the engine fully cools; top off the reservoir to FULL.
• Watch the temp gauge over the next few drives; any overheating means stop and recheck for trapped air or leaks.

---

💰 DIY vs Shop Cost

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

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

You Save: $210-$590 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.

---

🎯 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.