🔧 Highlander - Thermostat Replacement

Your Highlander’s thermostat controls engine temperature by opening and closing to regulate coolant flow. Replacing it usually fixes overheating, slow warm-up, or a “coolant temp” warning caused by a thermostat stuck open or closed.

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

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⚠️ Safety & Precautions

• ⚠️ Never open the radiator or coolant reservoir when hot—coolant can spray and burn.
• ⚠️ Support the vehicle with jack stands before working underneath (never rely on a jack).
• ⚠️ Keep coolant off belts and electrical connectors; wipe spills immediately.
• ⚠️ Coolant is toxic—catch it in a drain pan and dispose of it properly.
• Battery disconnect is not required for this job.

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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 10-quart)
• Funnel
• 10mm socket
• 12mm socket
• 1/4" ratchet
• 3/8" ratchet
• 6" extension
• Torque wrench (5–30 Nm range)
• Hose clamp pliers
• Pick tool
• Trim clip remover
• Flathead screwdriver
• Shop towels

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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
• Toyota Super Long Life Coolant (pink) pre-mixed - Qty: 2-3 gallons
• Replacement hose clamp(s) - Qty: 1-2

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📋 Before You Begin

• Park on level ground, set the parking brake, and chock the rear wheels.
• Let the engine cool fully (ideally overnight).
• Set the cabin heat to MAX HOT during bleeding later (this opens the heater circuit).
• Raise the front of your Highlander with a floor jack and support with jack stands.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the lower splash shield (undercover)

• Use a trim clip remover and 10mm socket to remove the fasteners from the front under-cover.
• Lower the panel and set it aside.

Step 2: Drain the coolant (enough to get below the thermostat level)

• Place a drain pan (at least 10-quart) under the radiator drain.
• Slowly open the drain cock using a flathead screwdriver (if equipped with a slotted head).
• Open the coolant reservoir cap to help it drain (only when the engine is cool).
• Tip: Drain 1–2 gallons; full drain isn’t always needed.

Step 3: Locate the thermostat housing

• The thermostat sits in the coolant inlet/outlet housing at the front side of the engine where a large radiator hose connects.
• Use a flashlight (from your shop area) and look up from below after removing the under-cover.

Step 4: Remove the radiator hose from the housing

• Use hose clamp pliers to compress and slide the clamp back. (Hose clamp pliers are pliers that lock the clamp open so you can move it safely.)
• Twist the hose to break it loose, then pull it off the housing.
• If stuck, carefully work the edge with a pick tool—do not gouge the plastic/metal nipple.

Step 5: Remove the thermostat housing bolts

• Put shop towels under the housing to catch remaining coolant.
• Use a 10mm socket, 1/4" ratchet, and 6" extension to remove the housing bolts.
• Pull the housing straight off and expect some coolant to spill.

Step 6: Replace the thermostat and gasket

• Note the thermostat orientation before removal (same direction going back in).
• Remove the old thermostat and gasket/O-ring by hand.
• Clean the mating surfaces using shop towels (no sanding discs).
• Install the new thermostat with the new gasket/O-ring (lightly wet the O-ring with fresh coolant).

Step 7: Reinstall the housing and torque bolts

• Reinstall the housing and start all bolts by hand to avoid cross-threading.
• Use a torque wrench (5–30 Nm range) and 10mm socket.
• Torque to 10 Nm (89 in-lbs)

Step 8: Reinstall the radiator hose

• Push the hose fully onto the housing nipple.
• Use hose clamp pliers to move the clamp back into its original position.

Step 9: Refill coolant

• Close the radiator drain cock.
• Use a funnel to refill with Toyota Super Long Life Coolant (pink) pre-mixed at the reservoir/radiator fill point (as equipped).
• Fill to the “FULL” line on the reservoir.

Step 10: Bleed air from the cooling system

• Start the engine and let it idle.
• Set the HVAC to HOT and fan to medium (this helps purge air from the heater core).
• Watch the temperature gauge and look for steady heat from the vents.
• As the engine warms, add coolant as the level drops (use a funnel and go slowly).
• Carefully squeeze the upper radiator hose with gloved hands to help push air out.
• Once the radiator fan cycles and the heat is consistent, shut the engine off and let it cool completely, then recheck the level and top off.

Step 11: Reinstall the under-cover

• Reinstall the splash shield using the 10mm socket and clips with the trim clip remover.

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✅ After Repair

• Check for leaks around the thermostat housing and hose connection with the engine running.
• Test drive 10–15 minutes and confirm the temperature gauge stays normal.
• After the engine cools fully, recheck coolant level and top off to the “FULL” mark.
• If a check-engine light appears, scan for codes (a stuck-open thermostat can set P0128).

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💰 DIY vs Shop Cost

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

DIY Cost: $60-$160 (parts only)

You Save: $190-$590 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2-3 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.