🔧 Highlander - Spark Plug Replacement

Replacing the spark plugs on your Highlander helps restore smooth starting, fuel economy, and ignition performance. On the V6 hybrid, the front bank is fairly accessible, but the rear bank requires removing the upper intake plenum to reach the plugs.

Difficulty Level: Intermediate | Estimated Time: 3-5 hours

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⚠️ Safety & Precautions

• ⚠️ This is a hybrid vehicle. Do not touch orange high-voltage cables or high-voltage components.
• ⚠️ Disconnect the 12V negative battery cable before removing ignition coils or intake components.
• ⚠️ Let the engine cool fully before starting. Aluminum cylinder heads can be damaged if spark plugs are removed hot.
• ⚠️ Keep dirt, bolts, and tools out of the intake ports after the intake plenum is removed.
• ⚠️ Do not use anti-seize on the new spark plug threads unless the plug manufacturer specifically requires it. Most modern iridium plugs are pre-coated.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 14mm socket
• 5/8-inch spark plug socket
• 3/8-inch drive ratchet
• 3/8-inch drive torque wrench
• 3-inch extension
• 6-inch extension
• Universal joint adapter
• Needle-nose pliers
• Plastic trim clip remover
• Flathead screwdriver
• Magnetic pickup tool
• Compressed air nozzle
• Clean shop towels
• Masking tape
• Permanent marker
• Safety glasses
• Nitrile gloves

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs - Qty: 6
• Upper intake plenum gasket set - Qty: 1
• Throttle body gasket - Qty: 1
• Dielectric grease - Qty: 1

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📋 Before You Begin

• Park your Highlander on level ground, shift to Park, and set the parking brake.
• Turn the ignition off and keep the key fob away from the vehicle so the hybrid system cannot wake up unexpectedly.
• Open the hood and let the engine cool completely.
• Disconnect the 12V negative battery cable using a 10mm socket. The 12V battery is located at the rear of the vehicle under the cargo area trim.
• Label hoses and electrical connectors with masking tape and a permanent marker before removing them. This helps everything go back in the correct place.
• A spark plug socket has a rubber insert or magnet inside that holds the plug so it does not fall into the engine bay.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the Engine Cover

• Use both hands to lift the plastic engine cover straight upward.
• If it feels stuck, gently wiggle it while pulling up. It is held by rubber grommets, not bolts.
• Set the cover aside where it will not get stepped on.

Step 2: Disconnect the 12V Battery Negative Cable

• Use a 10mm socket to loosen the negative battery terminal clamp.
• Move the cable away from the battery post so it cannot spring back and touch.
• Keep the key fob away.

Step 3: Remove the Air Intake Tube

• Use a flathead screwdriver to loosen the hose clamp at the throttle body.
• Use a flathead screwdriver to loosen the hose clamp at the air cleaner box.
• Use needle-nose pliers to release any small hose clamps attached to the intake tube.
• Disconnect the intake tube and remove it from the engine bay.

Step 4: Disconnect Throttle Body and Intake Plenum Connections

• Use your fingers to press the lock tabs and unplug the throttle body electrical connector.
• Use needle-nose pliers to release vacuum hose clamps and slide the hoses off carefully.
• Use masking tape and a permanent marker to label each hose before removal.
• A vacuum hose is a small rubber hose that carries engine air pressure signals to different components.

Step 5: Remove the Throttle Body

• Use a 10mm socket and 3/8-inch drive ratchet to remove the throttle body mounting bolts.
• Move the throttle body aside gently without pulling hard on any remaining hoses.
• Remove and discard the old throttle body gasket.
• During installation, tighten the throttle body bolts with a 3/8-inch drive torque wrench to Torque to 10 Nm (89 in-lbs).

Step 6: Remove the Upper Intake Plenum

• Use a 10mm socket, 12mm socket, 3-inch extension, and 3/8-inch drive ratchet to remove the upper intake plenum bolts and nuts.
• Use a 12mm socket to remove any rear support bracket bolts connected to the plenum.
• Lift the plenum slowly and check for any hose or connector still attached.
• Once free, remove the plenum and place it on a clean towel.
• Cover the exposed intake ports immediately with clean shop towels.
• The intake plenum is the large air chamber that feeds air into the engine cylinders.
• During installation, tighten the intake plenum fasteners evenly with a 3/8-inch drive torque wrench to Torque to 21 Nm (15 ft-lbs).

Step 7: Remove the Front Bank Ignition Coils

• Use your fingers to press each ignition coil connector lock tab and unplug the connector.
• Use a 10mm socket and 3/8-inch drive ratchet to remove each ignition coil hold-down bolt.
• Pull each ignition coil straight up with a gentle twisting motion.
• An ignition coil is the part that sends high voltage to the spark plug.
• Set the coils in order so each one goes back to the same cylinder.

Step 8: Remove the Rear Bank Ignition Coils

• With the intake plenum removed, use a 10mm socket, 6-inch extension, and 3/8-inch drive ratchet to remove the rear ignition coil bolts.
• Unplug the rear ignition coil connectors by pressing the lock tabs with your fingers.
• Pull each rear ignition coil straight up and set it in order.
• Work slowly near the firewall.

Step 9: Clean Around the Spark Plug Wells

• Use a compressed air nozzle to blow dirt out of each spark plug well.
• Wear safety glasses while using compressed air.
• Do not skip this step. Dirt can fall into the cylinder when the spark plug is removed.

Step 10: Remove the Old Spark Plugs

• Use a 5/8-inch spark plug socket, 6-inch extension, and 3/8-inch drive ratchet to loosen each spark plug.
• Turn counterclockwise slowly until the spark plug is free.
• Lift the spark plug out carefully with the spark plug socket.
• If a plug feels stuck, stop and gently tighten it slightly, then loosen again. Do not force it.

Step 11: Install the New Spark Plugs

• Place one new iridium spark plug into the 5/8-inch spark plug socket.
• Use the 6-inch extension by hand to thread the spark plug into the cylinder head.
• Start every spark plug by hand for several turns to avoid cross-threading. Cross-threading means the plug is going in crooked and can damage the cylinder head.
• Use a 3/8-inch drive torque wrench with the 5/8-inch spark plug socket to tighten each spark plug to Torque to 18 Nm (13 ft-lbs).
• Hand-start every plug first.

Step 12: Reinstall the Ignition Coils

• Apply a small amount of dielectric grease inside each ignition coil boot. Dielectric grease helps seal out moisture but does not conduct electricity.
• Push each ignition coil straight down onto its spark plug until seated.
• Use a 10mm socket and 3/8-inch drive torque wrench to tighten the ignition coil bolts to Torque to 8 Nm (71 in-lbs).
• Reconnect each ignition coil electrical connector until it clicks.

Step 13: Reinstall the Upper Intake Plenum

• Remove the clean shop towels from the intake ports.
• Install the new upper intake plenum gasket set.
• Lower the intake plenum into position carefully.
• Use a 10mm socket, 12mm socket, and 3/8-inch drive torque wrench to tighten the intake plenum fasteners evenly to Torque to 21 Nm (15 ft-lbs).
• Use a 12mm socket and 3/8-inch drive torque wrench to tighten the rear support bracket bolts to Torque to 21 Nm (15 ft-lbs).

Step 14: Reinstall the Throttle Body and Intake Tube

• Install the new throttle body gasket.
• Use a 10mm socket and 3/8-inch drive torque wrench to tighten the throttle body bolts to Torque to 10 Nm (89 in-lbs).
• Reconnect the throttle body electrical connector by pushing it in until it clicks.
• Use needle-nose pliers to reinstall hose clamps on all removed hoses.
• Reinstall the air intake tube.
• Use a flathead screwdriver to tighten both intake tube hose clamps snugly.

Step 15: Reconnect the 12V Battery and Reinstall the Engine Cover

• Use a 10mm socket to reconnect and tighten the negative battery terminal.
• Push the engine cover straight down onto its rubber mounts.
• Use a magnetic pickup tool to check for dropped bolts or sockets before starting the engine.

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✅ After Repair

• Start your Highlander and let it idle for a few minutes.
• The idle may be slightly uneven at first while the throttle body relearns airflow.
• Listen for hissing sounds near the intake plenum. A hiss may mean a vacuum hose or gasket is leaking.
• Check that no warning lights stay on after the engine runs.
• Take a short test drive and confirm smooth acceleration.
• If the check engine light comes on, recheck ignition coil connectors, vacuum hoses, and intake tube clamps first.

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💰 DIY vs Shop Cost

Shop Cost: $450-$750 (parts + labor)

DIY Cost: $90-$180 (parts only)

You Save: $360-$570 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2.5-4.0 hours.

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🎯 Ready to get started?

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