🔧 Forester - Spark Plug Replacement

Replacing the spark plugs on your Forester helps restore smooth idle, easy starts, fuel economy, and proper ignition performance. The 2.5L flat-four engine places the spark plugs low on each side of the engine, so the job needs patience, short extensions, and careful hand-threading.

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

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⚠️ Safety & Precautions

• ⚠️ Let the engine cool fully before starting; the plugs thread into aluminum cylinder heads that can be damaged when hot.
• ⚠️ Disconnect the negative battery cable to prevent accidental electrical shorts while working near ignition coils.
• ⚠️ Do not force spark plugs. Cross-threading means the plug is going in crooked and can damage the cylinder head.
• ⚠️ Work on one spark plug at a time so ignition coils and connectors stay in the correct location.
• ⚠️ Keep dirt out of the spark plug wells. Any debris can fall into the engine when the plug is removed.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 14mm thin-wall spark plug socket
• 3/8-inch drive ratchet
• 3/8-inch drive torque wrench
• 3-inch 3/8-inch drive extension
• 6-inch 3/8-inch drive extension
• 3/8-inch drive universal joint adapter
• Flat-blade screwdriver
• Needle-nose pliers
• Compressed air blow gun
• Spark plug gap gauge
• Magnetic pickup tool
• Fender cover
• Nitrile gloves
• Safety glasses

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs - Qty: 4
• Dielectric grease - Qty: 1

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📋 Before You Begin

• Park your Forester on level ground, shift to Park, and set the parking brake.
• Open the hood and allow the engine to cool completely.
• Use a 10mm socket to loosen the negative battery cable nut, then remove the cable from the battery post.
• A spark plug socket is a special deep socket with a rubber insert or magnet that grips the spark plug without dropping it.
• A universal joint adapter lets the socket angle slightly so it can reach plugs in tight spaces.
• Do not apply anti-seize to the new iridium spark plugs unless the plug manufacturer specifically requires it; most modern plugs have coated threads.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the Engine Cover and Intake Ducting

• Use your hands to lift the plastic engine cover upward from its rubber mounts, if equipped.
• Use a flat-blade screwdriver to loosen the intake duct clamp at the air box.
• Use a flat-blade screwdriver to loosen the intake duct clamp at the throttle body.
• Use needle-nose pliers to release any small hose clamps attached to the intake duct.
• Remove the intake duct and place it aside.
• Take a photo before removing hoses.

Step 2: Access the Driver-Side Ignition Coils

• Stand at the driver side of your Forester and look low along the side of the engine.
• The ignition coils are small black electrical modules attached directly over each spark plug.
• Use compressed air blow gun to clean around each ignition coil before removal.
• Press the connector tab by hand and unplug the ignition coil connector.
• If the connector is tight, use a flat-blade screwdriver gently under the locking tab. Do not pry hard.

Step 3: Remove One Driver-Side Ignition Coil

• Use a 10mm socket and 3/8-inch drive ratchet to remove the ignition coil retaining bolt.
• Twist the ignition coil gently by hand to break the rubber boot loose.
• Pull the ignition coil straight out of the spark plug tube.
• Set the coil aside in the same direction it came out.

Step 4: Remove the Old Spark Plug

• Use compressed air blow gun to clean inside the spark plug well before removing the plug.
• Install the 14mm thin-wall spark plug socket, 3-inch extension, universal joint adapter, and 3/8-inch drive ratchet onto the spark plug.
• Turn counterclockwise slowly to loosen the spark plug.
• Once loose, spin it out gently by hand using the extension.
• Remove the old plug with the spark plug socket or magnetic pickup tool.
• If the plug feels stuck, tighten it slightly, loosen again, and work slowly.

Step 5: Prepare the New Spark Plug

• Use a spark plug gap gauge to verify the new plug gap is about 0.039-0.043 in.
• Do not pry hard on an iridium plug tip. The fine center electrode is delicate.
• Apply a tiny amount of dielectric grease inside the ignition coil boot only.
• Do not put dielectric grease on the spark plug threads or metal tip.

Step 6: Install the New Spark Plug

• Place the new spark plug into the 14mm thin-wall spark plug socket.
• Use only the extension by hand at first. Thread the plug clockwise into the cylinder head.
• The plug should turn smoothly for several turns. If it does not, remove it and start again.
• Attach the 3/8-inch drive torque wrench and tighten the spark plug to Torque to 17.5 Nm (12.9 ft-lbs).
• Hand-start every plug to avoid damage.

Step 7: Reinstall the Ignition Coil

• Push the ignition coil boot straight onto the new spark plug until seated.
• Use a 10mm socket and 3/8-inch drive torque wrench to install the ignition coil bolt.
• Tighten the ignition coil bolt to Torque to 6.4 Nm (57 in-lbs).
• Reconnect the electrical connector by hand until it clicks.

Step 8: Repeat for the Second Driver-Side Plug

• Use the 10mm socket to remove the second driver-side ignition coil bolt.
• Use compressed air blow gun to clean the plug well.
• Use the 14mm thin-wall spark plug socket, extension, universal joint adapter, and ratchet to remove the old plug.
• Install the new plug by hand, then tighten to Torque to 17.5 Nm (12.9 ft-lbs).
• Use the 10mm socket and torque wrench to reinstall the coil bolt to Torque to 6.4 Nm (57 in-lbs).

Step 9: Access the Passenger-Side Ignition Coils

• Move to the passenger side of your Forester.
• If extra space is needed, use a 10mm socket to remove the air intake resonator or small brackets blocking access.
• Use compressed air blow gun to clean around the passenger-side ignition coils.
• Unplug one ignition coil connector by pressing the connector tab by hand.

Step 10: Replace the Passenger-Side Spark Plugs

• Use a 10mm socket and 3/8-inch drive ratchet to remove one ignition coil bolt.
• Twist and pull the ignition coil out by hand.
• Use the 14mm thin-wall spark plug socket, 3-inch extension, universal joint adapter, and ratchet to remove the spark plug.
• Check the new plug gap with the spark plug gap gauge at 0.039-0.043 in.
• Thread the new spark plug in by hand using the socket and extension.
• Use the 3/8-inch drive torque wrench to tighten the plug to Torque to 17.5 Nm (12.9 ft-lbs).
• Reinstall the ignition coil and tighten its bolt with a 10mm socket to Torque to 6.4 Nm (57 in-lbs).
• Reconnect the coil connector by hand until it clicks.
• Repeat this process for the remaining passenger-side spark plug.

Step 11: Reinstall Intake Ducting and Engine Cover

• Reinstall the intake duct by hand between the air box and throttle body.
• Use a flat-blade screwdriver to tighten both intake duct clamps until snug.
• Use needle-nose pliers to reconnect any small hose clamps removed earlier.
• If any brackets were removed, use a 10mm socket or 12mm socket to reinstall them snugly.
• Push the engine cover back onto its rubber mounts by hand.

Step 12: Reconnect the Battery

• Place the negative battery cable back onto the battery post.
• Use a 10mm socket to tighten the negative battery cable nut snugly.
• Do not overtighten the battery terminal; it only needs to be secure and unable to twist by hand.

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✅ After Repair

• ✅ Start your Forester and let it idle for 1-2 minutes.
• ✅ The engine may idle slightly high at first after the battery disconnect; this usually settles as the idle relearns.
• ✅ Listen for misfires, ticking, or air leaks near the intake duct.
• ✅ Make sure the check engine light stays off.
• ✅ Take a short test drive, then recheck that all intake clamps and connectors are secure.
• ✅ If the power windows lose auto-up/down, reset each window by fully lowering it, fully raising it, and holding the switch up for about 2 seconds.

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💰 DIY vs Shop Cost

Shop Cost: $300-$550 (parts + labor)

DIY Cost: $45-$120 (parts only)

You Save: $180-$430 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 1.5-2.5 hours.

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🎯 Ready to get started?

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