🔧 Forester - Spark Plug Replacement

On your Forester, spark plugs fire the air/fuel mix. Replacing worn plugs restores smooth idle, power, and fuel economy, and helps prevent misfires under boost.

Difficulty Level: Intermediate | Estimated Time: 2-4 hours

Assumption: Stock intake/engine bay parts installed.

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⚠️ Safety & Precautions

• ⚠️ Let the engine cool fully before starting; plugs thread into aluminum.
• ⚠️ Disconnect the negative battery terminal to avoid shorts and accidental cranking.
• ⚠️ Do not cross-thread spark plugs; always start by hand.
• ⚠️ Keep dirt out of the plug holes; blow debris away before removing plugs.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• 10mm socket
• 12mm socket
• 3/8" ratchet
• 3/8" torque wrench (5-30 ft-lbs range)
• 3/8" extension set (3", 6", 10")
• 3/8" wobble extension
• 3/8" universal joint
• 14mm thin-wall spark plug socket
• Flathead screwdriver
• Phillips screwdriver
• Pliers
• Trim clip remover
• Compressed air nozzle
• Flashlight
• Telescoping magnet pickup tool

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (OEM heat range for Forester 2.0XT) - Qty: 4
• Dielectric grease - Qty: 1

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📋 Before You Begin

• Park on level ground, shift to Park, and set the parking brake.
• Open the hood and remove the engine cover (pull up firmly by hand).
• Use a 10mm socket to disconnect the negative battery terminal and move it aside.
• Lay out parts in order (front-left, rear-left, front-right, rear-right) to avoid mixing anything up.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Create access to the coils (left/driver side)

• Use a 10mm socket to remove the battery hold-down and battery terminals, then lift the battery out.
• Use a 12mm socket to remove the battery tray bolts and remove the tray.
• Use a flashlight to locate the two ignition coils on the left cylinder head.
• More room = less chance of stripping threads.

Step 2: Create access to the coils (right/passenger side)

• Use a flathead screwdriver to loosen the intake hose clamps as needed.
• Use pliers to release any spring clamps on small hoses attached to the intake ducting.
• Use a 10mm socket to remove bolts/clips holding the air intake snorkel and air box, then lift the air box/ducting out as needed for working room.
• Take a quick photo before removing hoses.

Step 3: Unplug and remove one ignition coil

• Ignition coil = the pencil-shaped part that sits on top of each spark plug.
• Press the lock tab and unplug the coil electrical connector by hand (use a flathead screwdriver gently only if stuck).
• Use a 10mm socket to remove the coil hold-down bolt.
• Twist the coil slightly, then pull it straight out to remove it.
• Torque to 6.4 Nm (4.7 ft-lbs) when reinstalling the coil bolt.

Step 4: Clean around the spark plug hole

• Use compressed air nozzle to blow dirt/debris away from the plug well before loosening the plug.
• Use a flashlight to confirm the well looks clean and dry.

Step 5: Remove the spark plug

• Install the 14mm thin-wall spark plug socket on a 3/8" extension set (3", 6", 10").
• If access is tight, add a 3/8" wobble extension or 3/8" universal joint.
• Use a 3/8" ratchet to loosen the spark plug, then spin it out carefully.
• Use a telescoping magnet pickup tool if you drop the plug or bolt.

Step 6: Install the new spark plug

• Confirm the new plugs are the correct type; iridium plugs are typically pre-gapped—do not bend the center electrode.
• Start threading the plug by hand using the 14mm thin-wall spark plug socket and an extension only (no ratchet at first).
• Once fully hand-seated, use a 3/8" torque wrench (5-30 ft-lbs range) to tighten: Torque to 20.6 Nm (15.2 ft-lbs).
• If it won’t hand-thread, stop and realign.

Step 7: Reinstall the ignition coil

• Add a tiny amount of dielectric grease inside the coil boot (this helps prevent the boot from sticking; it does not improve electrical contact).
• Push the coil straight onto the plug until fully seated.
• Install the coil bolt using a 10mm socket, then tighten with a 3/8" torque wrench (5-30 ft-lbs range): Torque to 6.4 Nm (4.7 ft-lbs).
• Reconnect the coil electrical connector until it clicks.

Step 8: Repeat for the remaining 3 cylinders

• Repeat Steps 3–7 for each remaining coil/plug, doing one cylinder at a time.
• One-at-a-time prevents connector mix-ups.

Step 9: Reassemble intake and battery

• Reinstall the air box/intake ducting using a 10mm socket, and tighten clamps with a flathead screwdriver.
• Reinstall the battery tray using a 12mm socket.
• Reinstall the battery and terminals using a 10mm socket, then install the hold-down.
• Reinstall the engine cover (press down until it snaps in).

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✅ After Repair

• Start the engine and let it idle for 1–2 minutes. It should run smooth with no shaking.
• Listen for hissing (intake leak) and re-check hose clamps with a flathead screwdriver if needed.
• If the check engine light comes on or it runs rough, re-check that every coil connector is fully clicked in.
• Test drive gently, then do a few moderate accelerations once warm to confirm no misfire under boost.

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💰 DIY vs Shop Cost

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

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

You Save: $170-$590 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2.0-3.0 hours.

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🎯 Ready to get started?

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