🔧 Forester - Spark Plug Replacement

On your Forester, the spark plugs sit low on the sides of the engine and space is tight. The job is very doable at home, but it takes patience to avoid cross-threading and to get the coils and plugs out without forcing anything.

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

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⚠️ Safety & Precautions

• ⚠️ Work on a fully cool engine to avoid burns and damaged threads.
• ⚠️ Disconnect the negative battery cable before unplugging ignition coils.
• ⚠️ Support the vehicle with jack stands before removing wheels.
• ⚠️ Do not use anti-seize on spark plug threads (Subaru/NGK plated threads can over-tighten).

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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
• 21mm socket
• Breaker bar 1/2-inch drive
• 3/8-inch ratchet
• 3/8-inch extension set (3-inch, 6-inch, 10-inch)
• 3/8-inch universal joint swivel
• 14mm thin-wall spark plug socket (3/8-inch drive)
• 10mm socket
• 12mm socket
• Torque wrench (3/8-inch drive, 5–30 ft-lb range)
• Flat trim clip removal tool
• Needle-nose pliers
• Feeler gauge (0.028–0.031 inch range)
• Flashlight
• Compressed air blow gun
• Nitrile gloves
• Safety glasses

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (OEM-correct for Forester XT) - 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.
• Let the engine cool completely (warm aluminum threads strip easily).
• Use a 10mm socket to disconnect the negative battery terminal and set it aside.
• Plan your access method: the wheel-well method is usually easiest for first-timers.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Loosen the front wheels

• Use a 21mm socket and breaker bar to loosen (do not remove) the front lug nuts.

Step 2: Raise and support the front of the vehicle

• Lift with a floor jack and place the vehicle on jack stands.
• Place wheel chocks behind the rear wheels.

Step 3: Remove the front wheels

• Remove the lug nuts using a 21mm socket and take off both front wheels.

Step 4: Remove the front fender liners for access

• Use a flat trim clip removal tool and needle-nose pliers to remove the plastic clips/screws holding the front inner fender liners.
• Pull the liner back enough to reach the ignition coil and spark plug area on each side.
• Bag clips per side to avoid mixing.

Step 5: Unplug and remove the ignition coil (one cylinder at a time)

• Locate the ignition coil (the plug wire “boot” is built into this coil). Use a flashlight.
• Press the connector lock and unplug it by hand (use needle-nose pliers gently only if needed).
• Use a 12mm socket with a 3/8-inch ratchet to remove the coil hold-down bolt(s).
• Twist the coil slightly, then pull it straight off the spark plug.
• Do one coil/plug at a time.

Step 6: Clean the spark plug well

• Use compressed air blow gun to blow dirt out around the plug before removal.
• This prevents debris from falling into the cylinder.

Step 7: Remove the old spark plug

• Use a 14mm thin-wall spark plug socket, 3/8-inch extension, and a 3/8-inch universal joint swivel as needed to reach the plug.
• Turn counterclockwise to remove. If it feels stuck, stop and re-check tool alignment so you don’t crack the plug.

Step 8: Check the new plug gap (don’t bend iridium tips)

• Use a feeler gauge (0.028–0.031 inch range) to confirm the gap matches spec.
• If the plug is iridium, do not pry on the center tip; if the gap is off, exchange the plug rather than forcing it.

Step 9: Install the new spark plug by hand first

• Place the new plug into the 14mm spark plug socket and carefully start threading it in by hand using only the extension (no ratchet at first).
• If it doesn’t thread smoothly, back it out and start again. This avoids cross-threading.
• Once seated, use a torque wrench to tighten: Torque to 21 Nm (15 ft-lb).

Step 10: Reinstall the ignition coil

• Apply a tiny smear of dielectric grease inside the coil boot (this helps future removal and prevents moisture).
• Push the coil straight onto the plug until fully seated.
• Install the coil bolt(s) using a 12mm socket, then tighten: Torque to 7.5 Nm (5.5 ft-lb).
• Reconnect the coil electrical connector until it clicks.

Step 11: Repeat for the remaining 3 cylinders

• Repeat Steps 5–10 for each cylinder, one at a time.

Step 12: Reinstall fender liners and wheels

• Reposition the fender liner and reinstall clips using the flat trim clip removal tool and needle-nose pliers.
• Reinstall wheels and snug lug nuts using a 21mm socket.
• Lower the vehicle from jack stands using the floor jack.

Step 13: Torque the lug nuts and reconnect the battery

• Use a torque wrench and 21mm socket to tighten lug nuts: Torque to 120 Nm (88 ft-lb).
• Reconnect the negative battery terminal using a 10mm socket.

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✅ After Repair

• Start the engine and let it idle for 1–2 minutes. It should run smooth.
• If you get a flashing check engine light or shaking, shut it off and re-check each coil connector is fully clicked in.
• Test drive gently and confirm no misfire, hesitation, or fuel smell.

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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 2.0-3.5 hours.

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🎯 Ready to get started?

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