🔧 Forester - Spark Plug Replacement

On your Forester, the spark plugs sit on the sides of the engine (one per cylinder) and they wear over time, which can cause misfires, rough idle, and worse fuel economy. Replacing them is very doable, but access is tight, so you’ll remove a few under-hood items to make room.

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

---

⚠️ Safety & Precautions

• ⚠️ Let the engine cool fully before starting; hot aluminum threads can strip.
• ⚠️ Disconnect the negative battery terminal to prevent accidental shorts.
• ⚠️ Do not use anti-seize on modern plated spark plugs unless the plug manufacturer specifically requires it.
• ⚠️ Keep dirt out of the spark plug holes; contamination can damage the engine.

---

🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• Floor jack (rated 3-ton minimum)
• Jack stands (rated 3-ton minimum)
• Wheel chocks
• 10mm socket
• 10mm combination wrench
• Ratchet (3/8" drive)
• Extension set (3/8" drive)
• Swivel wobble extension (3/8" drive)
• Torque wrench (3/8" drive, 5–60 Nm range)
• 14mm thin-wall spark plug socket (3/8" drive)
• Trim clip remover
• Flathead screwdriver
• Needle-nose pliers
• Compressed air blow gun
• Flashlight
• Small pick tool

---

🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Spark plugs (iridium, correct OE-type) - Qty: 4
• Dielectric grease - Qty: 1

---

📋 Before You Begin

• Park on level ground, shift to Park, and set the parking brake.
• Open the hood and let the engine cool completely.
• Use a 10mm socket to disconnect the negative battery terminal and tuck it aside so it can’t spring back. Negative terminal is the “-” post.
• If you plan to access from the wheel wells, raise the front and support it on jack stands with wheel chocks at the rear wheels.

---

🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Make room to reach the ignition coils

• On the passenger side, loosen the intake duct clamps using a flathead screwdriver, then move the duct/airbox as needed for access.
• On the driver side, remove the battery hold-down using a 10mm socket, then lift the battery out carefully (it’s heavy).
• If any plastic clips block access, pop them out with a trim clip remover.

Step 2: (Optional but helpful) Open access at the wheel wells

• Turn the steering wheel to give more space (or remove the front wheels for maximum room).
• Remove the small access flap/liner fasteners using a trim clip remover and needle-nose pliers as needed.

Step 3: Unplug one ignition coil (do one cylinder at a time)

• Locate the ignition coil on the cylinder you’re starting with.
• Press the electrical connector tab and unplug it; use a small pick tool gently if the tab is stuck. Don’t pull on the wires.
• Remove the coil retaining bolt using a 10mm socket.

Step 4: Remove the ignition coil

• Twist the coil slightly to break it loose, then pull it straight out by hand.
• If it’s stubborn, keep twisting gently—avoid prying hard against plastic.

Step 5: Clean the spark plug well

• Blow out dirt around the plug hole using compressed air blow gun and aim away from your face.

Step 6: Remove the spark plug

• Install a 14mm thin-wall spark plug socket on a ratchet (3/8" drive) with an extension set and a swivel wobble extension if needed.
• Turn counterclockwise to loosen and remove the plug.
• Lift the plug out carefully with the socket (most spark plug sockets have a rubber insert to hold it).

Step 7: Install the new spark plug (start by hand)

• Check the new plug matches the old one in size and reach.
• Do not bend the tiny tip on iridium plugs; they are typically pre-gapped.
• Thread the new plug in by hand using the extension set (3/8" drive) with the 14mm thin-wall spark plug socket (no ratchet at first).
• If you feel resistance early, back it out and try again—this prevents cross-threading.
• Tighten with a torque wrench (3/8" drive, 5–60 Nm range): Torque to 21 Nm (15 ft-lbs).

Step 8: Reinstall the ignition coil

• Apply a very small amount of dielectric grease inside the coil boot (this helps future removal and seals out moisture).
• Push the coil straight onto the plug until fully seated.
• Install the coil bolt using a 10mm socket: Torque to 6.4 Nm (57 in-lbs).
• Reconnect the electrical connector until it clicks.

Step 9: Repeat for the remaining 3 cylinders

• Repeat Steps 3–8 for each cylinder, one at a time.
• One-at-a-time prevents mixing connectors.

Step 10: Reinstall removed components

• Reinstall the battery and hold-down using a 10mm socket.
• Reinstall the intake duct/airbox pieces and tighten clamps using a flathead screwdriver.
• Reinstall any wheel well access flaps/clips using a trim clip remover.

Step 11: Reconnect battery

• Reconnect the negative battery terminal using a 10mm socket.

---

✅ After Repair

• Start your Forester and let it idle for 1–2 minutes.
• Listen for a smooth idle (no shaking) and confirm no warning lights appear.
• If you get a flashing check engine light or misfire feel, shut it off and re-check coil connectors are fully clicked in.
• Take a short test drive and re-check for any loose intake clamps (whistling/hissing can indicate an air leak).

---

💰 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.

---

🎯 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.