🔧 Forester - Spark Plug Replacement

On your Forester, the spark plugs sit low on the sides of the engine. Replacing them restores smooth starts, idle quality, and fuel economy, and helps prevent misfires as mileage adds up.

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

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⚠️ Safety & Precautions

• 🧤 Work on a fully cool engine to avoid burns and thread damage in the aluminum cylinder heads.
• 🧯 Keep the key fob away from the vehicle; don’t press the Start button while hands are in the engine bay.
• 🔋 Battery disconnect is recommended if you remove the battery: disconnect negative (-) first, reconnect it last.
• 🧹 Do not let dirt fall into the spark plug holes; debris can damage the engine.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• 10mm socket
• 3/8" ratchet
• 3/8" torque wrench (10-30 ft-lbs range)
• 14mm thin-wall 12-point spark plug socket (specialty)
• 3/8" extension set (3", 6", 10")
• 3/8" wobble extension (specialty)
• Flathead screwdriver
• Needle-nose pliers
• Trim clip removal tool
• Compressed air blow gun
• Shop light

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Spark plugs (iridium, OEM-equivalent) - 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 (at least 1 hour after driving).
• If you’ll remove the battery for access, use a 10mm socket and disconnect the negative (-) terminal first.
• Tip: Take a quick photo of hoses/clamps first.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the engine cover and open up access

• Lift off the engine cover (it pulls upward from its rubber grommets).
• Use a flathead screwdriver to loosen the air intake hose clamp at the throttle body (just loosen, don’t remove the clamp).
• Use a trim clip removal tool to remove any intake snorkel clips/push-pins as needed.

Step 2: Create access to the passenger-side plugs

• Use a 10mm socket to remove the air cleaner housing fasteners/bolts as needed.
• Use a flathead screwdriver to loosen intake tube clamps, then move the intake duct/airbox out of the way enough to reach the passenger-side ignition coils.

Step 3: Create access to the driver-side plugs

• If access is tight, remove the battery for working room.
• Use a 10mm socket to disconnect the negative (-) terminal, then the positive (+) terminal.
• Use a 10mm socket to remove the battery hold-down and lift the battery out carefully.
• Move the washer fluid fill neck aside if it blocks access (release clips with needle-nose pliers as needed).

Step 4: Unplug and remove the ignition coil (one cylinder at a time)

• Find the ignition coil on the side of the engine (one per cylinder).
• Press the electrical connector lock tab and pull the connector off by hand. If stuck, gently help it with needle-nose pliers (don’t pull on the wires).
• Use a 10mm socket to remove the ignition coil hold-down bolt.
• Twist the coil boot slightly, then pull the coil straight out.
• Torque to 6.4 Nm (4.7 ft-lbs) when reinstalling the coil bolt.

Step 5: Clean the spark plug well

• Use compressed air blow gun to blow dirt out of the spark plug tube before removing the plug.
• Tip: Keep the nozzle slightly above the hole.

Step 6: Remove the old spark plug

• Install a 14mm thin-wall 12-point spark plug socket (specialty) onto a 3/8" extension.
• If the angle is tight, use a 3/8" wobble extension (specialty) to help the socket align.
• Use a 3/8" ratchet to loosen and remove the spark plug.

Step 7: Install the new spark plug (protect the threads)

• Start the new spark plug by hand using the spark plug socket and extension only (no ratchet at first) to avoid cross-threading.
• Once seated, use a 3/8" torque wrench to tighten the spark plug: Torque to 21 Nm (15.5 ft-lbs).
• Do not change the gap on iridium plugs unless the plug maker instructs it.
• Avoid anti-seize unless the plug manufacturer specifically requires it (most modern plated plugs do not).

Step 8: Reinstall the ignition coil

• Apply a small dab of dielectric grease inside the end of the coil boot (dielectric grease is a non-conductive grease that helps prevent moisture and makes future removal easier).
• Push the coil straight onto the spark plug until fully seated.
• Install the coil bolt with a 10mm socket, then tighten with a 3/8" torque wrench: Torque to 6.4 Nm (4.7 ft-lbs).
• Reconnect the coil electrical connector until it clicks.

Step 9: Repeat for the remaining 3 spark plugs

• Repeat Steps 4 through 8 for each cylinder, doing one at a time so nothing gets mixed up.

Step 10: Reassemble intake and battery

• Reinstall the airbox/intake pieces and tighten clamps using a flathead screwdriver and 10mm socket as needed.
• If removed, reinstall the battery hold-down using a 10mm socket.
• Reconnect the battery terminals: positive (+) first, then negative (-), using a 10mm socket.
• Reinstall the engine cover by pressing it down into the grommets.

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✅ After Repair

• Start the engine and let it idle for 1-2 minutes; it should be smooth with no shaking.
• Listen for intake air leaks (a loud hissing can mean a loose clamp); recheck clamps using a flathead screwdriver.
• Test drive 10 minutes, then recheck for any warning lights.
• If a check engine light appears with a misfire, recheck coil connectors and coil seating on each plug.

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💰 DIY vs Shop Cost

Shop Cost: $350-$600 (parts + labor)

DIY Cost: $60-$140 (parts only)

You Save: $210-$540 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?

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.