🔧 Forester - Spark Plug Replacement

On your Forester, the spark plugs sit on the sides of the engine under the ignition coils. The job is mostly about making room to reach them without damaging the coil connectors or cross-threading the new plugs.

Difficulty Level: Intermediate | Estimated Time: 1.5-3.0 hours

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⚠️ Safety & Precautions

• ⚠️ Let the engine cool fully before starting; the turbo/heat shields stay hot a long time.
• ⚠️ Disconnect the negative battery terminal to prevent accidental shorts and check-engine lights from unplugged coils.
• ⚠️ Do not work near the radiator fans with the key on; they can turn on by themselves.
• ⚠️ Never force spark plugs in; cross-threading the aluminum cylinder head is expensive.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• Fender cover
• 10mm socket
• 10mm wrench
• Ratchet (3/8")
• Torque wrench (3/8", 10-40 Nm range)
• Extension set (3", 6", 10")
• Universal joint adapter (3/8")
• Wobble extension (3/8")
• Spark plug socket 14mm thin-wall (3/8")
• Small flathead screwdriver
• Trim clip tool
• Needle-nose pliers
• Feeler gauge set (0.7-1.2 mm range)
• Compressed air blow gun
• Flashlight

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (OEM equivalent) - Qty: 4
• Dielectric grease (silicone) - 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 place a fender cover to protect paint.
• Make sure the engine is cold to the touch (especially near the turbo area).
• Use a 10mm wrench to disconnect the negative battery terminal and move it aside so it can’t spring back.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the engine cover (if equipped)

• Lift the cover straight up by hand; it pops off the mounting grommets.
• If it uses fasteners on your setup, remove them with a 10mm socket and ratchet (3/8").

Step 2: Make room on the passenger side (air intake/airbox area)

• Loosen the intake clamp(s) using a small flathead screwdriver.
• Unclip/remove the air duct or airbox lid using a trim clip tool as needed.
• Move the intake pieces aside enough to reach the passenger-side ignition coils.
• More room = less broken clips.

Step 3: Make room on the driver side (battery area)

• Remove the battery hold-down with a 10mm socket and ratchet (3/8").
• Disconnect the positive terminal using a 10mm wrench, then lift out the battery carefully.
• If a bracket or harness holder blocks access, remove the fasteners with a 10mm socket.

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

• Pick one coil to start with so parts don’t get mixed up.
• Press the connector lock tab and pull the connector straight off.
• If stuck, gently help the tab with a small flathead screwdriver (don’t pry hard).
• Pull the plug, not the wires.

Step 5: Remove the ignition coil

• Remove the coil bolt using a 10mm socket and ratchet (3/8").
• Twist the coil slightly, then pull it straight out.
• Set the coil somewhere clean and dry.

Step 6: Clean the spark plug well

• Blow out dirt around the plug using a compressed air blow gun.
• This prevents debris from falling into the cylinder when the plug comes out.

Step 7: Remove the old spark plug

• Install a spark plug socket 14mm thin-wall (3/8") onto a wobble extension (3/8") (or universal joint adapter (3/8")) and your ratchet (3/8").
• Turn counterclockwise to loosen and remove the plug.
• If it feels extremely tight, stop and work it out gently (small loosening/tightening motions) to avoid thread damage.

Step 8: Check and prep the new spark plug

• Verify the plug matches the old one (same reach and tip style) under a flashlight.
• Check the gap with a feeler gauge set (0.7-1.2 mm range).
• Do not force-bend the tiny iridium tip; if the gap is off, replace the plug.
• Do not apply anti-seize unless the plug manufacturer specifically requires it.

Step 9: Install the new spark plug by hand first

• Place the new plug into the spark plug socket 14mm thin-wall (3/8").
• Use an extension set (3", 6", 10") (no ratchet at first) to thread it in by hand.
• If it doesn’t spin in smoothly, back it out and start again (this prevents cross-threading).

Step 10: Torque the spark plug

• Set your torque wrench (3/8", 10-40 Nm range) and tighten the plug.
• Torque to 21 Nm (15 ft-lbs)

Step 11: Reinstall the ignition coil

• Apply a tiny smear of dielectric grease (silicone) inside the coil boot (the rubber part). Dielectric grease helps prevent moisture-related misfires.
• Push the coil straight onto the plug until fully seated.
• Install the coil bolt with a 10mm socket and ratchet (3/8"), then tighten with the torque wrench.
• Torque to 8 Nm (71 in-lbs)
• Reconnect the electrical connector until it clicks.

Step 12: Repeat for the remaining 3 cylinders

• Repeat Steps 4–11 for each cylinder, working one coil/plug at a time.
• Use the universal joint adapter (3/8") and wobble extension (3/8") as needed for the tight rear plugs.

Step 13: Reassemble what you removed

• Reinstall the air intake/airbox parts using the small flathead screwdriver for clamps and the trim clip tool for clips.
• Reinstall the battery and hold-down using the 10mm socket and 10mm wrench.
• Reconnect battery terminals (positive first, negative last) using a 10mm wrench.

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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 a ticking/air hiss near the intake that could indicate a loose clamp; tighten with a small flathead screwdriver if needed.
• Test drive 10–15 minutes, then recheck for any warning lights.
• If you had a misfire light before, clearing codes with a scan tool may be needed after the repair.

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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 1.5-3.0 hours.

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🎯 Ready to get started?

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