🔧 Impreza - Spark Plug Replacement

On your Impreza, the spark plugs sit on the sides of the flat (boxer) engine, so access is tighter than many cars. You’ll remove a few intake/battery parts for room, then replace all four plugs and torque them correctly to prevent misfires and stripped threads.

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

---

⚠️ Safety & Precautions

• ⚠️ Work on a fully cool engine to avoid burns and protect aluminum threads.
• ⚠️ Disconnect the negative battery terminal to prevent accidental shorts.
• ⚠️ Do not over-tighten spark plugs—stripped threads are expensive to repair.
• ⚠️ Keep dirt out of the spark plug holes; blow debris away before removing plugs.

---

🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• Fender cover
• 3/8" drive ratchet
• 3/8" drive torque wrench (10-60 ft-lbs range)
• 14mm thin-wall spark plug socket (3/8" drive)
• 3/8" drive 6" extension
• 3/8" drive 12" extension
• 3/8" drive wobble extension
• 3/8" drive universal joint
• 10mm socket
• 12mm socket
• Flathead screwdriver
• Needle-nose pliers
• Compressed air canister
• Small flashlight
• Magnetic pickup tool

---

🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (OE-equivalent, pre-gapped) - Qty: 4
• Dielectric grease - Qty: 1

---

📋 Before You Begin

• Park on level ground, set the parking brake, and open the hood.
• Let the engine cool completely (at least 1 hour if it was running).
• Use a 10mm socket to disconnect the negative battery terminal and tuck it aside so it can’t spring back.
• Assumption: stock intake and battery layout.

---

🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Make room on the right side (battery side)

• Use a 10mm socket to remove the battery hold-down hardware.
• Lift the battery out carefully (it’s heavy). Set it somewhere safe and upright.
• Use a 10mm socket to remove the battery tray if it blocks access.

Step 2: Make room on the left side (air box side)

• Use a flathead screwdriver to loosen the intake hose clamp(s).
• Unclip/remove the air intake snorkel/ducting as needed by hand.
• Use a 10mm socket to remove the air box bolts and lift the air box section(s) out for clearance.

Step 3: Unplug and remove the ignition coil (one cylinder at a time)

• Pick one coil to start with (doing one at a time prevents mix-ups).
• Press the electrical connector tab and unplug it by hand (use needle-nose pliers gently only if stuck).
• Use a 12mm socket to remove the coil retaining bolt.
• Twist the coil slightly and pull it straight out.
• “Ignition coil” = the part that sits on the plug.

Step 4: Clean around the spark plug hole

• Use a compressed air canister to blow dirt away from the plug tube before removing the plug.
• Use a flashlight to confirm the area is clean.

Step 5: Remove the old spark plug

• Install the 14mm thin-wall spark plug socket on a 3/8" drive extension and ratchet.
• If access is tight, use a wobble extension or universal joint to line up with the plug.
• Loosen the plug counterclockwise and remove it.
• If the plug feels extremely tight, stop and re-check alignment to avoid damaging threads.

Step 6: Install the new spark plug (critical)

• Put the new plug into the spark plug socket (the rubber insert holds it).
• Start threading the plug by hand using only the extension (no ratchet at first) until it spins smoothly.
• If it won’t thread in smoothly, back it out and try again—do not force it.
• Once seated, use the torque wrench to tighten: Torque to 21 Nm (15.5 ft-lbs).
• Do not adjust the gap on iridium plugs.

Step 7: Reinstall the ignition coil

• Apply a tiny dab of dielectric grease inside the coil boot.
• Push the coil straight onto the plug until fully seated.
• Install the coil bolt using a 12mm socket, then tighten with a torque wrench: Torque to 6.4 Nm (4.7 ft-lbs).
• Reconnect the electrical connector until it clicks.

Step 8: Repeat for the remaining three cylinders

• Repeat Steps 3 through 7 for each remaining spark plug.
• Use the magnetic pickup tool if you drop a bolt in a tight spot.

Step 9: Reassemble intake and battery

• Reinstall the air box and intake ducting using a 10mm socket and flathead screwdriver for clamps.
• Reinstall the battery tray and battery using a 10mm socket.
• Reconnect the negative battery terminal using a 10mm socket.

---

✅ After Repair

• Start the engine and let it idle for 30-60 seconds. It should run smooth.
• Listen for a steady idle; a rough idle can mean a coil connector isn’t fully clicked in.
• Take a short test drive and re-check for any warning lights.
• If the check engine light is on for misfire, re-check coil connectors and plug torque.

---

💰 DIY vs Shop Cost

Shop Cost: $250-$450 (parts + labor)

DIY Cost: $50-$120 (parts only)

You Save: $130-$400 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.

---

🎯 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.