🔧 Impreza - Spark Plug Replacement

On your Impreza, the spark plugs sit on the sides of the flat engine (boxer), so access is tight. The job is very doable at home, but you’ll be working in cramped spaces and you must be careful not to cross-thread the new plugs.

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

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⚠️ Safety & Precautions

• 🔥 Work on a fully cool engine to avoid burns and damaged threads.
• 🧯 Keep ignition OFF and keys away from the car while unplugging coils.
• 🔋 If you remove the battery (recommended for access), disconnect the negative terminal first and reconnect it last.
• 🧼 Blow dirt away from the plug wells before removing plugs to prevent debris from falling into the cylinders.
• 🧵 Start spark plugs by hand only; cross-threading the aluminum head is expensive to fix.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• 10mm socket
• 12mm socket
• 3/8" drive ratchet
• 3/8" drive torque wrench (10–30 Nm range)
• 3/8" drive extension set (3", 6")
• 3/8" drive universal joint (swivel)
• 14mm thin-wall spark plug socket (magnetic or rubber insert)
• Flat trim tool
• Phillips screwdriver
• Pick tool
• Compressed air blow gun
• Telescoping magnet pickup tool
• Flashlight

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Spark plugs (iridium, correct spec for Impreza 2.0L) - 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.
• Open the hood and let the engine cool completely.
• If you plan to remove the battery for access: use a 10mm socket to disconnect the negative terminal first.
• Lay out the 4 new plugs and keep them clean. Don’t drop iridium plugs.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Create working room (top-side access)

• Use a 10mm socket to disconnect the battery negative terminal (if removing battery).
• Use a 10mm socket to remove the battery hold-down, then lift the battery out.
• Use a Phillips screwdriver and flat trim tool to loosen the intake snorkel/air duct clips as needed.
• Use a 10mm socket to loosen the air box fasteners and move the air intake assembly for more space on the passenger side.

Step 2: Locate the ignition coils

• Each cylinder has an ignition coil bolted to the side of the engine (one coil per plug).
• Use a flashlight to identify the coil electrical connector and the single coil hold-down bolt.
• Do one plug at a time to avoid mix-ups.

Step 3: Unplug and remove an ignition coil

• Use a pick tool to gently lift the connector lock tab (if equipped), then unplug the coil connector by hand.
• Use a 10mm socket to remove the ignition coil bolt.
• Twist the coil slightly and pull it straight out by hand.

Step 4: Clean the spark plug well

• Use compressed air blow gun to blow debris away from the spark plug hole before removal.
• If anything falls, use a telescoping magnet pickup tool to retrieve it.

Step 5: Remove the old spark plug

• Install the 14mm thin-wall spark plug socket on a 3/8" drive extension and 3/8" drive universal joint (swivel) as needed for the angle.
• Use a 3/8" drive ratchet to loosen and remove the spark plug.
• Lift the plug out with the socket (or use the telescoping magnet pickup tool if it slips).

Step 6: Install the new spark plug (by hand first)

• Place the new plug into the 14mm thin-wall spark plug socket.
• Carefully thread it in by hand using only the extension (no ratchet) until it seats. This prevents cross-threading.
• Use a 3/8" drive torque wrench (10–30 Nm range) to tighten the spark plug: Torque to 21 Nm (15.5 ft-lbs).
• Do not use anti-seize on modern plated plugs.

Step 7: Reinstall the ignition coil

• Apply a tiny amount of dielectric grease inside the rubber coil boot (a thin film only).
• 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" drive torque wrench: Torque to 6.4 Nm (4.7 ft-lbs).
• Reconnect the coil electrical connector until it clicks.

Step 8: Repeat for the other 3 spark plugs

• Repeat Steps 3–7 for each remaining cylinder.
• Reposition the intake/air box as needed to reach both sides.
• Patience matters more than force on this engine.

Step 9: Reassemble and reconnect battery

• Reinstall the air intake/air box hardware using a 10mm socket and Phillips screwdriver.
• Reinstall the battery and hold-down using a 10mm socket.
• Reconnect the battery negative 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 with no shaking.
• If the check engine light comes on or it runs rough, re-check that every coil connector is fully clicked in.
• Take a short test drive and confirm normal power and no flashing check engine light.

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💰 DIY vs Shop Cost

Shop Cost: $300-$550 (parts + labor)

DIY Cost: $45-$120 (parts only)

You Save: $180-$450 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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