🔧 Ascent - Spark Plug Replacement

This is doable at home, but tight access makes it a patient beginner project. You’ll remove the battery-side obstructions and intake airbox, then replace one plug at a time using a thin-wall socket and short extensions.

Difficulty Level: Intermediate | Estimated Time: 3-5 hours

Assumption: These instructions cover the stock 2.4L turbocharged engine. Subaru’s flat engine puts the plugs beside the frame rails, so tool selection matters more than brute force. howtoo.com

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⚠️ Safety & Precautions

• ⚠️ Let the engine become completely cold before removing any spark plug.
• ⚠️ Disconnect the negative battery terminal before unplugging the ignition coils.
• ⚠️ Support a removed battery securely. It’s heavy and contains corrosive acid.
• ⚠️ Blow debris from each plug well before removing the plug.
• ⚠️ Start every new plug by hand. Cross-threading the aluminum cylinder head can become a major repair.
• ⚠️ Don’t apply anti-seize to the plated NGK spark-plug threads.
• ⚠️ Never use an impact wrench on spark plugs.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 14mm thin-wall spark plug socket
• 1/4-inch drive ratchet
• 3/8-inch drive flex-head ratchet
• 3/8-inch drive torque wrench rated 7-30 ft-lbs
• 3-inch 3/8-inch drive extension
• 6-inch 3/8-inch drive extension
• 3/8-inch drive universal joint
• Flat-blade screwdriver
• Plastic trim-clip removal tool
• 0.024-inch wire spark plug gap gauge
• Compressed-air blow gun
• Battery lifting strap
• LED work light
• Safety glasses
• Nitrile mechanic gloves

A universal joint is a hinged socket adapter that allows the extension to approach the plug at a slight angle.

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• NGK OE Series Laser Iridium spark plugs - Gap: 0.024 inch - Qty: 4
• Dielectric ignition grease - Qty: 1 small tube

The listed NGK plugs use a 14mm hex, M12 x 1.25 threads, and a factory gap of 0.024 inch or 0.6mm. Replace all four together. howtoo.com

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📋 Before You Begin

• 📋 Park your Ascent on level ground, select Park, and engage the parking brake.
• 📋 Switch off the ignition, remove the key from the work area, and open the hood.
• 📋 Allow at least one hour for the engine and turbocharger area to cool.
• 📋 Photograph the intake hose, clamps, clips, and battery connections before disassembly.
• 📋 Keep bolts from the battery area and airbox in separate labeled containers.
• 📋 Replace one plug at a time so the ignition coils and connectors stay organized.
• 📋 No infotainment service mode, scan-tool command, coding, or battery registration is required.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Disconnect and remove the battery

• Use a 10mm socket to loosen the negative terminal and lift it from the battery post.
• Move the negative cable aside so it cannot spring back onto the post.
• Use the 10mm socket to disconnect the positive terminal.
• Use the appropriate 10mm or 12mm socket to remove the battery hold-down bracket.
• Use the battery lifting strap to lift the battery straight out.
• Negative off first, negative on last.

Step 2: Remove the intake duct and airbox

• Use a flat-blade screwdriver to loosen the intake-hose clamp.
• Disconnect any attached breather hose by releasing its clamp by hand.
• Use the plastic trim-clip removal tool and 10mm socket to remove the intake snorkel and airbox fasteners.
• Lift the intake pieces and airbox from the engine compartment without pulling on wiring.
• Cover the exposed intake opening with a clean, lint-free cloth.

Step 3: Remove the first ignition coil

• Use the LED work light to locate the ignition coils on the sides of the engine.
• Press the electrical connector lock and pull on the connector body, not its wires.
• Use the 10mm socket and 1/4-inch drive ratchet to remove the coil retaining bolt.
• Rotate the coil gently, then work it outward from the spark-plug well.
• Don’t pry against the plastic coil housing.

Step 4: Clean the spark-plug well

• Put on safety glasses.
• Use the compressed-air blow gun to remove sand and debris from around the plug.
• Look inside with the LED work light and verify that the well is clean and dry.
• If oil is present, stop. A leaking seal should be diagnosed before installing new plugs.

Step 5: Remove the old spark plug

• Fit the 14mm thin-wall spark plug socket to the shortest extension that reaches the plug.
• Add the 3/8-inch drive universal joint only where the frame clearance requires it.
• Use the 3/8-inch drive flex-head ratchet to loosen the plug counterclockwise.
• Once loose, turn the extension by hand and withdraw the plug carefully.
• If the plug remains tight after breaking loose, turn it slightly inward and outward while removing it.

Step 6: Inspect and prepare the new plug

• Use the 0.024-inch wire spark plug gap gauge to verify a 0.024-inch or 0.6mm gap.
• Don’t force a flat blade between the fine iridium electrode and ground strap.
• If the gap is substantially incorrect or the ceramic is cracked, replace the plug instead of bending it.
• Leave the threads clean and dry. Don’t add anti-seize.
• Place a tiny smear of dielectric grease inside the coil boot, not on the plug terminal or threads.

Step 7: Hand-thread the new spark plug

• Place the plug into the 14mm thin-wall spark plug socket.
• Lower it gently into the well using only the extension.
• Turn the extension clockwise with your fingers until the plug seats.
• If it doesn’t turn easily for several complete rotations, back it out and start again.
• Finger-starting protects the aluminum threads.

Step 8: Torque the spark plug

• Attach the 3/8-inch drive torque wrench while keeping the extension aligned as straight as possible.
• Tighten the plug to 21 Nm (15.5 ft-lbs). howtoo.com
• Stop immediately when the torque wrench clicks.
• Don’t click the wrench repeatedly. That can overtighten the plug.

Step 9: Reinstall the ignition coil

• Push the coil straight onto the spark plug until the boot is fully seated.
• Start the retaining bolt by hand.
• Use the 3/8-inch drive torque wrench with the 10mm socket to tighten the coil bolt to 8 Nm (71 in-lbs or 5.9 ft-lbs). howtoo.com
• Reconnect the electrical connector and verify that its lock clicks.

Step 10: Replace the remaining three plugs

• Repeat Steps 3 through 9 for each remaining cylinder.
• Use the shortest extension combination that provides enough working room.
• Confirm every coil bolt and connector before moving to the next cylinder.
• Patience wins this job. The frame rails aren’t interested in negotiating.

Step 11: Reinstall the intake system

• Remove the clean cloth from the intake opening.
• Use the 10mm socket and plastic trim-clip removal tool to reinstall the airbox, snorkel, and retaining clips.
• Reconnect any breather hose removed earlier.
• Use the flat-blade screwdriver to secure the intake-hose clamp.
• Check that the intake hose is fully seated around its entire connection.

Step 12: Reinstall and connect the battery

• Use the battery lifting strap to lower the battery into its tray.
• Use the appropriate 10mm or 12mm socket to reinstall the battery hold-down securely.
• Use the 10mm socket to connect the positive terminal first.
• Use the 10mm socket to connect the negative terminal last.
• Make the terminals secure without crushing or deforming them.

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✅ After Repair

• ✅ Check the engine compartment for sockets, extensions, loose connectors, and uncovered openings.
• ✅ Start the engine and allow it to idle for two minutes.
• ✅ A brief uneven start may occur after the battery disconnect, but the idle should quickly become smooth.
• ✅ If the check-engine light flashes or the engine shakes strongly, switch it off and inspect every coil connector.
• ✅ Listen around the intake hose for hissing that could indicate a loose clamp.
• ✅ Take a short test drive and confirm smooth acceleration without hesitation or warning lights.
• ✅ Reset the clock and any convenience settings lost during the battery disconnect.

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💰 DIY vs Shop Cost

Shop Cost: $350-$650 equivalent (parts + labor)

DIY Cost: Approximately $107 for four listed NGK plugs, plus tools and supplies

You Save: Approximately $240-$540 by doing it yourself!

Labor pricing around Sonīpat can differ considerably from U.S. estimates. Tight boxer-engine access is why this service carries substantial labor time.

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