Why Change Spark Plugs?

Worn spark plugs cause poor fuel economy, rough idle, hard starting and reduced engine power. Replacing them is one of the most cost-effective maintenance jobs you can do — plugs typically cost $5–15 each and a set of 4 takes about an hour. A garage charges $150–300 for the same job.

What You Need

  • Spark plug socket (usually 16mm or 21mm — check your car manual)
  • Ratchet wrench and extension bar
  • Torque wrench (recommended for correct tightening)
  • New spark plugs (check your car manual for the exact part number)
  • Spark plug gap tool (to verify the gap)
  • Anti-seize compound (optional but recommended)
  • Dielectric grease (for the plug boots)
Safety firstOnly work on a cold engine — spark plugs get extremely hot and can burn you badly. Wait at least 2 hours after running the engine before starting this job.
  1. 1

    Identify and buy the right plugs

    Check your car manual or look up your make, model and year at a parts store. The correct part number matters — using the wrong plug can damage your engine. Buy quality plugs (NGK, Bosch, Denso, Champion) not the cheapest available.

  2. 2

    Check the spark plug gap

    New plugs are often pre-gapped but verify the gap with a gap tool matches your manual's specification. The gap is the distance between the centre and ground electrode — usually 0.8–1.0mm for most modern cars.

  3. 3

    Locate the spark plugs

    On a 4-cylinder car, the spark plugs are usually in a row along the top of the engine. Follow the thick ignition leads (or coil packs on modern cars) to find them. Some engines require removing a cover or airbox.

  4. 4

    Change one plug at a time

    This is important — do not remove all leads at once. On older cars, mixing up lead order causes misfires. Work on one cylinder, complete it, then move to the next.

  5. 5

    Remove the lead and old plug

    Pull the ignition lead off the plug (twist gently, pull straight out — never yank). Use the spark plug socket to unscrew the old plug anti-clockwise. Inspect it for signs of oil fouling, white deposits or heavy wear.

  6. 6

    Install the new plug

    Thread the new plug in by hand first to avoid cross-threading. Then tighten with the socket wrench — do not overtighten. A torque wrench set to your manual's specification (usually 20–25 Nm) is the safest approach.

  7. 7

    Reattach the lead or coil pack

    Push the lead firmly onto the new plug until you feel or hear it click. Apply a small amount of dielectric grease inside the lead boot to prevent future sticking.

  8. 8

    Repeat for each cylinder

    Repeat steps 4–7 for each remaining plug. Start the engine and check for smooth idle.

Frequently Asked Questions

Rough idle, harder starting, reduced fuel economy, engine misfires (shaking at idle or during acceleration) or the check engine light with a misfire code. Even without symptoms, follow your car's service schedule — most manufacturers specify replacement every 30,000–60,000km for standard plugs or 100,000km+ for iridium plugs.
Yes. Since they all have the same mileage, replacing one means the others will fail soon anyway. Replacing all at once saves labour time and ensures even performance across all cylinders.