How to Change Spark Plugs: A Beginner's Guide

To change spark plugs, let the engine cool completely, remove the ignition coil or plug wire from one cylinder, unscrew the old plug with a spark plug socket, check the new plug's gap, thread it in by hand and tighten it to the spec in your manual. Then refit the coil and move on to the next cylinder. On many four-cylinder engines it's a very manageable first DIY job with basic hand tools.

What you need

• The correct new plugs. Use the part number from your owner's manual or a parts catalog for your exact engine.
• A spark plug socket in the right size. Most car plugs take 16 mm (5/8 inch), but some engines use 14 mm and older ones 21 mm. A 16 mm magnetic spark plug socket with swivel has a thin 1 mm wall for deep plug wells and a magnet that holds the plug.
• A 3/8-inch drive ratchet and one or two extensions
• A 3/8-inch drive torque wrench (strongly recommended)
• A wire-type gap gauge
• Small sockets or Torx bits for the engine cover and coil bolts. A 1/4-inch socket and bit set with 4 to 14 mm sockets and T10 to T40 Torx bits is the right kind of tool for these small, light-torque fasteners.
• Compressed air or a vacuum to clean around the plug wells
• A short length of rubber hose that pushes snugly onto the top of the plug (handy for starting plugs by hand)
• Eye protection

Before you start

• Work on a cold engine. Plug makers say to remove and install plugs only when the engine is cool, because metal expands when hot and the threads in an aluminum cylinder head are easy to damage. A hot engine will also burn you.
• Ignition off. Take the key out, or keep a keyless fob away from the car.
• Check your manual. Note the plug part number, the gap and the tightening torque. On some V6 engines the rear plugs sit under the intake manifold, which turns this into a much bigger job.

How to change spark plugs step by step

Step 1: Find the plugs and remove the engine cover

On most modern engines you'll see a row of ignition coils on top of the engine, one per cylinder, often under a plastic cover. Older engines have thick plug wires running to each plug instead. Remove the cover; it usually pulls off rubber mounts or has a few small bolts.

Step 2: Clean around each plug

Blow out the area around each coil and plug well with compressed air, wearing eye protection. Grit that falls into an open plug hole can end up inside the cylinder.

Step 3: Remove one coil or plug wire

Work on one cylinder at a time so nothing gets mixed up.

1. Coil-on-plug engines: press the release tab and unplug the electrical connector. Remove the coil's hold-down bolt (often a small hex or Torx bolt). Then pull the coil straight up, twisting it gently to free the rubber boot.
2. Plug wires: grip the boot, not the wire, and twist as you pull. Pulling on the wire itself can damage it.

Put the bolt somewhere safe. If one drops into the engine bay, see how to get a dropped bolt out of the engine bay.

Step 4: Remove the old plug

1. Slide the spark plug socket down the well, on an extension, until it seats fully on the plug's hex.
2. Turn counterclockwise. The first crack can take some force; after that it should unscrew smoothly.
3. If it tightens partway out, stop (see troubleshooting below).
4. Lift the plug out. A magnetic socket brings it up with the socket, so you don't have to fish for it.

Step 5: Look at the old plug

Light tan or gray deposits are normal. Heavy dry black soot suggests rich running or misfires. Oil on the firing end means oil is getting into that cylinder. A white, blistered tip can mean overheating. Anything other than normal is worth mentioning to a mechanic.

Step 6: Check the gap on the new plug

1. Find the gap spec in your manual or the plug maker's catalog.
2. Many plugs come pre-gapped, but plug makers still recommend checking it against your engine's spec.
3. If it needs adjusting, bend only the ground electrode (the curved outer one). Never pry against the center electrode. On iridium and platinum plugs the fine center tip is easy to damage.
4. NGK recommends moving the gap no more than about 0.008 inch (0.2 mm) from the factory setting. Some service manuals say not to adjust iridium plugs at all, but to replace any that are out of spec.
5. If you drop a new plug on a hard floor, recheck the gap and look for cracks in the white insulator. If in doubt, use a new one.

Step 7: Thread the new plug in by hand

Start the plug by hand for several full turns before any tool touches it. In a deep well, push the rubber hose onto the plug's top and use it as a long handle, or turn the socket and extension with your fingers only. If the plug binds in the first few turns, back it out and start again. A swivel socket helps you reach plugs behind pipes and covers, but keep the socket straight while you start the threads so the plug doesn't go in crooked.

Step 8: Tighten to spec

Use a torque wrench set to the value in your manual. Your car maker's figure always comes first. As a reference, NGK's chart lists about 25 to 30 Nm (roughly 18 to 22 lb-ft) for a common 14 mm thread, gasket-type plug in an aluminum head, and about 15 to 20 Nm for a 12 mm thread gasket-type plug in the same kind of head. Taper-seat plugs, which have no gasket, take less.

Don't add anti-seize unless your manual calls for it. NGK and Denso both advise against putting lubricant on their plugs' threads, because it changes how much the plug actually tightens at a given torque. NGK says that if you do use it, use very little and reduce the torque by about 30%.

Step 9: Refit the coil and repeat

Push the coil or plug wire boot down until it seats on the plug, refit the hold-down bolt snugly (it only needs light torque), and plug the connector back in until it clicks. Then move on to the next cylinder.

Step 10: Start the engine and check

Refit the engine cover, start the engine and listen. It should idle smoothly with no warning lights. A rough idle or a flashing check engine light usually means a connector that didn't click home or a coil that isn't fully seated.

How to change spark plugs without a torque wrench

A torque wrench is the better way, especially in aluminum heads. If you don't have one, plug makers give a turn-angle method instead: thread the plug in by hand until it seats, then turn it a set amount further with the wrench.

• New gasket-type plugs: NGK gives about 1/2 to 2/3 of a turn for its 14 mm thread plugs, while Denso gives about 1/4 to 1/2 of a turn. Smaller plugs generally need less.
• Taper-seat plugs: only about 1/16 of a turn.
• Reused gasket-type plugs: NGK says only about 1/12 of a turn, because the gasket is already crushed.

The figures differ by brand and design, so use the angle printed on your plug's box if there is one, and your manual over both. Too loose and the plug can overheat and cause engine damage; too tight and you can stretch the plug or strip the threads.

Troubleshooting and mistakes to avoid

• A plug won't come out, or gets tight partway. Don't force it. Forcing a seized plug can strip the threads in an aluminum head, which is an expensive repair. Some engines have known plug-removal problems with their own procedures, so check a service manual for your engine first.
• The new plug won't thread in smoothly. Stop and back it out. Never use a ratchet to push through resistance on the first turns.
• The socket insert stays on the plug. Rubber-insert sockets sometimes leave the insert stuck down the well. A magnetic socket avoids that.
• Misfire after the job. Check every connector and coil first, then the gap, then whether the right plug part number was fitted.

When to call a pro

Hand it to a mechanic if the plugs sit under the intake manifold and you're not comfortable removing it, if a plug is seized or breaks, if threads are damaged, or if a misfire continues after new plugs. A stripped plug hole is fixable, but it's not a first DIY job.

FAQ

How often should you change spark plugs?

Follow the schedule in your owner's manual, because it varies a lot by engine and plug type. Long-life iridium and platinum plugs fitted at the factory often last somewhere around 60,000 to 100,000 miles (roughly 100,000 to 160,000 km), while standard nickel plugs need changing sooner. Hard use can shorten the interval.

Do I need to change the ignition coils with the spark plugs?

Not usually. On most cars, coils are replaced when they fail rather than on a schedule. While they're out, check the rubber boots for cracks or tears, as a damaged boot can cause misfires.

Can you change spark plugs without a torque wrench?

Yes, with the turn-angle method above. A torque wrench is more reliable, especially in aluminum heads.

What size socket do I need for spark plugs?

Most car plugs take 16 mm (5/8 inch). Some newer engines use 14 mm, and older engines often 21 mm (13/16 inch). See spark plug socket sizes explained to check yours.

Should I put anti-seize on spark plugs?

Only if your vehicle manual tells you to. NGK and Denso both advise against it because it changes the effective torque (see Step 8).

Ready to start? Make sure the 16 mm spark plug socket matches your plugs before you order.

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