You can replace a spark plug in 30 minutes to an hour with basic tools

Replacing a spark plug is one of the most straightforward car repairs you can do yourself. You need a spark plug socket, a ratchet wrench, a gap tool (sometimes), and the correct replacement spark plug for your vehicle. The whole job usually takes between 30 minutes and an hour, depending on how many plugs you're replacing and how straightforward they are to reach in your engine.

The hardest part is often just locating the spark plugs under the hood and knowing which one to buy. Once you have those two things figured out, the actual removal and installation is mechanical and repetitive — the same steps for each plug.

Key Takeaways

  • Find your vehicle's year, make, and model in the owner's manual or on the driver's door jamb to buy the exact spark plug you need.
  • Locate the spark plugs by following the thick wires or coils on top of the engine; each one screws straight out and back in.
  • Remove one wire or coil at a time, unscrew the old plug, check the gap on the new one, and screw the new plug in hand-tight before using the socket wrench.
  • Reconnect the wire or coil to the new plug and move to the next one; do not remove all wires at once or you may reconnect them in the wrong order.

Finding the right spark plug for your vehicle

The spark plug you need depends on your specific vehicle. Open your owner's manual and look for the maintenance or specifications section — it will list the exact spark plug type, usually something like "NGK BPR6ES" or "Champion RC12YC." If you do not have the manual, you can find the same information on a sticker inside the driver's door jamb, or you can give your vehicle's year, make, model, and engine size to an auto parts store and they will look it up for you.

Write down the spark plug type before you go to the store or order online. Buying the wrong type will not damage anything, but the plug straightforward will not fit or will not work correctly. Most spark plugs cost between $3 and $15 each, depending on the brand and type.

Gathering your tools and preparing the engine

You need a spark plug socket (a special socket sized for spark plugs, usually 5/8 inch or 13/16 inch), a ratchet wrench that fits the socket, and a spark plug gap tool (a small metal or ceramic tool with slots for measuring the gap between the plug's electrodes). Many new spark plugs come pre-gapped, so you may not need the gap tool, but it is worth having one on hand for about $5.

Let the engine cool for at least 15 minutes before you start. A hot engine can burn you, and the metal parts contract slightly as they cool, making the plugs easier to remove. Once the engine is cool, open the hood and locate the spark plugs. You will see thick wires or plastic coils sitting on top of the engine — each one connects to one spark plug underneath.

Removing the old spark plug

Start with the spark plug closest to you or the easiest to reach. Gently twist and pull the wire or coil straight up and off the top of the plug. Do not yank it by the wire itself; grip the rubber boot (the connector at the end) and wiggle it side to side as you pull. If it is stuck, you can use a spark plug wire puller tool, but your fingers usually work fine.

Once the wire or coil is off, place your spark plug socket over the old plug and attach it to your ratchet. Turn the ratchet counterclockwise to unscrew the plug. It should come out easily — if it is stuck, do not force it; explore a little penetrating oil and wait a few minutes. Once the old plug is out, set it aside and look at the new one.

Checking the gap and installing the new plug

The gap is the space between the center electrode and the side electrode on the tip of the spark plug. Most new plugs come pre-gapped to the correct distance, but it is worth checking. Use your gap tool to measure the space — it should match the specification in your owner's manual, usually between 0.028 and 0.060 inches. If the gap is wrong, you can carefully bend the side electrode to adjust it.

Hold the new spark plug in your hand and thread it into the hole by hand first, turning it clockwise. This prevents you from cross-threading it (screwing it in at an angle, which damages the threads). Once it is hand-tight, use your ratchet and socket to tighten it the rest of the way. Do not over-tighten — snug is enough.

Reconnect the wire or coil by pushing it straight down onto the top of the new plug until you hear or feel a click. You should see a small mark or notch on the coil that aligns with a mark on the plug to help you get it on correctly.

Finishing the remaining spark plugs

Repeat the same steps for each remaining spark plug: remove the wire or coil, unscrew the old plug, check the gap on the new one, screw in the new plug by hand, tighten with the socket, and reconnect the wire or coil. Do one plug at a time rather than removing all the wires at once — if you mix up which wire goes where, the engine will not run correctly.

Once all the plugs are in and the wires or coils are reconnected, start the engine and listen for any misfires or rough running. The engine should start normally and idle smoothly. If it does not, turn it off and double-check that each wire or coil is fully seated on its plug.

When to replace your spark plugs

Most spark plugs last between 30,000 and 100,000 miles, depending on the type. Your owner's manual will tell you the recommended interval for your vehicle. Some cars use standard copper plugs that need replacing every 30,000 miles, while others use platinum or iridium plugs that can go 100,000 miles or more. Check your manual or ask your mechanic at your next service to find out when yours are due.

You can also inspect your spark plugs yourself if you remove them. A healthy plug has a light tan or gray color on the tip. If the tip is black and sooty, wet, or has a white crusty buildup, the plug may need replacing sooner than the recommended interval.

Frequently Asked Questions

What if I cannot get the old spark plug out?

Spark plugs can seize in the hole if they have been in the engine for many years. Spray penetrating oil around the base of the plug, wait 15 to 30 minutes, and try again. If it is still stuck, rock the socket gently side to side rather than pulling straight up. Do not force it hard enough to break the plug or strip the threads in the hole.

Can I replace just one spark plug, or do I have to do them all?

You can replace just one if only one is failing, but most mechanics recommend replacing all of them at the same time. They wear at roughly the same rate, so if one is worn, the others are probably close behind. Replacing them all at once saves you from having to do the job again in a few thousand miles.

What does the gap tool do?

The gap tool measures the space between the two electrodes at the tip of the spark plug. Most new plugs come pre-gapped correctly, but the gap can shift during shipping or storage. If the gap is wrong, the plug will not fire correctly and the engine may misfire or run rough.

Do I need to replace the spark plug wires too?

Spark plug wires wear out over time and can crack or corrode. If your wires are more than 10 years old or show visible damage, replacing them at the same time as the plugs is a good idea. However, if your car has coil-on-plug ignition (a coil sitting directly on each plug instead of a wire), you do not have wires to replace.

What if my engine has a coil pack instead of individual coils?

Some engines have one coil pack that connects to all the spark plugs through wires. The process is the same — disconnect the wire from each plug, remove the old plug, install the new one, and reconnect the wire. The only difference is that you are disconnecting one wire at a time from a central coil pack rather than removing individual coils.