What you need to do before you start

Replacing spark plugs is a job most car owners can do at home with basic tools and about an hour of time. You will need new spark plugs (the correct type for your vehicle), a spark plug socket, a ratchet wrench, a gap tool or feeler gauge, and possibly a spark plug wire puller or boot removal tool depending on your car's age. Before you buy anything, check your owner's manual or search online for your vehicle's year, make, and model to find out which spark plugs fit and how many your engine has — this varies widely.

Let the engine cool completely before you start. A hot engine can burn you, and you want to work safely. If you have recently driven the car, wait at least 30 minutes. Open your hood and locate the spark plugs — they sit under thick wires or coils on top of the engine. Take a photo with your phone so you can remember which wire or coil goes where when you reinstall them.

Key Takeaways

  • You will need the correct spark plugs for your vehicle year and model, which you can find in your owner's manual or by searching online.
  • Remove one wire or coil at a time, unscrew the old spark plug, check the gap on the new one, and screw it in before moving to the next one.
  • Reconnect each wire or coil firmly before removing the next one so you do not mix up the order.
  • The whole job usually takes 30 minutes to an hour and costs between $20 and $100 in parts, depending on your vehicle.

Removing the old spark plugs one at a time

Start with the first spark plug. Gently twist and pull the wire or coil straight up and away from the plug — do not yank on the wire itself, as that can damage it. Grip the boot (the rubber or plastic connector) and wiggle it side to side as you pull. If it is stuck, a spark plug wire puller tool costs about $5 and makes this step easier.

Once the wire or coil is off, use your spark plug socket and ratchet to unscrew the old spark plug by turning counterclockwise. The socket slides over the plug and holds it as you turn. Lift the old plug out and set it aside. Look at the tip — if it is black and sooty, your engine has been running rich (too much fuel). If it is white, it has been running lean (too little fuel). Either extreme means something else in your engine may need attention, but you can still replace the plugs.

Checking and installing the new spark plug

Before you screw in the new spark plug, check the gap — the space between the two metal points at the tip. Your owner's manual or the box the plugs came in will tell you what the gap should be, usually between 0.028 and 0.060 inches. Use a gap tool or feeler gauge to measure. If the gap is wrong, gently bend the outer electrode (the curved piece) until it matches. This takes only a few seconds and matters because the wrong gap can cause misfires or poor fuel economy.

Thread the new spark plug into the hole by hand first, turning clockwise slowly. This prevents you from cross-threading it, which damages the hole. Once it is hand-tight, use your ratchet to snug it down — do not over-tighten. A spark plug should be firm but not forced. Push the wire or coil back onto the plug until you hear or feel a click, or until it is seated firmly. You should not be able to pull it off by hand without effort.

Moving through the remaining plugs

Repeat the process for each remaining spark plug, one at a time. The reason you do them one at a time instead of removing all the wires first is that it is straightforward to mix up which wire goes where, and putting them back in the wrong order will cause the engine to misfire or not start. By finishing each plug completely before moving to the next, you avoid that mistake.

If your car has six or eight spark plugs, you will work your way around the engine. Some engines have plugs that are harder to reach — you may need to remove an air intake hose or other parts to get to them. If you get stuck, stop and look up a video for your specific vehicle model. Mechanics' videos on YouTube often show exactly which parts to move out of the way.

Starting the engine and checking your work

Once all the plugs are in and all the wires or coils are reconnected, close your hood and get in the car. Turn the key to start the engine. It should start normally and run smoothly. If it does not start, or if it starts but runs rough or misfires, turn it off and check that each wire or coil is pushed all the way onto its plug. A loose connection is the most common reason for problems after a DIY spark plug job.

Let the engine idle for a minute or two and listen. You should not hear any unusual knocking or pinging. If the engine runs smoothly, you are done. Drive the car normally — there is no break-in period needed for new spark plugs. If the check engine light comes on, it usually means a wire was not seated properly or a plug was cross-threaded. Turn off the engine, check the connections, and start again.

When to call a mechanic instead

Some vehicles make spark plug replacement difficult. If your engine has spark plugs buried under the intake manifold or other large components, or if you do not have the right tools, a mechanic can do the job faster and with less risk of damage. Labor usually costs $75 to $200 depending on the shop and how hard the plugs are to reach. If you are not comfortable working on your car, that cost is worth the peace of mind.

If you start the job and find that a spark plug is stuck or cross-threaded, stop when ready and take the car to a mechanic. Forcing a stuck plug can crack the cylinder head, which is an expensive repair. A mechanic has tools to remove stuck plugs safely.

Frequently Asked Questions

How often do spark plugs need to be replaced?

Most modern spark plugs last between 30,000 and 100,000 miles, depending on the type and your driving habits. Check your owner's manual for the recommended interval for your vehicle. Some cars use long-life plugs that go much longer; others need them sooner.

What if I do not know which spark plugs to buy?

Write down your vehicle's year, make, model, and engine size, then take that information to an auto parts store. The staff can look up the correct plugs for you, or you can search online using that information. The box will also tell you the correct gap setting.

Can I replace just one spark plug?

You can, but it is not ideal. Spark plugs wear at roughly the same rate, so if one is worn, the others probably are too. Replacing all of them at once costs more upfront but saves you from doing the job again in a few months.

What does a misfire feel like?

A misfire feels like a hesitation or stumble when you accelerate, or a rough idle when the car is stopped. It usually means a spark plug is not firing correctly, often because the wire is loose or the gap is wrong. Check all your connections first.

Do I need to replace the spark plug wires too?

Not always. Wires last longer than plugs and do not need to be replaced every time. If your wires are cracked, burned, or more than 100,000 miles old, replace them. Otherwise, reuse them. Coil packs on newer cars do not need replacement unless they fail.