What you need before you start

Replacing spark plugs is a task most car owners can do at home with basic tools and about 30 minutes to an hour, depending on your vehicle. You will need new spark plugs (the correct type for your car's year, make, and model), a spark plug socket, a ratchet wrench, a gap tool or feeler gauge, and possibly a spark plug wire puller if your car uses older plug wires instead of coils. Check your owner's manual or search online for your specific vehicle to find the exact spark plug part number and gap setting — the small space between the electrodes that the manufacturer sets for your engine.

Before you buy anything, locate your engine's spark plugs. Open your hood and look for thick wires or rubber boots connected to the top of the engine. If you see four, six, or eight of these (depending on your engine size), those are your spark plug locations. Take a photo with your phone so you remember which wire or coil goes where — this matters when you put everything back together.

Key Takeaways

  • You will need the correct spark plug type for your vehicle, which you can find in your owner's manual or by searching your car's year, make, and model online.
  • Remove one spark plug wire or coil at a time, then unscrew the old plug and screw in the new one, checking that the gap matches your manufacturer's specification.
  • Reconnect each wire or coil to the same location before moving to the next plug — doing them one at a time prevents mixing up the order.
  • The entire job takes 30 minutes to an hour for most vehicles, and you can save $100 to $300 in labor costs by doing it yourself.

Removing the old spark plugs one at a time

Start with the first spark plug. Grasp the wire or rubber boot connected to it and twist gently while pulling away from the engine — do not yank on the wire itself, as this can damage it. If the boot is stuck, use a spark plug wire puller (a small tool that costs a few dollars) to remove it safely. Once the wire is off, use your spark plug socket and ratchet to unscrew the old plug by turning counterclockwise.

Before you throw away the old plug, look at its condition. A plug that is wet, black, or covered in oil tells you something else might be wrong with your engine — a mechanic should check this before you assume the new plugs will solve the problem. A plug that is tan or light brown and dry is normal wear.

Work on one plug at a time rather than removing all of them at once. This prevents you from mixing up which wire goes where, which can cause your engine to run poorly or not start at all.

Installing the new spark plugs

Before you screw in the new plug, check the gap. The gap is the space between the two electrodes at the tip of the plug. Your owner's manual or the box the plugs came in will tell you the correct gap measurement — common gaps are 0.028 to 0.044 inches, but this varies by engine. Use a gap tool or feeler gauge to measure the space. If the gap is too wide or too narrow, adjust it by carefully bending the outer electrode until the tool slides through with slight resistance.

Screw the new spark plug into the hole by hand first, turning clockwise slowly. This prevents you from cross-threading the plug, which damages the hole and creates an expensive repair. Once the plug is hand-tight, use your ratchet and socket to tighten it snugly — you do not need to crank hard. Spark plugs are small and can break if over-tightened.

Reconnecting wires and coils in the right order

Reconnect the wire or coil to the new plug when ready after installation. Push it onto the plug until you hear or feel a click, or until it sits flush against the engine. The wire should not wiggle or come off easily. If you removed multiple wires before installing plugs, you risk putting them back in the wrong order, which causes misfires and poor engine performance.

If your vehicle uses coil packs (individual coils that sit directly on top of each plug), straightforward push the coil pack back onto the new plug and find any clips or bolts that hold it in place. Coil packs are harder to mix up because they only fit one way, but wires on older vehicles can go on any plug if you are not careful.

Finishing the job and testing

Once all spark plugs are installed and all wires or coils are reconnected, close your hood and start the engine. It should start normally and run smoothly. If the engine hesitates, misfires, or runs rough, stop and check that each wire or coil is seated firmly on its plug. A loose connection is the most common reason for problems after a spark plug replacement.

Let the engine run for a minute or two, then turn it off. You are done. Most new spark plugs last 30,000 to 100,000 miles depending on the type — your owner's manual will tell you when to replace them again.

When to call a mechanic instead

Some vehicles make spark plug replacement difficult or impossible without special tools. If your engine has spark plugs buried under intake manifolds, fuel injectors, or other large components, a mechanic's labor is worth the cost. Similarly, if you remove a spark plug and the hole is stripped (the threads are damaged), or if the plug breaks while you are removing it, stop and take the car to a shop — attempting to fix this yourself can cause more damage.

If you are uncomfortable working on your engine or do not have the tools, there is no shame in taking your car to a mechanic. The cost of a professional replacement is usually $100 to $300 depending on how many plugs your engine has and how difficult they are to reach.

Frequently Asked Questions

How do I know which spark plug to buy?

Search online for "[your car year] [make] [model] spark plug" or check your owner's manual. The manual lists the exact part number and gap setting. You can also take a photo of your old plug to an auto parts store and they will match it for you. Do not guess — using the wrong plug can damage your engine.

What if I cannot get the old spark plug out?

If a plug is stuck, spray penetrating oil like WD-40 around the base where it screws into the engine and wait 15 to 30 minutes. Try again gently. If it still will not budge, stop — forcing it can break the plug inside the hole, which requires professional removal. A mechanic has tools to extract broken plugs safely.

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

You can replace one plug if only one is bad, but most mechanics suggest replacing all of them at the same time if they are near the end of their lifespan. Plugs wear at similar rates, so doing them all together saves you from having to do the job again in a few months. Check your owner's manual for the recommended replacement interval.

Why does my engine run rough after I replaced the spark plugs?

The most common cause is a loose wire or coil connection. Check that each wire is pushed firmly onto its plug until it clicks or sits flush. If all connections are tight and the problem continues, one of the new plugs may be defective — try swapping it with a plug from a different cylinder to see if the problem moves.