What you need to do before you start
Replacing spark plugs is a task most car owners can do themselves 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 engine type. Before you begin, let your engine cool completely — a hot engine can burn you.
Find your vehicle's manual or look up the specifications online using your car's year, make, and model. You need to know how many spark plugs your engine has (usually four, six, or eight), what type to buy, and the correct gap setting — the space between the electrodes. Most auto parts stores can tell you the right plug if you give them this information, and many will set the gap for you at no cost.
Locate your spark plugs under the hood. On most cars, you will see thick wires or coils sitting on top of the engine in a row. Each one connects to a spark plug underneath. Take a photo of the wire arrangement with your phone before you disconnect anything — this helps you reconnect them in the right order.
Key Takeaways
- You will need the correct spark plug type and gap setting for your vehicle, which you can find in your owner's manual or by asking an auto parts store.
- Remove one wire or coil at a time, replace the spark plug underneath, and reconnect before moving to the next one — this prevents mixing up the wire order.
- The spark plug should sit snug in its socket but not so tight that you force it; hand-tighten first, then use the wrench for a final quarter-turn.
- Reconnect each wire or coil by pushing straight down until you hear or feel a click, and check that it sits flush against the engine.
Removing the old spark plugs one at a time
Start with the spark plug closest to you. Grasp the thick wire or coil at its base — not the thin wire itself — and twist gently while pulling straight out. If it is stuck, a spark plug wire puller tool can help, but most come free with a firm, steady pull. Once the wire is off, you will see the spark plug sitting in a hole in the engine head.
Fit your spark plug socket over the top of the spark plug. The socket has a rubber insert inside that grips the plug. Push down firmly and turn the ratchet counterclockwise to loosen the old plug. Once it is loose enough, you can usually unscrew it by hand. Set the old plug aside — you may need it later if something goes wrong.
Do not remove all the wires at once. Work on one spark plug at a time, from start to finish, before moving to the next. This prevents you from mixing up which wire goes where, which can cause the engine to run poorly or not start.
Installing the new spark plug
Before you install a new spark plug, check that the gap is correct. The gap is the space between the center electrode and the side electrode at the tip of the plug. Use a gap tool or feeler gauge to measure it — slide the gauge into the gap until you feel slight resistance. If the gap is wrong, carefully bend the side electrode until the measurement matches your vehicle's specification.
Thread the new spark plug into the hole by hand first. This prevents you from cross-threading it, which damages the hole and creates an expensive repair. Once it is hand-tight, fit the spark plug socket back on and use the ratchet to tighten it. Turn clockwise until the plug is snug — you want it tight enough that it will not vibrate loose, but not so tight that you strip the threads. A quarter-turn past hand-tight is usually correct.
If you are unsure about the tightness, err on the side of looser rather than tighter. A plug that is too tight can crack, and a plug that is slightly loose will usually tighten itself as the engine heats up.
Reconnecting the wires or coils
Once the new spark plug is in place, reconnect the wire or coil. Push it straight down onto the top of the spark plug until you hear or feel a click. The connection should be firm and flush — there should be no gap between the wire boot and the engine. If the wire does not click or feels loose, pull it off and try again.
Now move to the next spark plug. Remove its wire, replace the plug, and reconnect the wire before moving on. This method takes longer than removing all the wires at once, but it is the safest way to avoid mixing them up.
Checking your work before you start the engine
Once all the spark plugs are in and all the wires are reconnected, take a moment to look at the engine. Make sure every wire or coil is seated firmly and no wires are pinched or touching hot surfaces. If you have a photo from before you started, compare it to what you see now — the arrangement should be identical.
Start the engine and listen. It should turn over smoothly and run without misfiring or rough idling. If the engine hesitates, stalls, or sounds wrong, turn it off when ready. The most common cause is a wire that is not fully seated or in the wrong position. Turn off the engine, let it cool for a few minutes, and check each connection again.
When to call a mechanic instead
Some vehicles make spark plug replacement difficult. If your engine has coils instead of wires, or if the spark plugs are buried deep under other components, the job takes longer and requires more tools. If you do not feel confident after reading these steps, or if your vehicle's manual warns that the job is complex, a mechanic can do it for you in about an hour.
If you start the job and find that a spark plug is stuck or will not come out, stop and do not force it. Forcing a stuck plug can break it inside the engine, which requires professional removal. Call a mechanic rather than risk that damage.
Frequently Asked Questions
How often should I replace my spark plugs?
Most modern spark plugs last 30,000 to 100,000 miles depending on the type and your vehicle. Check your owner's manual for the recommended interval. Some plugs are designed to last longer than others, so the cost per mile is often similar even if the replacement interval is different.
What if I mix up the wire order?
The engine will misfire or run very poorly. If this happens, turn it off and reconnect the wires in the correct order using your photo or your manual. The wires must go to the spark plugs in the exact sequence the engine fires them, which is specific to your vehicle.
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 mechanics usually recommend replacing all of them at the same time. They wear at similar rates, and doing them all at once saves you the labor cost of opening the engine again in a few months.
Do I need to gap new spark plugs?
Most new spark plugs come pre-gapped from the factory, but it is worth checking against your vehicle's specification. Some plugs are gapped for a different vehicle or may have shifted during shipping. Checking takes two minutes and prevents a misfire.
What should I do with the old spark plugs?
Most auto parts stores will take old spark plugs for recycling. You can also check with your local waste management facility. Do not throw them in the trash — they contain materials that should be recycled.