What you need to do before you start
Replacing spark plugs is a job most car owners can do in an hour or two with basic tools. You will need a spark plug socket (a deep socket with a rubber insert), a ratchet wrench, a spark plug gap tool or feeler gauge, new spark plugs matched to your car's year and model, and possibly a spark plug wire puller if your car has older ignition wires instead of coils. Check your owner's manual or a parts store to confirm which spark plugs fit your engine.
Before you touch anything under the hood, let the engine cool completely. A hot engine can burn you, and you risk damaging components. Park on level ground, set the parking brake, and disconnect the negative terminal of your battery by loosening the bolt with a wrench. This prevents electrical shock and accidental engine start.
Locate your spark plugs. On most cars, they sit under a plastic cover labeled "engine cover" or you will see thick wires or coils running to the top of the engine. Take a photo with your phone before you remove anything so you can see where each wire or coil goes back. Spark plugs must be reconnected in the exact same order, or your engine will misfire.
Key Takeaways
- Let the engine cool completely and disconnect the negative battery terminal before you begin work.
- Remove one spark plug wire or coil at a time, then pull out the old spark plug with a socket wrench, and check the gap on the new one before installation.
- Push the new spark plug in by hand first to avoid cross-threading, then tighten it snugly with the socket wrench — not so tight that you strip the threads.
- Reconnect the wire or coil to each new spark plug in the same order you removed them, using your phone photo as a reference.
- Reconnect the battery terminal, start the engine, and listen for any rough running or misfires that would mean a wire was put back in the wrong spot.
Remove the first wire or coil pack
Grasp the spark plug wire or coil pack by the boot (the thick rubber connector), not by the wire itself, and twist it gently side to side while pulling straight up. If it is stuck, use a spark plug wire puller tool — a small plastic tool that grips the boot and pulls it free without tearing. Do not yank hard on the wire; you can damage the connection inside.
Set the wire or coil on a clean surface where you can see it. If your car has multiple coils, mark which cylinder it came from using a piece of tape and a pen. This matters because the firing order must be exact.
Remove the old spark plug
Insert the spark plug socket onto the ratchet wrench and lower it over the spark plug hole. The rubber insert in the socket grips the old spark plug so it does not fall into the engine. Turn the ratchet counterclockwise to loosen the spark plug, then unscrew it the rest of the way by hand once it is loose enough to grip.
Look at the old spark plug. A normal plug has a tan or light gray coating on the electrode. If it is black and sooty, your engine is running rich (too much fuel). If it is white, your engine is running lean (too little fuel). Neither means the plug is bad — you are just seeing how your engine has been running. Discard the old plug.
Check the gap on the new spark plug
The gap is the space between the center electrode and the side electrode at the tip of the plug. Your owner's manual or the box the plugs came in will tell you the correct gap measurement, usually between 0.028 and 0.060 inches depending on your engine. Use a spark plug gap tool or a feeler gauge to measure the gap on your new plug.
If the gap is too wide, gently tap the side electrode against a hard surface to close it slightly. If the gap is too narrow, use the gap tool to pry the side electrode outward. Check the gap again after adjusting. A correct gap ensures the spark jumps reliably and your engine runs smoothly.
Install the new spark plug
Lower the new spark plug into the hole by hand first, turning it clockwise slowly. This prevents cross-threading, which happens when you start the plug at an angle and the threads bind. Once the plug is hand-tight and seated properly, attach the socket wrench and tighten it snugly. You want it firm enough that it will not vibrate loose, but not so tight that you strip the aluminum threads in the cylinder head. If you are unsure, tighten until you feel resistance, then give it a quarter turn more.
Remove the socket wrench and set it aside. Do not reconnect the wire or coil yet.
Repeat for each remaining spark plug
Move to the next spark plug and repeat the entire process: remove the wire or coil, remove the old plug, check the gap on the new one, install the new plug, and set the wire or coil aside. Work on one spark plug at a time rather than removing all the wires first. This method keeps you from mixing up which wire goes where, which is the most common mistake that causes rough running after the job.
Most cars have four, six, or eight spark plugs. Take your time with each one. There is no speed advantage to rushing, and a mistake means you will have to remove the plugs again to fix it.
Reconnect the wires or coils in the correct order
Once all the new spark plugs are installed, reconnect each wire or coil to its corresponding plug. Use your phone photo to match the position. Push the connector straight down onto the plug until you hear or feel a click. The boot should seat fully against the plug — if there is a gap, the connection is not complete and the plug will not fire.
If you are working on an older car with spark plug wires, the wires run from the distributor cap to each plug in a specific order called the firing order. Your owner's manual shows this order. If you put even one wire in the wrong spot, the engine will run rough or not start at all. Double-check your photo before you push each connector home.
Reconnect the battery and test
Reconnect the negative battery terminal by tightening the bolt with a wrench. Make sure it is snug so the connection does not come loose while you drive. Close the engine cover if your car has one, and close the hood.
Start the engine and listen. The engine should turn over smoothly and settle into a steady idle. If it runs rough, hesitates, or misfires, turn it off when ready and check that each wire or coil is pushed fully onto its spark plug. A loose connection is the most common cause of rough running after a plug replacement. If the problem persists after you have checked all the connections, one of the wires may be in the wrong cylinder. Refer to your owner's manual to verify the firing order and swap the wires as needed.
Frequently Asked Questions
How do I know which spark plugs to buy?
Check your owner's manual for the exact part number, or take your car's year, make, and model to an auto parts store and they will look it up for you. Spark plugs are specific to your engine, and using the wrong ones can cause poor performance or damage. The box will also show you the correct gap setting.
What if I cannot get the old spark plug out?
If a plug is stuck, explore a penetrating oil like WD-40 around the base of the plug and wait 15 minutes, then try again. If it is still stuck, you may need a spark plug extractor socket, which has reverse threads and grips the plug as you turn it out. A mechanic can remove it if you do not have the tool.
Can I replace just one spark plug instead of all of them?
You can, but it is not recommended. Spark plugs wear at roughly the same rate, so if one is worn enough to need replacement, the others are close behind. Replacing all of them at once prevents you from having to do this job again in a few months. The cost difference between one plug and a full set is small compared to the labor time.
What does it mean if my spark plugs are wet or oily?
Wet plugs mean oil is leaking into the combustion chamber, usually from a worn valve cover gasket or piston rings. Oily plugs will foul quickly and cause misfires. Have a mechanic inspect your engine for oil leaks. You can replace the plugs, but they will get dirty again until the leak is fixed.
How often do spark plugs need to be replaced?
Most modern spark plugs last 30,000 to 100,000 miles depending on the type and your driving habits. Check your owner's manual for the recommended interval. Older cars with copper plugs may need replacement every 10,000 to 20,000 miles, while newer cars with iridium or platinum plugs can go much longer.