What you need before you start
Changing a spark plug means removing the old one from your engine and screwing in a new one. The job takes 15 minutes to an hour depending on your car's layout, and you can do it with basic tools you may already own. You will need a socket wrench sized to fit your spark plug (usually 5/8 inch or 13/16 inch), a ratchet handle, a spark plug gap tool (optional but useful), and the correct replacement spark plug for your vehicle.
Before you buy a new spark plug, find out which type your car needs. Check your owner's manual, look under the hood for the part number stamped on the old plug, or search your vehicle's year, make, and model online. Spark plugs cost between $3 and $15 each, and most cars need one to eight of them replaced at the same time. Buying the wrong size or heat range will not damage your engine, but the plug will not fire correctly and your car may run rough.
Make sure your engine is cold before you start. A hot engine can burn your hands, and you risk cracking a ceramic insulator if you remove a plug while the metal is still expanding from heat.
Key Takeaways
- You need a socket wrench, ratchet, and the correct replacement spark plug for your vehicle's year, make, and model.
- Remove one wire or coil at a time, unscrew the old plug, and screw the new one in by hand first to avoid cross-threading.
- Tighten the new plug snugly with your socket wrench, but do not over-tighten, which can strip the threads in the cylinder head.
- Work on one spark plug at a time and reconnect the wire or coil before moving to the next one, so you do not mix up the firing order.
Locate the spark plugs under your hood
Open your hood and look for thick wires or rubber boots running from a central coil pack or distributor cap down into the engine. Each wire or boot connects to one spark plug. On newer cars (roughly 2000 and later), you will see a coil pack sitting directly on top of each plug. On older cars, wires run from a distributor or coil to each plug.
If your engine is crowded and you cannot see all the plugs easily, consult your owner's manual or a repair guide for your specific model. Some cars require you to remove other parts—an air intake, a heat shield, or a plastic cover—to reach certain plugs. This adds time but does not change the basic process.
Remove the wire or coil from the first spark plug
Grasp the wire or coil boot (the rubber connector) closest to the engine block, not the wire itself. Twist it gently side to side while pulling straight out. If it is stuck, rock it back and forth slowly; do not yank hard, or you may tear the boot or the connector inside.
Once the wire or coil is free, set it aside where you can see which plug it came from. If your car has multiple plugs, you will reconnect this wire to the same plug after you install the new one. Some people mark the wires with tape or a pen to avoid confusion, especially on cars with six or eight plugs.
Unscrew the old spark plug and install the new one
Insert your socket wrench over the spark plug and turn counterclockwise to loosen it. Once it is loose enough to turn by hand, unscrew it the rest of the way and pull it out. Look at the old plug for signs of wear or damage—a black coating, oil buildup, or a gap that is too wide—which confirms it was time to replace it.
Before you install the new plug, check the gap (the space between the center electrode and the side electrode). Most new plugs come pre-gapped to the correct distance for your engine, but it is worth confirming with your owner's manual. If the gap is wrong, use a spark plug gap tool to adjust it by bending the side electrode slightly.
Thread the new spark plug into the hole by hand first, turning clockwise. This prevents cross-threading, which happens when you start the plug at an angle and strip the threads in the cylinder head. Once it is hand-tight, use your socket wrench to tighten it snugly. Do not over-tighten; a firm snug fit is enough. Over-tightening can crack the ceramic insulator or strip the threads.
Reconnect the wire or coil and move to the next plug
Push the wire or coil boot back onto the new spark plug until you hear or feel it click into place. The connection should be firm and not wiggle when you tug on it gently. If it does not feel find, remove it and check for dirt or damage on the connector, clean it if needed, and try again.
Now repeat the entire process for the next spark plug. Work on one plug at a time rather than removing all the wires at once. This method keeps you from mixing up which wire goes where, which can cause your engine to misfire or not start.
What to do if something goes wrong
If a spark plug is stuck and will not unscrew, explore a penetrating oil like WD-40 around the base of the plug and wait 10 to 15 minutes. Try again gently; forcing it may break the plug inside the hole. If it still will not budge, stop and consult a repair manual for your vehicle or take it to a mechanic.
If you cross-thread a plug (you hear grinding or feel resistance before it is fully seated), stop when ready and back it out. Do not force it in. You may need a helicoil repair kit or professional help to fix the damaged threads.
If your engine does not start after you finish, or it runs rough, the most common cause is a wire or coil connected to the wrong plug. Double-check that each wire is firmly seated on its plug and that you did not accidentally swap two wires during the job.
When to replace spark plugs and what comes next
Most cars need new spark plugs every 30,000 to 100,000 miles, depending on the plug type and your vehicle's age. Copper plugs wear out faster (30,000 to 50,000 miles), while iridium or platinum plugs last longer (60,000 to 100,000 miles). Your owner's manual will specify the interval for your car.
After you install new plugs, your engine should start smoothly and run without hesitation or misfiring. If you notice rough idling, poor fuel economy, or difficulty starting within a few days of changing the plugs, one of the wires or coils may not be seated properly, or you may have installed the wrong plug type. Check the connections first, and if that does not solve it, verify that you bought the correct plugs for your vehicle.
Frequently Asked Questions
Do I have to replace all the spark plugs at once?
No, but most mechanics recommend replacing them all together if they are due. Plugs wear at roughly the same rate, so if one is worn, the others probably are too. Replacing them all at once saves you from having to open the hood again in a few months.
Can I reuse the old wires if I am just changing the plugs?
Yes, as long as the wires are not cracked, burned, or loose. If a wire is damaged or the connection is weak, replace it at the same time as the plug. A bad wire will cause misfiring even with a new plug.
What if my car has a coil pack instead of wires?
The process is the same. Remove the coil pack by unbolting it or unclipping it, pull out the old plug, install the new one, and bolt or clip the coil pack back down. Coil packs are more reliable than wires and rarely need replacement unless they fail.
How tight should I make the spark plug?
Tighten it until it is snug and will not wiggle, then stop. If you are using a socket wrench, a firm quarter-turn after hand-tight is usually enough. Over-tightening is a common mistake and can damage the plug or the cylinder head threads.
What does a spark plug gap mean?
The gap is the space between the center electrode (the thin rod in the middle) and the side electrode (the bent piece next to it). This gap is where the spark jumps to ignite the fuel. If the gap is too small, the spark may not form. If it is too large, the plug may not fire at all. Most new plugs come pre-gapped correctly for your engine.