Spark plugs wear out and need replacement on a schedule
Spark plugs don't last forever. Over time, the electrode gap widens, the insulator cracks, or carbon builds up, and the plug stops firing reliably. When that happens, your engine misfires, fuel economy drops, and starting becomes harder. Replacing them is a straightforward maintenance task that most car owners can learn to do, though some vehicles make it easier than others.
How often you replace them depends on the plug type and your vehicle. Copper spark plugs typically last 25,000 to 30,000 miles. Iridium plugs can go 60,000 to 100,000 miles or more. Check your owner's manual for the interval your manufacturer recommends—it's printed there because the engine design affects how quickly plugs degrade. Some cars also have a maintenance schedule sticker under the hood.
You'll know it's time when the check engine light comes on, the engine hesitates under acceleration, or you notice rough idling. A mechanic can also inspect the plugs during a routine service and tell you whether they're worn.
Key Takeaways
- Copper spark plugs last roughly 25,000 to 30,000 miles; iridium plugs last 60,000 to 100,000 miles or longer, depending on your vehicle and driving conditions.
- Your owner's manual specifies the exact replacement interval and the correct plug type and gap setting for your engine.
- You can replace spark plugs yourself if you have basic tools and your engine layout allows straightforward access, though some vehicles require removing other components first.
- Buying the wrong plug type or gap setting will cause misfires and poor performance, so verify the specifications before you purchase.
- A mechanic can replace all plugs in 30 minutes to an hour, and the labor cost is usually less than the cost of the plugs themselves.
Finding the right replacement plug for your vehicle
The correct spark plug is not universal. Your engine needs a specific type, heat range, and electrode gap. Using the wrong one causes misfires and can damage the ignition system. The safest way to find the right plug is to look it up by your vehicle's year, make, model, and engine size.
Start with your owner's manual—it lists the plug type and gap. If you don't have the manual, you can find this information on the manufacturer's website, at an auto parts store (give them your vehicle details and they'll look it up), or on the engine bay sticker under the hood. Write down the part number and specifications before you go shopping.
When you buy the plugs, you'll see options at different price points. Copper plugs are cheapest but wear fastest. Iridium and platinum plugs cost more but last longer. For most drivers, iridium plugs offer the best value over time because you replace them less often. Buy the quantity your engine needs—most cars need four, six, or eight plugs depending on the number of cylinders.
Replacing spark plugs yourself versus hiring a mechanic
Whether you can replace your own plugs depends on your engine layout and your comfort level with tools. On some vehicles, the plugs sit on top of the engine under a cover you can remove in five minutes. On others, you have to remove the intake manifold, air filter box, or other components to reach them. Check a repair manual or video for your specific vehicle before you decide.
If you do it yourself, you'll need a spark plug socket (a deep socket with a rubber insert), a ratchet wrench, a gap tool to check the electrode spacing, and possibly a plug wire puller if your vehicle uses plug wires instead of coil packs. The job usually takes 30 minutes to an hour. The main risk is dropping a plug into the engine bay or cross-threading it into the cylinder head, both of which are fixable but frustrating.
If you take it to a mechanic, expect to pay for labor plus the cost of the plugs. Labor typically runs $75 to $150 depending on how hard the plugs are to reach. Many shops charge a flat rate for the job rather than hourly labor. Getting a quote before you commit lets you compare the cost of doing it yourself against paying someone else.
The step-by-step process for replacing spark plugs
If your plugs are accessible and you decide to replace them yourself, follow this order. First, let the engine cool completely—hot engines can burn you. Remove the cover or components blocking access to the plugs. Unplug one wire or coil pack by twisting gently and pulling straight out; don't yank on the wire itself.
Use the spark plug socket and ratchet to unscrew the old plug and lift it out. Check the gap on the new plug using a gap tool—the space between the electrodes should match your manual's specification. If it's off, bend the outer electrode slightly to adjust it. Screw the new plug in by hand first to avoid cross-threading, then tighten it snugly with the socket. Don't over-tighten; you'll crack the insulator.
Reconnect the wire or coil pack by pushing it straight down until it clicks. Repeat for each remaining plug. Start the engine and listen for any misfires or rough running. If it runs smoothly, you're done. If it misfires, stop and check that all connections are seated firmly and that you used the correct plug type.
Common mistakes that cause problems
The most frequent error is using the wrong plug type or gap. If the gap is too wide, the spark won't jump across. If it's too narrow, the plug fouls quickly. Always verify the specification in your manual before you buy.
Another common mistake is cross-threading the plug into the cylinder head. This happens when you screw it in at an angle instead of straight down. Always start by hand and feel for resistance; if it doesn't thread smoothly, back it out and try again. Forcing it will strip the threads and require a repair shop to fix.
Pulling on the plug wire instead of the boot (the rubber connector) can tear the wire inside, leaving you with a bad connection. Always grip the boot and twist gently before pulling. If you have coil packs instead of wires, the same rule applies—twist the pack gently and pull straight out.
Reconnecting the wires or coils in the wrong order causes the engine to misfire because the spark fires in the wrong cylinder at the wrong time. Take a photo of the original setup before you disconnect anything, or label each wire with tape so you know where it goes back.
Signs that your spark plugs need replacement soon
Before your plugs fail completely, your engine usually sends signals. A rough idle—the engine shaking or running unevenly when stopped—often means one or more plugs are fouling. Hesitation or stumbling when you accelerate, especially when the engine is cold, is another sign. Poor fuel economy that gets noticeably worse over time can also point to worn plugs.
The check engine light may come on if a plug misfires enough to trigger the oxygen sensor. This doesn't always mean the plug is bad—it could be a coil pack, fuel injector, or compression issue—but a mechanic can read the code and tell you what's happening. If you ignore these signs, the misfire can damage the catalytic converter, which is expensive to replace.
Frequently Asked Questions
How do I know if my spark plugs are actually bad?
A mechanic can remove and inspect them visually—worn plugs show a wide gap, heavy carbon buildup, or a cracked insulator. You can also have the ignition system tested with a scope to see if the plugs are firing correctly. If you're unsure, a shop can check for free or for a small diagnostic fee.
Can I replace just one spark plug, or do I have to do them all?
You can replace one if only one is bad, but mechanics usually recommend replacing all of them at the same time. They wear at roughly the same rate, so if one is failing, the others are close behind. Doing them all at once saves you labor costs later.
What's the difference between plug wires and coil packs?
Older vehicles use plug wires that carry the spark from the distributor to each plug. Newer vehicles use coil packs mounted directly on top of each plug. The replacement process is similar, but coil packs are more reliable and don't need replacing as often as wires do.
Will replacing my spark plugs improve my gas mileage?
If your plugs are worn and causing misfires, replacing them will restore fuel economy to normal. You won't see an improvement beyond what the engine was designed for, but you'll stop losing efficiency to bad combustion.
Is it safe to drive with a misfiring engine?
Short trips are usually fine, but extended driving with a misfire can overheat the catalytic converter and cause permanent damage. If you notice rough running or hesitation, have the plugs or ignition system checked soon rather than waiting until the next scheduled service.