Most spark plugs last between 30,000 and 100,000 miles, depending on the type
The distance your spark plugs will run before needing replacement depends almost entirely on what kind you have. Copper spark plugs typically last 25,000 to 30,000 miles. Iridium spark plugs often go 60,000 to 100,000 miles or more. Platinum spark plugs fall somewhere in between, usually 30,000 to 100,000 miles depending on the brand and your engine.
Your vehicle's owner manual will tell you the exact interval for your car — this is the most reliable number to follow. If you have lost the manual, you can find it online by searching your vehicle's year, make, and model plus "owner manual PDF", or you can call your dealership with your vehicle identification number (VIN) and ask them directly.
The interval also varies by driving conditions. If you spend most of your time in stop-and-start city traffic, your plugs may wear faster than if you drive mostly on highways. Towing, extreme heat, or very cold climates can also shorten the life of your plugs.
Key Takeaways
- Check your owner manual for the replacement interval specific to your vehicle — this is more reliable than a general mileage range.
- Copper plugs need replacement sooner than iridium or platinum, so the type of plug in your car matters as much as the miles.
- Engine misfires, rough idle, difficulty starting, and reduced fuel economy are signs your plugs may be worn before the scheduled interval.
- You can visually inspect spark plugs yourself to see if they are fouled or heavily worn, though you do not need to remove them to know when to replace them.
Warning signs that spark plugs are failing before the scheduled interval
Even if you have not reached the mileage in your manual, certain symptoms mean your plugs are not firing correctly. The most common is a rough idle — the engine shakes or vibrates when you are stopped at a red light or in park. This happens because one or more cylinders are not igniting fuel properly.
An engine misfire while driving feels like a brief stutter or hesitation, as if the engine momentarily lost power. You may also notice the check engine light turning on. A misfire can be caused by several things, but worn spark plugs are a frequent culprit, especially if your vehicle has high mileage.
Other warning signs include difficulty starting the engine in the morning, a noticeable drop in fuel economy, or a lack of power when accelerating. If your vehicle is hard to start and you have not had the plugs checked in several years, they are a reasonable place to start troubleshooting.
How to visually inspect your spark plugs
You can remove and inspect your spark plugs yourself if you are comfortable working under the hood, though you do not need to do this to know when to replace them — your manual interval is your guide. If you do want to look, you will need a spark plug socket (a deep socket with a rubber insert), a ratchet, and a socket extension.
Locate the spark plugs by following the thick wires or coils on top of the engine — each one connects to a spark plug underneath. Remove one wire or coil by twisting gently, then use the spark plug socket to unscrew the plug. A healthy spark plug has a light tan or gray deposit on the tip. A fouled plug is covered in black soot or wet with oil, or the electrode is burned away and the gap is too wide.
If you see heavy black buildup, oil, or significant erosion on the electrodes, the plugs need replacement. If they look relatively clean and the gap (the space between the electrodes) is still tight, you can reinstall them and wait until the scheduled interval. Always reinstall the wire or coil in the same position you removed it from.
What happens if you ignore worn spark plugs
Driving on worn spark plugs does not cause when ready catastrophic failure, but it does damage your engine over time. Misfires mean unburned fuel enters the exhaust system and can damage the catalytic converter, which is an expensive repair — often $800 to $2,000 or more depending on your vehicle.
Worn plugs also reduce fuel economy noticeably. You may find yourself filling up more often without driving more miles. The engine has to work harder to compensate for weak ignition, which increases wear on other components and can eventually lead to more serious problems.
In some cases, a severely fouled plug can cause the engine to run so poorly that you cannot drive the vehicle safely. This is rare, but it is another reason to replace plugs at the interval in your manual rather than waiting for a crisis.
The difference between copper, platinum, and iridium plugs
Copper spark plugs are the least expensive and have the shortest lifespan. They work well in older vehicles and are still used in some modern engines. If your vehicle came with copper plugs from the factory, you can replace them with copper plugs of the same type.
Platinum spark plugs last longer than copper and cost more. They have a platinum disc welded to the center electrode, which resists erosion better. Many vehicles from the 1990s onward came with platinum plugs.
Iridium spark plugs last the longest and cost the most. They have an iridium tip on the center electrode and a platinum disc on the ground electrode. If your manual specifies iridium plugs, use iridium replacements — mixing types can cause misfires. You can usually upgrade from copper to platinum or iridium if you want longer intervals, but check your manual first to make sure the vehicle is compatible.
When to replace plugs at the scheduled interval versus waiting for symptoms
The safest approach is to follow your owner manual. Replacing plugs at the recommended interval prevents the misfires, rough idle, and catalytic converter damage that come with waiting until they fail. The cost of a spark plug replacement — typically $100 to $300 depending on how many plugs your engine has and how difficult they are to access — is far less than repairing a damaged catalytic converter.
If you are unsure whether your vehicle is due, check the maintenance records from your last service. Many repair shops note the mileage and date of the last plug replacement on your invoice. If you have owned the vehicle for several years and have no record of plug replacement, and your manual calls for replacement every 30,000 to 50,000 miles, you are likely overdue.
If you are experiencing symptoms like rough idle or misfires and your vehicle is near the scheduled interval anyway, replacing the plugs is a reasonable next step. If your vehicle is running smoothly and you have not reached the mileage in your manual, you can wait.
Frequently Asked Questions
Can I replace spark plugs myself, or do I need a mechanic?
It depends on your vehicle and your comfort level. On many cars, spark plugs are straightforward to access and removal takes 30 minutes to an hour. On others, the engine design makes them difficult to reach and you may need to remove other components first. If you have never done it, a mechanic is the safer choice — the cost of labor is worth avoiding a mistake that damages the engine.
What does it mean if my check engine light is on?
A check engine light can mean many things, including worn spark plugs, a loose gas cap, a faulty oxygen sensor, or a catalytic converter problem. You can read the code with a diagnostic scanner (many auto parts stores will scan for free) to narrow down the cause. If the code points to a misfire, spark plugs are a likely culprit.
Do I have to use the exact brand of spark plug that came in my car?
No. You need to match the type (copper, platinum, or iridium) and the heat range, which your manual specifies. Different brands of the same type and heat range will work fine. Your mechanic or auto parts store can help you find the correct replacement if you provide your vehicle's year, make, and model.
Why do some cars need spark plugs more often than others?
Engine design, fuel type, and the type of spark plug all affect lifespan. Older engines and those that run on lower-octane fuel may need plugs more often. Copper plugs wear faster than iridium. Some high-performance engines are also tuned to run hotter, which shortens plug life.
Can bad spark plugs damage my engine permanently?
Worn spark plugs themselves do not usually cause permanent damage, but the misfires they cause can. Unburned fuel in the exhaust damages the catalytic converter over time, and the extra strain on the engine increases wear on pistons and valves. This is why replacing plugs at the scheduled interval is cheaper than ignoring them.