When to replace spark plugs and signs they're worn
Spark plugs are easy to forget. They sit out of sight and slowly wear out. When they finally get bad, you feel it: hard starts, rough idle, weak acceleration. Here is how to know when it is time to change them, and what signs to watch for.
Start with your owner's manual
There is no single mileage that fits every car. NGK says there is "no single answer" to how often plugs should be replaced. The best guide is the interval your vehicle maker sets. You will find it in the maintenance schedule in your owner's manual.
Intervals vary a lot between vehicles. NGK's own service interval list runs from 25,000 miles on some vehicles to 75,000 miles on others. Even vehicles with the same type of plug can have different intervals. Some vehicles also have a shorter interval for "severe service," so check which schedule in your manual fits how you drive. That is why the manual matters more than any rule of thumb.
Typical ranges by plug type
If you want a rough idea, the plug type gives a clue. These are typical figures from the plug makers, not promises for your engine:
- Copper (nickel) plugs – Champion says most copper plugs need changing about every 20,000 miles.
- Performance iridium plugs – NGK says its Iridium IX plugs are designed to last 40,000 to 50,000 miles.
- Original-equipment long-life iridium plugs – NGK says its Laser Iridium (OE-style) plugs are typically designed to last 80,000 to 100,000 miles.
Driving habits, engine condition and fuel all affect real plug life. When in doubt, follow the manual.
Why plugs wear out
Every spark takes a tiny bit of metal from the electrodes. Over time, the gap between them gets wider. NGK explains that a wider gap needs more voltage to make a spark. Eventually the ignition system can't keep up, some fuel doesn't burn, and the engine starts to suffer.
Iridium and platinum tips wear more slowly than nickel, which is why long-life plugs can go longer between changes.
Signs your spark plugs are worn
Denso and NGK both list symptoms that can point to worn or fouled plugs:
- Hard starting – the engine cranks longer than usual before it catches.
- Rough or unstable idle – the engine shakes or the idle speed wanders.
- Hesitation when you accelerate – a stumble or sag when you press the pedal.
- Misfires and lost power – the engine feels weak or uneven.
- Worse fuel economy – unburned fuel means fewer miles per gallon.
These symptoms don't always mean bad plugs. Worn ignition parts, wrong ignition timing and other faults can cause them too. If your plugs are near or past their interval, though, they are a good place to start.
What the old plug can tell you
When you pull a plug out, look at the firing end. NGK says a brown or light gray tip generally means the plug has been working well. Heavy black deposits can point to a rich fuel mixture, a vacuum leak or low compression. NGK says a melted electrode is caused by overheating. A broken insulator means the plug is damaged. In either case, replace the plug and have the cause checked.
Denso also notes that rounded, worn electrodes won't work like new again just by re-gapping, so replacing the plug is the better choice.
Tips for replacing spark plugs
- Match the plug exactly. Use the number on your old plug or the one in your owner's manual.
- Don't downgrade. If your engine came with iridium or platinum plugs, don't switch to copper.
- Check the gap. NGK notes the factory gap may not be the right gap for your engine. Compare it to your manual.
- Torque to spec. NGK says to follow the manufacturer's torque spec. Too loose or too tight can both cause problems.
- Skip the anti-seize on NGK plugs. NGK says not to use anti-seize or lubricant on its plugs, because it can change the torque by up to 20 percent.
Find the right plug for your engine
We list spark plugs from ACDelco, Champion, NGK, Denso and Motorcraft by exact part number, in single plugs and multi-packs. Check the number on your old part or your owner's manual before you order.
Find your part: search spark plugs by part number.