Quick answer: Use the plug type and interval in your owner’s manual. Many modern engines use iridium or platinum plugs rated for roughly 60,000–120,000 miles. Install them in a cool engine at the specified gap and torque.
Signs of worn plugs
- Rough idle, hesitation, or stumbling when you accelerate.
- A flashing or steady check engine light with misfire codes. P0300 means random misfires, and P0301–P0308 point to a specific cylinder.
- Harder starting and lower fuel economy.
Misfires can also come from ignition coils, injectors, vacuum leaks, or low compression. If one cylinder keeps misfiring, a common test is to swap that coil with one from another cylinder and see if the misfire moves with it.
Plug types
| Type | Typical use | Notes |
|---|---|---|
| Copper / nickel | Older engines, some performance applications | Cheapest, with the shortest replacement interval |
| Platinum / double platinum | Many 1990s–2000s engines | Longer life. Double platinum is often required for waste-spark ignition |
| Iridium | Most modern engines | Longest life, with a fine center electrode |
Don't "upgrade" against the spec without checking. Use the type and heat range the engine maker calls for.
Installing them right
- Work on a cool engine, especially with aluminum cylinder heads.
- Blow debris away from the plug wells before removing plugs so nothing falls into the cylinder.
- Gap: many plugs come pre-gapped. Verify against your spec with a wire-style gauge, and avoid bending fine-wire iridium or platinum tips.
- Start threads by hand to avoid cross-threading. A length of rubber hose on the plug works well.
- Torque to spec, or follow the plug maker's turn-angle instructions. Over-tightening can damage threads, and under-tightening can cause overheating.
- Many plug makers say their plated plugs don't need anti-seize, and anti-seize changes the effective torque. Follow the plug maker's guidance.
- A small amount of dielectric grease inside the coil boot helps it seal and come off easily next time.
What old plugs tell you
Light tan or gray deposits are normal. Black, dry soot can mean running rich. Oily deposits can mean oil getting into the cylinder. White, blistered tips can mean overheating or the wrong heat range. Bring an odd-looking plug to a pro if you're unsure.
FAQ
How often should spark plugs be changed?
Follow your owner’s manual. Many modern iridium plugs are rated for about 60,000–120,000 miles; older copper plugs may need changing much sooner.
Do I need to gap new spark plugs?
Always check the gap against your engine’s spec, even with pre-gapped plugs, but be careful not to damage fine-wire tips.
Should I replace all spark plugs at once?
Yes, replace them as a full set so every cylinder fires consistently.
References
This guide is general information, not professional repair advice. Always follow your vehicle’s owner’s manual and service specs, and use proper safety equipment.