Warning Signs
- Rough idle — engine shakes or vibrates at standstill.
- Poor acceleration — hesitation or stumbling when you press the throttle.
- Increased fuel consumption — worn plugs cause incomplete combustion.
- Engine misfires — noticeable jerking or engine management light.
- Difficulty starting — especially in cold weather.
- Failed emissions test — incomplete combustion raises exhaust emissions.
Recommended Intervals
| Plug Type | Change Interval |
|---|---|
| Copper/Nickel | Every 20,000–30,000 miles |
| Platinum | Every 40,000–60,000 miles |
| Iridium | Every 60,000–80,000 miles |
| Double Iridium | Every 80,000–100,000 miles |
Always check your handbook. Some manufacturers specify shorter intervals than the plug type would suggest, due to engine-specific factors.
Frequently Asked Questions
Do spark plugs wear out gradually or suddenly?
Gradually. Performance deteriorates slowly over thousands of miles, which is why you might not notice until the plugs are severely worn.
Can worn spark plugs damage my engine?
Persistent misfires from worn plugs can damage the catalytic converter (£500+). It’s much cheaper to change plugs on schedule.
Do diesel cars have spark plugs?
No — diesel engines use glow plugs instead. Glow plugs heat the combustion chamber to aid cold starting but don’t produce a spark.
