Spark plugs are out of sight and out of mind — which is exactly why most people run them 20,000 miles past their replacement interval. By the time you feel a misfire or rough idle, you've been wasting fuel and stressing your catalytic converter for months.
Replacement intervals
- Copper plugs: 20,000-30,000 miles. Cheap but wear fast. Common in older cars.
- Platinum plugs: 60,000 miles. Standard on most 2000s-era vehicles.
- Iridium plugs: 80,000-100,000 miles. Most modern cars come with these.
- Double platinum/iridium: up to 100,000+ miles on some engines.
Check your owner's manual — the interval varies by engine, not just plug type.
Signs they're overdue
- Rough idle — the engine shakes or stumbles at a stoplight
- Hard cold starts — takes an extra crank or two to fire up
- Misfires — a flashing check engine light or a rhythmic stumble under acceleration
- Worse fuel economy — a worn plug needs more voltage and burns fuel less completely
- Sluggish acceleration — the engine feels flat or hesitates when you step on it
Why it matters
Worn plugs don't just cause rough running. Incomplete combustion sends unburned fuel into the catalytic converter, which overheats it. A set of iridium plugs costs $30-60. A catalytic converter costs $800-2,500. The math is straightforward.
Quick action
Look up your car's spark plug interval in the owner's manual and check it against your current mileage. If you're within 10,000 miles of the interval — or past it — put plugs on your next service list. If you're handy with a ratchet, most 4-cylinder engines are a 30-minute DIY job.
