Copper plugs need changing every 20,000-30,000 miles (32,000-48,000 km), while iridium or platinum plugs common in modern cars last 60,000-100,000 miles (100,000-160,000 km) — check your owner's manual, since intervals vary a lot by engine. Warning signs include rough idle, hesitation on acceleration, poor fuel economy, and a check-engine light with a misfire code (P0300-P0308). DIY replacement costs $20-$100 for plugs; a shop job with labor runs $100-$300 ($60-£220), more on engines where the rear plugs sit under the intake manifold.
At a glance
| Plug type | Typical interval |
|---|---|
| Copper | 20,000-30,000 miles (32,000-48,000 km) |
| Platinum | 60,000-80,000 miles (96,000-128,000 km) |
| Iridium / double platinum | 80,000-100,000+ miles (128,000-160,000+ km) |
| DIY parts cost | $20-$100 (£15-£75) for a full set |
| Shop replacement | $100-$300 (£70-£220) parts + labor |
Why spark plugs wear out
Every ignition cycle erodes the metal electrode slightly and the gap widens over time. A wider gap needs more voltage to jump, which stresses the ignition coil and eventually causes misfires. Iridium and platinum plugs resist this erosion far longer than old-style copper plugs because the harder metal wears more slowly, which is why manufacturers now spec them at 60,000+ mile intervals as standard equipment.
Signs your spark plugs need replacing
- Rough or uneven idle, especially noticeable at stoplights
- Hesitation, sluggishness, or a flat spot when accelerating
- Noticeably worse fuel economy over a tank or two
- Engine misfires — a stumble you can feel, sometimes with a check-engine light
- Hard starting, particularly in cold or damp weather
How to check your interval
The single best source is your owner's manual or manufacturer maintenance schedule — it specifies the exact plug type and mileage interval your engine was designed around. Aftermarket "lifetime" claims on some iridium plugs are optimistic; most techs still recommend inspecting by 100,000 miles even if the plug looks intact, since the gap can widen beyond spec before it visibly misfires.
DIY replacement basics
- Buy the exact plug type and gap specified for your engine — don't substitute a hotter or colder heat range.
- Let the engine cool completely to avoid cross-threading a hot aluminum cylinder head.
- Remove the coil pack or spark plug wire from one cylinder at a time to avoid mixing up firing order.
- Use a spark plug socket with a rubber insert to avoid dropping the plug into the well.
- Hand-thread the new plug fully before using a wrench, then torque to the spec in your manual — overtightening can crack the head or strip threads.
- Reconnect the coil/wire and repeat one cylinder at a time.
Never use anti-seize compound on plugs that already come with a factory coating (most modern plugs do) — it can throw off torque readings and cause overtightening. Check the plug packaging first.
Cost and where it varies
V6 and V8 engines with rear cylinders tucked under the intake manifold can add 1-3 hours of labor versus a simple inline-4, which is why quotes vary so widely. In the UK and India, iridium plugs for common family cars typically run £8-£15 or ₹300-₹800 each; labor-only jobs at independent garages are usually cheaper than main dealers for this task since it rarely needs manufacturer diagnostic tools.
Frequently asked questions
How often should spark plugs be replaced?
What happens if you don't change spark plugs?
Can I replace spark plugs myself?
Do new spark plugs improve gas mileage?
What's the difference between iridium and platinum plugs?
Sources & further reading
- U.S. Department of Energy — FuelEconomy.gov Maintenance Tips
- NGK Spark Plugs — Technical Resource Library
Figures, prices and policy details were current at the last-updated date above. Automotive pricing, incentives and regulations change frequently — verify time-sensitive details with the linked primary sources. Read our editorial policy and fact-checking standards.
