How often should spark plugs be replaced?

It depends on the plug type: standard copper/nickel plugs every 20,000-30,000 miles, platinum plugs every 60,000 miles, iridium plugs every 80,000-100,000 miles. Always check your owner's manual as some modern engines with long-life plugs can go up to 100,000 miles.

What are the signs of worn spark plugs?

Symptoms include: rough idle, engine misfires, poor acceleration, increased fuel consumption, difficulty starting (especially in cold weather), and a check engine light. A misfiring engine can also damage the catalytic converter, which is much more expensive to replace.

What is the difference between spark plug types?

Standard copper/nickel plugs are cheapest (£2-£5 each) but wear fastest. Platinum plugs (£5-£10) last longer as platinum is harder than nickel. Iridium plugs (£8-£20) last longest as iridium is the hardest commonly used electrode material. Double iridium or iridium/platinum plugs offer the best longevity.

Should I upgrade to iridium spark plugs?

If your car was designed for standard plugs, iridium plugs can be a good upgrade. They last 3-4 times longer, provide a more consistent spark, and can slightly improve fuel economy. However, on very old or simple engines, the benefit may not justify the cost. Always use the type recommended by your manufacturer.

Can worn spark plugs damage my engine?

Worn plugs themselves rarely cause direct engine damage, but the symptoms they cause can. Persistent misfires send unburnt fuel into the exhaust, which can overheat and destroy the catalytic converter (a £500-£1,500 repair). They can also cause rough running that stresses engine mounts and other components.

CarCalcs

Spark Plug Calculator — Replacement Interval

Calculate spark plug replacement intervals based on plug type and usage.

Current Mileage

Last Plug Change (mileage)

Spark Plug Type

Number of Cylinders

Annual Mileage

Cost Per Plug

How We Calculate This

Replacement Intervals by Plug Type

Standard (copper/nickel): Every 20,000-30,000 miles
Platinum: Every 60,000 miles
Iridium: Every 80,000 miles
Double iridium / long-life: Every 100,000 miles

Cost Calculation

Total cost = Number of cylinders x Cost per plug

Labour adds £30-£100 depending on engine accessibility. Some engines (e.g., boxer/flat engines, V6/V8) require significantly more labour due to difficult access.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Actual figures may vary based on driving conditions, vehicle specifications, and other factors.