When to Replace Spark Plugs

Spark plugs are small, cheap, and easy to forget — until a misfire turns up. How long they last depends almost entirely on what they're made of. Here's the breakdown. Last updated: June 2026 · 7 min read.

Copper vs. Platinum vs. Iridium

The single biggest factor in plug life is the electrode material. Copper plugs conduct well and run cheap, but the soft electrode wears fast — expect 20,000 to 30,000 miles. Platinum plugs use a harder, heat-resistant electrode and typically last 60,000 to 80,000 miles. Iridium plugs — the standard fit on most modern engines — use an extremely durable fine-wire electrode and commonly go 80,000 to 120,000 miles, with a more consistent spark. Always match the exact plug type and heat range your manufacturer specifies.

Symptoms of Worn Spark Plugs

Rough idle and engine misfires — a weak, inconsistent spark causes a shaky idle and dumps unburned fuel into the exhaust
Hard starting or no start — fouled plugs may not generate enough spark to light the mixture, especially on cold mornings
Dropping fuel economy — a plug that misfires or burns inefficiently wastes fuel; a quiet MPG slide is a common sign
Sluggish acceleration and lost power — incomplete combustion means less power at the wheels and a doughy feel

What Wears Plugs Out Faster

Lots of short trips — frequent cold starts foul plugs with carbon before they get hot enough to burn it off
Oil or coolant leaking into the cylinder — a leaking valve seal or head gasket fouls the plug early
A too-rich fuel mixture — a faulty sensor or injector leaves sooty deposits that shorten plug life
Hard, high-RPM driving — sustained heat and pressure erode the electrode gap faster than gentle commuting

Should You Replace the Coils Too?

Spark plugs and ignition coils work as a team. If a coil has failed and caused a misfire, you'll usually replace it along with the plug. On high-mileage engines many people leave good coils alone, but if one dies of age the others often aren't far behind. Replacing plugs is the ideal moment to inspect coils and boots for cracks and oil contamination.

What Replacement Costs

The plugs themselves are cheap — roughly $4 to $15 each, with iridium at the higher end. On a simple four-cylinder with easy access, a shop job runs about $100 to $250. On a V6 or V8 where the intake manifold has to come off to reach the rear bank, labor can push the bill to $300-450. It's a satisfying DIY job on many engines with a torque wrench and the right gap.

Log the Plug Type and Mileage Once

Note which plugs you fitted and at what mileage in Velox Virtual Garage, and the right replacement window is waiting for you next time — no digging through receipts to remember if they were copper or iridium.

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