How often should I change my spark plugs?
It depends entirely on the plug material. Iridium: 60,000–100,000 miles (100,000–160,000 km). Platinum or double platinum: around 60,000 miles (100,000 km). Copper or nickel: 20,000–30,000 miles (30,000–50,000 km). Your manual names the type the engine was built for.
The material is the whole answer
A spark plug wears by erosion: every spark takes a few atoms off the electrode, the gap widens, and the coil has to work harder to jump it. Harder metals erode more slowly, and that is the only reason iridium plugs last three times as long as copper ones.
So the useful question is not "how many miles" but "which plug is in there". Fitting long-life plugs to an engine specified for copper does not always work — some engines want the heat range and conductivity of a softer plug, and older distributor ignition systems in particular are happier with them. Fit what the manual specifies and use its interval.
Turbocharged and highly boosted engines are the exception in the other direction: cylinder pressure erodes electrodes faster, and many manufacturers halve the interval for the same plug in a boosted application. Some performance engines call for 30,000 miles on iridium.
What shortens the interval
- Turbocharging, supercharging, or a tune that raises boost or timing
- An engine that burns oil or coolant — deposits foul plugs long before the electrode wears
- A rich-running fault, which sooties the plug and leaves black deposits
- Consistently short trips, which never burn the plug clean
- Poor fuel quality, or long storage with fuel in the tank
How to tell it is due
Unlike most items on this list, spark plugs do give clear warnings, though usually well after the ideal replacement point:
- A rough or lumpy idle, or a stumble that comes and goes
- Hesitation or a flat spot under acceleration, especially uphill or under load
- Harder starting, particularly when cold
- Fuel economy drifting worse over a few thousand miles with no other explanation
- A misfire fault code (P0300 for a random misfire, P0301–P0308 for a specific cylinder) or a flashing check-engine light, which means unburnt fuel is reaching the catalytic converter — that one is expensive and needs attention now, not next month
Two cautions before you start
Removing plugs from a hot aluminium head can pull the threads out of it. Work on a cold engine. Blow or brush debris out of the plug wells before pulling the plugs, so nothing falls into the cylinder.
Torque matters more than people expect — a plug that is under-tightened does not transfer heat into the head, and one that is over-tightened distorts and can seize. Use the figure in the manual and a torque wrench, and pre-gap plugs only if the manufacturer says to; most modern plugs come gapped and the fine electrodes bend easily.