Spark plugs are small, cheap and easy to ignore right up until they cause a misfire, a rough idle or a check-engine light. They light the fire that runs your engine thousands of times a minute, and worn plugs drag down power, smoothness and fuel economy.
What They Do
Each spark plug delivers a precisely timed spark that ignites the fuel-and-air mixture in a cylinder. The tiny gap at the tip is where that spark jumps, and over tens of thousands of miles it erodes, widening until the spark gets weak or inconsistent. A weak spark means incomplete combustion, which you feel as lost power and waste as fuel.
Signs They're Worn
- Rough idle or misfires: the engine stumbles or shakes, sometimes with a flashing check-engine light.
- Hard starting: a weak spark makes the engine reluctant to fire up.
- Poor economy and sluggish acceleration: combustion isn't as complete as it should be.
Replacement Intervals
It depends heavily on the plug type. Older copper plugs may need changing around 30,000 miles, while modern platinum and iridium plugs often last 60,000 to 100,000 miles or more. The owner's manual gives the figure for your engine. Use the exact plug type and heat range specified, and set the gap correctly if the plugs don't come pre-gapped.
A Manageable DIY Job
On many engines, spark plugs are accessible and the job is satisfying: remove the coil or wire, unscrew the old plug, and thread in the new one carefully so you don't cross-thread the aluminum head, then torque it to spec. On engines where the plugs sit deep or under the intake, a shop may be the easier route. Either way, fresh plugs restore smooth, efficient running, often for a surprisingly small parts cost.