When to Change Spark Plugs
The mileage interval on the box is a starting point, not a deadline. Here is how to actually know when your plugs need changing.
Change Intervals by Plug Type
Copper
30,000 miles
$3-$5 each per plug
Older vehicles, high-performance applications with special requirements. Shortest lifespan but cheapest to replace.
Platinum
60,000 miles
$7-$12 each per plug
Standard on most vehicles made from the mid-1990s onwards. Double platinum plugs (electrode on both ends) last even longer.
Iridium
100,000 miles
$12-$20 each per plug
Factory fitment on most modern cars. Much harder electrode material. Do not use a coin-style gap tool on them - it damages the fine tip.
Check the sticker under your hood or your owner manual for the exact spec. The plug type in the car from the factory is usually the right choice to stick with.
Change Them Early If You Notice These
Rough idle
The engine should be smooth at a stoplight. If it shakes or vibrates, a misfiring plug is the most common cause. Even one bad plug in a 4-cylinder affects the whole engine.
Poor fuel economy
Worn plugs do not combust fuel as efficiently. Fuel economy drops noticeably before other symptoms appear. If your MPG has slipped 10-20%, plugs are worth checking.
Hard starting
Longer cranking before the engine catches. Common in cold weather when spark plugs that are marginal in normal conditions cannot produce enough spark.
Engine misfires
Feels like a brief stumble under acceleration or a jerky idle. Your check engine light may come on with a P030X code (where X is the cylinder number).
Check engine light
P0300 (random misfire), P0301-P0308 (cylinder-specific misfire). These codes do not always mean plugs - coils and wires can cause the same codes - but plugs are the cheapest thing to check first.
Emissions failure
Misfires send unburned fuel into the exhaust, failing emissions tests. New plugs sometimes fix a borderline failure. Worth trying before replacing more expensive components.
Visual Inspection: Pull One Out
You only need a spark plug socket to do this. Pull one plug out - ideally from the cylinder you suspect - and look at the electrode end.
Tan / light brown
Normal combustion. Plug is healthy.
Black and dry / sooty
Rich fuel mixture or excessive idling. Engine is burning too much fuel.
Black and wet / oily
Oil burning in the combustion chamber. Indicates a more serious engine issue.
White or chalky deposits
Running lean or overheating. Could indicate a coolant leak or wrong heat range.
Worn / rounded electrode
Plug is past its service life even if it looks otherwise clean. Replace it.
Bottom line
Follow the manufacturer interval unless symptoms appear first. If you see any of the warning signs above before you hit the mileage threshold, change the plugs and inspect the coils at the same time. The plugs are the cheapest thing to rule out.