Spark plugs ignite the air-fuel mixture in each engine cylinder. Modern iridium and platinum-tipped plugs last 60,000-100,000 miles, far longer than older copper plugs (30,000 miles).
Symptoms of failing spark plugs: rough idle, misfires (especially under load), reduced fuel economy, check engine light with misfire codes.
Replacement cost varies by vehicle: $80-$150 at a shop for easy-access plugs on a 4-cylinder, $400-$800 for hard-to-reach plugs on a V8 with coil-on-plug ignition.
Don't skip the recommended interval. Failed plugs can damage catalytic converters ($1,500-$2,500 replacement) if misfires go uncorrected.
Check the gap on replacement plugs. Pre-gapped plugs are usually correct but worth verifying with a gap gauge.