Ignition
Spark plugs, coils and the intervals nobody can agree on.
The ignition system turns a low-voltage signal into an arc strong enough to light a compressed fuel charge, once per cylinder per two crankshaft revolutions. When it degrades it rarely fails outright — it fails intermittently, under load, and it shows up as a misfire, a rough idle or hesitation long before it becomes a no-start.
This is also the maintenance area with the least reliable public information. The copper-30,000, platinum-60,000, iridium-100,000 table that appears on hundreds of sites is not published by any spark plug manufacturer. NGK publishes no mileage figure at all, and DENSO's global platinum figure is 100,000 kilometres — about 62,000 miles, not 100,000. Sourced vehicle schedules run from 45,000 miles to 120,000 miles within a single model year.
For the cost side of the same job — why quotes vary from $115 on an accessible four-cylinder to $700 on a V6 that needs the intake off — see spark plug replacement cost.