Why the interval varies so much
Spark plugs wear slowly as the electrode erodes with every spark. Harder metals like iridium and platinum erode far more slowly than copper, which is why modern engines can go so long between changes. Engine design matters too: some turbocharged engines are harder on plugs.
Always replace with the type the manual specifies. A cheaper plug may fit but won’t last the scheduled interval.
Signs your spark plugs need attention
- Rough idle or the engine shaking at a stop
- Misfires, hesitation or poor acceleration
- Harder starting than usual
- Worse fuel economy
- The check engine light — a flashing check engine light usually means a severe misfire; ease off and get it checked promptly
These symptoms can also come from ignition coils, fuel delivery or sensors, so a diagnosis beats guessing.
Why some spark plug jobs cost more
On many four-cylinder engines the plugs are easy to reach. On some V6 engines the rear plugs sit under the intake manifold, which has to come off — so the job takes much longer. Ask for a quote that says how many hours of labor are included.
Spark plugs are often scheduled alongside other long-interval items. See what the 100K mile service usually includes.
Common questions
Can I change spark plugs early?
You can, but there’s little benefit if the engine runs well and the plugs aren’t due. Follow the schedule unless there’s a symptom.
Should I replace the ignition coils too?
Not automatically. Coils are replaced when they fail, though a shop may suggest it if they’re old and the labor overlaps.
What if I don’t know when they were last changed?
Treat it as unknown rather than assuming. On a used car near the interval, replacing them gives you a known starting point — see used cars without records.