Symptoms
- No record of spark plug replacement at the recommended interval
- Rough idle or occasional misfire feeling
- Reduced fuel economy or sluggish acceleration
- Check engine light with a misfire code
Likely causes
- CommonInterval simply overdueModern iridium/platinum plugs can go 60,000–100,000 miles, but copper plugs need changing much sooner, and people often assume the longer interval applies.
- CommonCarbon buildup or foulingShort trips and stop-and-go driving can foul plugs faster than highway driving.
- PossibleIgnition coil issue mimicking worn plugsA failing coil can cause similar misfire symptoms even with healthy plugs.
What to do
- Check your manual for the exact spark plug type and replacement interval for your engine.
- Note any symptoms like rough idle, hesitation, or reduced fuel economy that suggest plugs may already be affecting performance.
- If a check engine light is on, get the code read first; a misfire code (P0300 series) supports plugs being the cause but isn't definitive on its own.
- Have the plugs replaced with the exact type and heat range specified, since substituting the wrong plug can cause performance or engine issues.
- Ask that the ignition coils be inspected at the same time, since a failing coil can look like a plug problem.
- Note the mileage of the new plugs so you know the next interval.
Tools and parts
- Spark plug socket and ratchet
- Gap gauge (if not pre-gapped)
Spark plugs specified for your engineSearch: “spark plugs for [your car year make model]”
When to call a pro
If misfires continue after new plugs, have ignition coils and fuel injectors checked by a mechanic.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.