Bad Spark Plugs Symptoms: How to Tell It's Time
Worn spark plugs typically cause a rough idle, misfires and a check engine light, hard starting, sluggish acceleration and lower fuel economy. Modern iridium plugs usually last 60,000 to 120,000 miles; the interval is in your owner's manual.
7 signs your spark plugs are worn
- Rough or shaky idle
- Engine misfires, felt as stumbling or jerking
- Check engine light with codes P0300–P0308
- Hard starting, especially in cold or damp weather
- Hesitation or weak acceleration
- Fuel economy dropping without another explanation
- You have passed the replacement interval in the manual
How often spark plugs need replacing
| Plug type | Typical interval |
|---|---|
| Copper | 20,000–30,000 miles |
| Platinum | 60,000 miles |
| Iridium | 60,000–120,000 miles |
What a worn plug can damage
As the electrode wears, the gap widens and the ignition coil must produce more voltage to fire it. That extra load is a common reason coils fail. Persistent misfires can then overheat the catalytic converter. Replacing plugs on schedule is one of the cheapest ways to protect both.
Reading an old spark plug
- Light tan or gray deposits: normal wear
- Black, dry soot: running rich or a weak spark
- Oily deposits: oil getting into the cylinder
- White, blistered tip: running too hot, possibly lean
Frequently asked questions
Should I replace all spark plugs at once?
Yes. Plugs wear at similar rates, and replacing them as a set keeps the engine running evenly.
Can bad spark plugs cause a car not to start?
Severely worn or fouled plugs can make starting very hard, especially in cold weather.
Why is replacing plugs on a V6 so expensive?
On many V6 engines the intake manifold must come off to reach the rear plugs, which multiplies labor time.
Related repair costs
More guides
- How Long Does a Car Battery Last?
- Bad Alternator Symptoms: 7 Signs Your Alternator Is Failing
- Why Does My Car Shake When Braking?
- How Long Do Brakes Last?
Sources and methodology
Estimates combine typical book labor times, current aftermarket and OEM parts prices, and 2026 US shop labor rates. See exactly how we calculate them.
- U.S. Bureau of Labor Statistics: Occupational Employment and Wage Statistics, automotive service technicians (49-3023), by state
- U.S. Bureau of Labor Statistics: Automotive Service Technicians and Mechanics (pay and duties)
- Federal Trade Commission: Auto Repair Basics (estimates, warranties, your rights)
- NHTSA: Check for recalls before paying for a repair (our free recall check uses NHTSA data)
- ASE (National Institute for Automotive Service Excellence): technician certification