Spark Plugs
What it is
Spark plugs screw into the top of each cylinder and produce the spark that ignites the air-fuel mixture. A four-cylinder engine has four; many V6/V8s have one per cylinder (some have two).
What it does
Each plug fires precisely many times per second to start combustion. As the electrode wears and the gap grows, the spark gets weaker — causing misfires, rough running and poor economy. Worn plugs are the single most common cause of misfire codes (P0300–P0304).
Symptoms of failure
- Misfire codes P0300–P0308
- Rough idle, shaking or hesitation
- Hard starting
- Reduced fuel economy and power
- Flashing check engine light (active misfire)
Common fault codes
Which vehicles need it
Every petrol engine. Iridium and platinum plugs last far longer than old copper plugs.
Replacement cost
| DIY (part only) | $15–$80 |
|---|---|
| At a shop (parts + labor) | $120–$400 |
| Replacement interval | Copper ~30,000 miles; platinum ~60,000 miles; iridium ~90,000–100,000 miles (check your manual). |
| DIY difficulty | Easy–Moderate (DIY) — a spark-plug socket, gap gauge and torque wrench; access varies by engine |
| Recommended brands | NGK, Denso, Bosch, ACDelco |
Where to buy the part
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Frequently asked questions
How often should I change spark plugs?
It depends on the plug type: copper around 30,000 miles, platinum around 60,000, and iridium 90,000–100,000. Always follow the interval in your owner's manual.
Should I replace all spark plugs at once?
Yes. They wear at a similar rate, so replacing them as a set keeps the engine smooth and avoids a return visit when the next old plug fails.
Can worn spark plugs cause a misfire code?
Absolutely — worn or wrongly gapped plugs are the most common cause of P0300-series misfire codes. They are also the cheapest thing to check first.