What are the symptoms of bad spark plugs?

Spark plugs are vital components for gas-powered engines. However, they have a limited lifespan. Here are some common signs that it's time to replace yours.
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I bought a certified pre-owned car, and a week later the check engine light came on. Is the dealer responsible?

So, you finally did it. After weeks of researching, test-driving, and maybe even a little […] ...