Spark plugs are small, but a worn one can quietly wreck an engine's performance long before the check engine light comes on. Swift's Automotive handles spark plug jobs across Southeast Texas and sees the same pattern again and again — a rough idle here, a few extra trips to the gas station there — until it adds up to real damage. Here are the five clearest signs it's time for a replacement, what happens if it's put off too long, and how the job gets done on-site.