Reasons for Far left dashboard light on?

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I don't have my Manuel, but the light on far left stays on. I think it said something somewhere about "gas valves?". Anyway, I took it to Autozone for a free Diagnostics test, and just my luck, the light went OFF on the way there. They tested it anyway, saying they may not be able to determine the problem since the light is not on. They finally said I needed Spark plugs! Well, I'm sure I do, but now the light is on 100% of the time. Should I take it back to Autozone ? I can start from scratch and even go to a different location. Thank you ...