engine light

I have a 99 El Dorado with 64 thousand miles on it, today the engine light went on, it also feels like it skips when I am stopped but when driving it still runs smooth, do I need a tune up already? if so what is the estimate
for a tune up on this year of car, it seems 4 of the plugs are under a cover and 4 show and are easy to get to.

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