A well-tuned engine delivers the best balance of power & fuel economy, & produces the lowest level of emissions. Modern engines compensate for worn parts to a degree, giving you the sense that everything is fine with your car. A routine tune-up will restore your car to its normal operating state, & contribute to the overall efficiency of the engine and emissions system. If the “check engine” or “service engine” light comes on, it is saying its time for a tune up.