Engine management light fiesta 53 Zetec. Came on last night took to garage today. Got charged lots for something I've never heard of supposed to regulate the petrol flow high or low octane. Drove home perfect. Left for an hour got back in light on again? Did I read somewhere that these lights are prone to be faulted? Garage say no bother bring it back and drive it. But do not drive if it flashes? Anyone any ideas?