About a year ago I filled up with some bad gas in my SLk 200. After spluttering its way to the Mercedes garage to be tested, they said it was ok and off I eventually drove. But the car has not been 100% since then, sometimes it shudders when idling and the fuel injection light has been on for awhile. Today my local garage tested it and the diagnostics showed up a couple of faults. I want to know how to turn the light off on the dashboard or is there an economical way of fixing this fault?