check engin light on

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Hey guys I bought this ms6 from a major dealer, they told me it had ran lean before they fixed it. Now 2 weeks later the check engine light is on and I assume its cause its running lean. What can cause it? How do I find out? The car is bought as is so I can't go back to the dealer. The car is running great still, there has been no change in its driving performance. Please help!