Fill it with premium fuel and disconnect the negative battery terminal for a few seconds. The check engine lite will go out and if it was the fuel issue then the check lite should not come back on. It will come on if something else triggered it after which time you will have to dig a little deeper into the diagnostics. I am not sure if this is the same system as my E320 - you will have to get some additional input on that. Make SURE you have your radio code prior to disconnecting that battery - you will need it!!
The check engine light only comes on when there is an issue with the emissions system.