That is BS that BioD set off limp home mode because the emissions were too low. It was probably the poor fuel quality that made the CEL go on, as well as putting the car into Limp Home Mode. Once you mixed it with diesel it diluted the poor bio quality. I do not see how low emissions would make a CEL come on. I could however see how poor fuel quality could make the CEL come on and once you added diesel it lessened the effects of the poor quality bio and made it acceptable quality.

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2005 SL65
1999 E300
1995 E300
1994 SL320
1988 560SL
1987 300TD
1982 300D
1955 300SL