E60 2007 Pre lci M/S mode problem

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Hi all BMW sifus, recently today While on the road, after switching to m/s mode, normally the 'D' will change to 'DS', and there will be a small light at the m/s mode near the gear but this time it just stays at D and there is no light, the revs and gear changes is as though it's still In normal D mode as well. I'm just wondering what is the cause of this problem and will it be expensive to fix?

Thank you everyone for any information given.