Can only echo the comments about the fact that it’s most likely the battery being the reason why the stop start is not working. My 2016’s stop start didn’t work for months until I had to do an essential work-related 100 mile round trip journey and hey presto it started to work again. Also the 2016 X260 models do not have a secondary stop start battery as is found in the X250 models. The X260 XFs only have one battery.