The ECO stop start function on my two years old SL350 AMG Sports has a mind of its own. Some days it becomes operative after a journey of a mile but on others I can drive five or six miles and I have driven ten or more miles without it activating. Even when it has activated it's just as likely to de-activate without warning. For instance it will often just start the engine when stopped at traffic lights. I know several parameters have to be met before it will work but I haven't identified any set of circumstances which might affect it although it seems to activate more promptly after it's been parked up for a short time after being driven for only a few miles. The dealership agree there's a problem and eventually fitted a new battery under warranty which seemed to improve the situation marginally but now it's as bad as ever it was.