I've a 2012 W204 C250cdi. The start/stop function is gradually becoming slower to engage, and lasts less time. It occasionally turns green after a mile or so, but more usually it takes 10+ miles. The time it will operate in stop mode has gradually decreased to between 10 and 60 seconds. I suspect it may be the auxiliary battery on its last legs, but no errors have been flagged. Am I looking in the right area, or are there other things to check. Thanks.