I've noticed recently several queries over non function of start stop. The answer given is usually low battery charge. However, I 'believe it is not as simple as that. As the owner of a Mercedes C class I have experienced this problem over the winter period and carried out a little research. On Mercedes in particular there are several things can cause this including coolant temperature too high or too low, outside temp, high current drain, low vacuum, seatbelt not fastened, and strangely, cabin temp not reaching the set temp, and probably the ngine management just having a "moment". Normally the system works (annoyingly) well in summer after a few minutes driving. All through winter it has been either non functional or intermittent. Sometimes when it is working it suddenly decides to start the engine while standing still which can be a little disconcerting.