I bought a new Mercedes- Benz A180 in 2014 and had no problems for 12 months until the first service when a message appeared, "Add coolant, if on a slope". On a longer drive the temperature gauge would fluctuate quickly from normal to 90 degrees, then down to 75, then slowly back again, sometimes 20-30 minutes apart. I've been back to the dealer seven times, with four days off the road, for pressure checks, thermostat replacement etc. Now they report Benz has said it is OK and not to worry about it. This makes me think they don't know the cause.
Mercedes-Benz Australia spokesman David McCarthy says: "The issue has been, first, a coolant leak from the thermostat therefore displaying the low coolant level light on a slope. After this repair was completed the customer has started to watch his temperature gauge and found the operation different to what he has possibly experienced on other vehicles. The design of the cooling system has located the temperature sensor in an area where the coolant temperature does fluctuate as the thermostat operates. Stop-start operation can also influence this temperature variance because if the engine is not running no heat is generated. This is normal on the A-Class and has been documented from Germany for our dealers."