andorra

I have a 2004 E500 4matic wagon. I had a pinhole leak on the seam of the coolant reservoir so I replaced it. If I drive to the store etc., park and come out later (with the engine still warm) and begin driving, I get the "Coolant! visit workshop" warning. The coolant is fine, no leaks and no level fluctuation. I think it might be the level sensor inside the reservoir. Anyone have a similar issue? how to fix?