Hi there,
I have a 2003 Mercedes E500 that I just purchased. I was running errands this evening and had the heater on and it was working then all of a sudden it started blowing cold air. Does anyone have any idea what this could be???

I have only had the car 3 weeks and I am hoping that I am not going to start having all kinds of problems with this car.....

yes, the heater shut off valve has failed. it blocks the coolant flow to the heater core. They fail all the time. It is located under the wiper motor so you will have to remove the wiper motor and windsheild cowling to replace.