My van overheated the other day and i relized it was because i was out of anti-freeze so i filled it up with anti-freeze but my heater still doesn't work. I thought maybe my heater wasn't working because it was out of anti-freeze but then I filled it up and it still didn't work. Then today it worked on the way home but when i went back out to use my van again it didn't work.Why doesn't my heater work?