Last week sometime I noticed that while driving to work with the heat on, it did not seem to be getting warm in the car even though I had the fan on and the temp turned up and the commute is more than sufficent to warm the engine up. When I stopped the car at the office and let the car idle, the air from the vents got warm as normal?!? Heater control valve? Core going bad? No leaks or smells that I can detect and it was working up until a week ago....