Hey forum members! My son has a 2001 Honda Civic DX, (with 147K miles), that has a strange issue. It blows cold air through the heater while stopping at a light or idling. The engine temp is up to normal and when he's driving, it blows "hot" air just fine. I don't think the heater core would be plugged 'cause when the car is moving / when he's driving it, it works just great!

What can I check or do to fix this problem?

Thanks much!