Stratus heating

Hello everyone, I have a problem with the heater in my '97 Stratus. When I turn it on, hot air comes out at first, but after a minute it starts blowing cold air. The heater core isn't circulating water inside. It has a thermostat, and I recently replaced the water pump about 30,000 km ago. I would greatly appreciate any help.
Thank you for your attention.

Hi Robert, it's most likely that the heater core is clogged. You'll have to remove it and have it unclogged. Good luck and best regards.