Stratus heating

Hello everyone, I have a problem with the heating of my Stratus 97 it makes it and at the beginning the hot air comes out but after a minute the cold air begins to come out the nucleus of the heating does not pull water inside, it brings thermostat and I have little to change the water pump approx. About 30,000km to those who can help me would
thank you very much for your attention

Hi Robert, most likely the heater radiator is covered, you will have to disassemble and send it to uncover, luck and greetings.