Our furnace was pushing out cold air, my husband replaced the thermocouple and now the furnace will lite, then about 20 seconds later the light will go out and the whole furnace will stop working. It is a Coleman forced heat. Please give us some help

When the pilot lites, is it a strong flame that you can hear burn or is it a lazy yellow flame? When the main burners lite is the pilot flame going out or moving away from the thermocouple? Give us moore info and we will try to help.

Roof jack (Flue pipe) could be partially blocked causing the flame to extinguish itself.