1993 Sedan Deville Cadillac

On heater control panel, I push (auto) and it always indicates "econ". The blower is working fine, but there isn't any heat coming from heater duct.
I have had the coolant system flushed and new thermostat put in and I will get heat for a very short time (around the block) and then it stops.
What is wrong with this?
Thanks so much.