heater blows cold on 1997 Chevrolet Tahoe

i forgot i also change the heater control valve it worked great for a few days then started blowing cold again

Asked by for the 1997 Chevrolet Tahoe
check for full of radiator fluid (low fluid = no heat), if full may need new thermostat - you can check the old one by placing in freezer till cold and run under hot water and watch to see if it opens