heater

Super late to the party here but I'm experiencing the same issue on my 2001 Silverado 1500 LS 4WD. G A's answer above is solid. Lower radiator hose is straight-up cold while the upper radiator hose is really hot. Same for the two hoses going into the firewall (I believe to the heater core). One of those is really hot, the other is barely warm. Engine temp runs normal all the time. I've been told that there is a high probability that the thermostat is stuck in the open position. I will be replacing the thermostat this morning (Dec 22, 2019... First official shortest day of the year). It's the easiest and cheapest fix. If that does not solve the issue I may have to consider the actuator blend door or worse... the heater core. If the heat does not work after the thermostat replacement, I've been told to try using the garden hose to clean out the two lines that go into the firewall (to the heater core) as G A above suggests. I've watched enough vids to hope that it is the thermostat. The blend door requires taking the dash apart for access. The heater core looks to require even more in-depth dash removal. Neither look too daunting other than the time and patience that is needed.