If you've changed the coolant recently, it could be air locked in the heater core. To remove park the truck with the front higher than the back and remove the radiator cap. Start the engine and let it run while watching for a big air bubble burp coming out the radiator neck. Top off the coolant and continue.
On my 2002 Toyota Tundra, there are 2 temperature control cables. One controls the valve on the firewall to control the water coming into the heater core, the other controls the air gate on the heater blower unit. My cable came off of the air gate which would allow the valve to work fine but still not blow hot air into the cab of the truck. Find where is came off of the air gate and reconnect it and your heater should be fine.
There is a known issue with the heat control cable, behind the heater control panel. The post that the heater valve connects to, on the temperature control, can break off. To check this, have an assistant move the temperature control from cold to hot. See if the arm on the heater control valve, under the hood, moves. If it moves, are both hoses the same temperature, with the engine running? They shoud, in the HOT position, be so warm that you cannot hold your hand on them for more than a few seconds. If the hoses are lukewarm, and the same temperature as the top radiator hose, I would suspect your thermostat needs replaced. Hope this helps you.
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