2000 gmc sierra 5.3l heater does not get hot

It's easy to flush your heater core. I use a garden hose. My same truck as yours, lost heat in the cab. I used the garden hose trick, and flushed the problem away. It's as easy as removing both heater hoses from the heater core, and using an old garden hose that I cut the end off. Luckily, it slipped right onto the heater core inlet. Hooked the garden hose to the water spigot and turned it on. Let it run for a couple minutes. The plug should wash away.

when replacing coolant, did you bleed the air from system! you can remove radiator or overflow cap when cold. start vehicle and let it run until thermostat opens,when all air is out hoses should get hot. but before starting, turn heater on to high setting, if you vehicle does not have a air bleeder on it. do as above. just be sure radiator is full of coolant. after hoses get hot, turn off heater, then put cap back on radiator or overflow. good-day! the first thing to do is to check for a stuck closed thermostat. do this first ok!

could be thermostat but also could be clutch fan,or a bad head gasket.

check the antifreeze level, if it is ok change the thermostat

This could possibly be a block in your heater core or the thermostat may be going bad and not circulating the antifreeze the way it should once the engine has warmed up. Have you had your antifreeze system flushed? If its a 2000 then its 10 years old and the system should be flushed and refilled

sounds like you have a bad actuator motor that is what moves the door in heater box to change from hot to cold

Check in the one heater hose will be a control valve. Check the operation of the valve(this is the heater control valve). If the lever on it appears to be moving. Take the valve out and manually check that the valve is opening and closing the whole way. If not replace the valve and recheck. If it is the heater core may be clogged not allowing the hot coolant to flow thru so you can get heat.

THERMOSTAT REPLACE IF NOT SEE IF HOSES ARE CLOGGED SHOT OF AIR COULD HELP OR DISSCONNECT BATTERY CABLE 10-15 MIN CONNECT BACK TRY HEATER

make sure antifreeze is full. run engine 10 min. verify engine is around 195 degrees. if not, try replacing thermostat. if it is running at 195, then maybe heater core problem or heater control not functioning properly

I experienced the same head ache. First attempt was to replace the thrmostat and after taking it apart I realized that there was nothing wrong with it. I then removed the plastic cover/panel located under the dash board on the passenger's side. (only a few 4-5 smalls screws) Removal will enable you to see the on/off switch/gate moving/rotating as you turn the knob to hot or cold positions, on the control panel. If that switch/gate seems to work ok, then the trouble is within the heater core. Many mechanincs suggested that I replace the heater core. A very expensive proposition.
I checked the heater core supply/intake hose and found it hot to the touch, while the return hose was nearly cold. It was definitely a heater core blockage. I removed both ends (supply and return) hose connections/clamps. They are located near the fire wall in the engine compartment. A special tool is very helpful to remove those clamps easily and to replace them as well. It can be done the old fashion way but this tool was a life saver and reduced work time to just minutes. (Tool is $70.00 bucks at auto parts store,so I borrowed mine from a mechanic/friend)
I then hooked up a garden hose to the return end of the heater core and reversed the flow of coolant/water through the heater core. My truck is a Sierra 2000 and I refuse to see how a heater core could get clogged up with clear coolant alone. To my surprise a flat piece of rubber about the size of a quarter blew out of it and the flow of water became unobstructed. I reconected the ends of the hoses to the heater core, replaced the lost antifreeze & everything returned back to normal. No money spent for something most mechanics suggested I replace control valves/switches ($280.00) replace heater core ($500.00) etc etc. I am glad I was curageous enough to tackle this myself and save a few pennies. Good luck, and I hope this will work for you.