I have a 2005 GMC Sierra regular cab. 4.3 v6 The heater has only been blowing cold air. Checked coolant and replaced the thermostat. No change. AC worked fine this summer. Heat worked fine last winter? Suggestions?

Coolant in system ??
Coolant flowing through 'Heater Core' ??
Code(s) present w/Scanned ??
Possible HVAC issue(s);Blend air door n HVAC box (Under Dash)-Temp Actuator-HVAC control head @ dash.....

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Before assuming the heater core is blown check the following.

1. Check coolant level while engine is cold.
2. Leave the radiator cap off and let the engine run. See if you can see the coolant circulate
when the engine is brought up to temperature.
3. Check the temperature gauge inside and note if the engine temp is correct.
4 Does the engine overheat?
5. Feel the heater hoses and check if they get hot.
6. Can you smell antifreeze inside the cab?

Start here and let me know.

Make sure the coolant level is full. If coolant levels are ok you made need to replace the thermostat .You can disconnect the heater hoses going to the heater core and flushing water through with a garden hose at low pressure. If the vehicle has the 4.3L engine these are quite prone to heater cores plugging up. Would try flushing the heater core first.If that does not work most likely the thermostat is causing your issue. Be sure to fill coolant back up after servicing and allow air to bleed from the cooling system.

check the coolant level in radiator. if its low it will only circulate while driving, and not while idling. May also be a bad water pump. See a professional for that.

Find the two heater hoses going through the firewall to the heater core both need to be hot at operating temp. If both are hot the problem will be inside with the heater control. I only one heater hose is hot the
problem will be the heater core. I am assuming you have bled all the air out of the cooling system.

Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.

you have a blend door actuator that may be bad. Its a little motor that can be a bear to swap out. Unless your pretty mechanically inclined and have the right tools you may want a shop to diagnose this to be sure.

Check the water control valve behind the engine close to the firewall. This valve opens and closes to allow hot coolant to enter the heater core. It is usually connected to the heater controls with a cable. When you adjust the heater control for heat, a cable activated lever on the valve should move and allow coolant to move through the valve. The cable from the heater control can come loose from the lever, or the valve itself can become corroded and hard to move.

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.

You need to verify the blend door is operating and the heater core is not plugged or coolant flow to it is restricted.