The heater only blows hot air

try locating the physical tap that connects your heater hoses to the interior of the car. note what position that tap is in before and after you turn up the temp dial inside. whatever connects the dial to the tap may be damaged.

if the tap itself turns then the heater side of the blower may be blocked.

check all your fuses first if they are all good you may need to replace the heater control

bad heater control switch or resistor.

If the engine is reaching normal temp, and both heater hoses are hot, you may have a problem with one of the blend doors under the dash.

Hi John,
Your right with your diagnostics, you have a blend door problem. If your switching to cool air and it is still hot then you also have a heat exchange unit fault, this is the component that changes the water flow from hot to cool by shutting off. If it is not operating correctly this will give you constant hot air no matter where you temperature switch is positioned.
As for the warm air while the engine is cold, sounds like the blend door is still open to atmosphere tempreture and if it is hot outside will pull that hot air into the inside of the vehicle.
Have the heat exchange valve checked, and the only way the check blend door operation is to remove the wiring from the blend doors and have the circuit tested. If the circuit is ok then the blend doors will need to be replaced.
Hope this helps

there should be a temperature door, but I don't know if there is a cable or if its electric. turn the temp knob, if it feels like it moves anything? I dont have a diagram here at home, but will monday/

that year / series of GM sonoma and s-10 where famous for having clogged heater cores - is temp of hoses going in and out the same ? if not try to flush out ther core, if they are, ck to see if the hot water valve is woking, and finally operation of the doors inside the heater box itself

HAVE THE OPERATION OF THE TEMPERATURE CONTROL BLEND CHECKED, IT IS LIKELY STUCK IN THE HEATER POSITION, THAT IS WHY THE HOT AIR, TO REPLACE THIS PART THE DASH MUST BE REMMOVED

You have a broken wire connection between the temperature control knob and the circuit board behind the center dash console. Vibration causes the failure and once the control knob gets loose the solder connection fails.

For someone that knows how to remove the dash unit and can re-solder a couple of new wires this fix should take no more than 1 hour. Make sure that you apply loctite to the heat control knob spindle nut when you re-assemble it.

The dealer quoted me 750 dollars to fix this issue and I found out that it's a common failure for this year. I fixed mine virtually for free but it took some tools and about an hour of my time.

Everything that is heat and cooling in a vehicle is controlled by vacuum from the engine. It appears to me that your blend door vacuum line to the temp control switch is either off or cracked/broken and not making things work the way you want/expect. It's the vacuum friend.