Why has my 02 Suburban's heat suddenly quit?

Yes, this sounds like a failed MDA (mode door actuator) you should try the control head calibration procedure first!

1. Turn the key off.

2. Remove the "Radio" fuse from the Underhood Electrical Center (UHEC).

3. Wait 60 seconds minimum.

4. Reinstall the "Radio" fuse.

5. Turn the key on, engine off. DO NOT CHANGE ANY VALUE ON THE HVAC PANEL.

6. Wait 60 seconds.

7. Start the engine. At operating temperature verify if the HVAC system is working correctly, if not you will need to replace the mode actuator assembly.

NOTE: The Blend door controls the temperature of the discharge air, and the Mode door controls where the discharge air is directed!

Yes, you have a blend door problem which is common on those models. One of the heater blend doors is suppose to direct the flow of air through the heater core or a/c evaporator or just outside air depending on the temp setting /fresh air setting and other blend doors divert the flow of air to the various vents depending on the setting for defrost, or floor heat or a/c dash vents.

Get under the right side dash while someone else operates the heater controls and see where the clicking/thumping sound is coming from...that is probably the blend door for the heater core/evap ...either the linkage is broke or the door is jammed or the actuator is bad. The other blend doors could have problems as well if the air only comes out the upper defrost vents or the heater selector switch is bad.

A Chilton or Haynes shop manual will show how to replace the parts...parts stores sell them .

One company offers an upgrade kit to fix the blend door pivot...

http://www.heatertreater.net/chevy-suburban-00-12

Both hoses are hot, huh? Is the blower motor running?

If so, I think you're right. Sounds like the mixing door may be shut. Turn on the A/C and lower thermostat to lowest setting. Do you get cool air?

I would cycle through all the programs, heat, defrost, split-level, etc and see if the door moves.

Is it blowing at all. If not it's the blower motor, resistor pack, switch, blend doors. I would need more info on what exactly does it do, and also what it does when you try the AC. At all temperatures and settings. Then you might get a diagnosis.

You need a new heater element