2003 Suburban LT - AC blows hot...like full blast heater on....from the dash vents regardless of temp setting, but normal from the rear passenger vents.

It has dual side controls that are controlled with the computer via selecting a temperature.

What could this be?

 

Welcome to the the forum...
Most likely the blend door is not operating correctly, you could try the calibration function as a place to start and its free to try... if it doesn't work then you will likely have to replace the actuator

to try the calibration
Remove the HVAC fuse for 10 seconds, re-install fuse
Turn key to start no crank and wait at least 40 seconds for the doors to calibrate, do not make any adjustments to the AC during this time...
After the 40 seconds are done start the vehicle and see if it is operating correctly...
I think this is for the 2006 and up but its worth a try...

 

Considering its dual side control, is there a single blend door actuator? is it the one under the passenger dash?