My 2001 chevy suburban 1500 will not blow heat from the front unit will blow col on 2001 Chevrolet Suburban 1500

air back unit will blow heat

maybe the blend door is broken
Mine is doing this now. They are telling me 1st to get a flush, and have it cleaned. told it's a 50/50 chance that will solve it. won't even tell you the cost! [coolant to refill is @$32.00/gal], then if this doesn't work...told it's the heater core.