Why is front A/C blowing hot air?

the front a.c. has two air condition senors located under dash. one or both is bad . i had the same problem last month , took my gmc to a certified gmc dealer , cost me about $800.00 . now i have cold air on both sides of my vehicle. do not allow a shade tree mechanic to do the repairs, i tried that first. god blee and be safe out there. rev. carr, dadeville,al.