Turned on heater to blow on floor and heard a pop. Now air blows from floor only on 1999 GMC Sierra 1500

Air stuck blowing from floor and defrost only

Asked by for the 1999 GMC Sierra 1500
1 answer 2 comments
thanks for the direction to start . could you tell me where blend door is located. Have not been under a dash in 20 yrs. Im a fork lift mechanic now.
there are 3 blend doors rt, left and middle. 1 is by gas pedal one is center of dash amd pne is under glove box area.