06 rendezvous that the digital temp display for outside temp is wrong. The display reads 32 degree and it is actually 80 degrees so the A/C does not cool vehicle down. What is the problem?

SOURCE: temperature control sensor?

Sounds like a loose connection -38 is probably the open circuit position. Not sure where the sensor would be though.

SOURCE: external temp reading is wrong... can it be reset?

i belive thats a default reading for that sensor replace it there cheep buy from dealer and locate it in front grill som times debris hit tese and mess them up simple repair probably only one screw holding it on

SOURCE: A/C system does not cool vehicle compartment completely

Needs to be properly charged

SOURCE: the gauge in the cabin says the outside temp is

call a buick dealer and ask the parts dept...gl

SOURCE: ext temp is reading wrong external temps on dash

Sounds like the sensor is playing up, your local dealer should be able to assist.