Whats the temp gauge showing?

 

Any recent cooling system work? When was coolant last changed? Any leaks? Wonder if you have air in the system blocking flow thru the heater core.

 

Do not know if a 2005 user 100% electric controls for the heat & A/C ? Or if some of the controls were still vacuum operated ?

If vacuum operated , you may have a vacuum leak or something similar ?

I drive a 2008 Silverado company truck & pretty sure it is all electric / electronic .

Any check engine lite codes ?

God bless
Wyr

 

Did you check your coolant level? That's the most common cause.

 

Excellent point ! :-)

God bless
Wyr