2005 GMC Sierra heater blowers work fine but only blows cold air until you start into a good continuous acceleration. It blows cold though no matter how long the truck sits idle after cranking. Never done this before in 3 years ownership. What could the problem be?

you have a problem with the heater/a/c controls there obviouly stuck on a/c you will need to strip out the control panel to see why the direction/vent , hot/cold , fan speed controls are not working , before doing this have you checked the fuses as there may be a seperate fuse for the control panel ,if this all checks out take to an a/c specialist as you dont want to mess around with a/c gasses etc

First check that the blower has power to it and when turned on a varying ground signal when the speed switch is moved between the speeds. You will need a DVOM to test this.

One of two things has happened. Either your coolant has gotten really low (easy enough to check) or you heater core is starting to plug up.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

sounds like you have a bad temperature door actuator. This is a motor that connects to the door and uses inputs from the sensor in itself and the heater control module to open the door from hot to cold. I does take some knowledge on recalibrating this actuator and installing it. if it isn't install correctly it could damage the new part, so just take care

replace the blower motor resistor located on the passenger side under the dash around the blower motor

may need mode door actuator

Check your rad and make sure it is full. Wait til truck is cold before removing rad cap.

Check coolant level. Check for heat at the heater hose going from water pump to heater core at firewall. Check to see that cable is moving coolant control valve from the closed to open position. Low mileage so it shouldn't be plugged heater core.

check your coolant level on the radiator

wiggle the blower resistor wire harness it is next to blower motor bad about connector melting