01 heater blowing cold

[aggar]

On the 01 I just got the Heater has stuck on cold. No matter where I turn the temp knob it is cold. That truck can run for an hour and still blowing cold. Which Actuator should I look into first?

Thanks.
Ryan

 

[aggar]

Hey it's F-n cold out and I need some input! Any help would be appreciated.