Heat worked fine 12/19-27th. As of 12/28 I am blowing barely lukewarm air to all vents on all speeds. Blower comes on/off as directed and blows hard. I can't seem to get any real heat however. I also must note we've had an old issue we lived with for around 6 years now of not being able to adjust the temperature while driving, only can choose before starting the truck, your start and that's the temperature you get for that trip. Wanna turn the heat down? Gotta stop, change the dial and restart. Even this began to fail in the last 12-14 months. I really think this could be the blend door which has finally just stopped getting over to the warm side and is stuck on the cool side. I am NOT A MECHANIC! I'm a nut-turner as they say...know my way around a fair amount, been using tear-down manuals for years to maintenance my cars/trucks. I've been lurking on these forums for advice and I've come up with this: Please help me to understand if I'm correct, or give me other instructions to troubleshoot. 1)Let the truck warm up and then check the heater hoses to the heater core. One warm at first, the other cool and then they should both be warm/finally hot after a couple more minutes. Note: I did experience a low coolant condition just last week... and had to add about 3/5 of a gallon to top off the system... checked yesterday and there was no noticeable loss. 2) This is a procedure question... if the warm intake hose indicates hot coolant going to the core... does hot coolant in the return hose indicate that the heater core is functioning ok? i.e. passing the coolant successfully through the heater core? 3)Check the blend door actuator.... I've seen a bit on this...mine is not clicking at all or ever which leads me to believe I have an issue with the heater core or coolant getting to the heater core.